(https://jknightandkarr.altervista.org/Computers/zx81-schematic-1.png)
https://jknightandkarr.altervista.org/Computers/zx81-schematic-1.png (https://jknightandkarr.altervista.org/Computers/zx81-schematic-1.png)
Well, starting post here on repairing my Timex Sinclair 1000. So here is the schematics. I replaced my NPN transistor, still nothing on the screen at all. I busted out the probes for my GW Instek GOS-620 and probed around aling with using my EEVBlog 121GW DMM and I have the folowing results:
-Tested the transistor with my cheap-o LCR meter, showed good, replaced unknown status NPN.
-Last night, changed the input signal on the composite circuit to the correct wire. I traced, from the ULA pin 16, through D9 and R32 which goes into the RF modulator.
-Hooking the scope to the Pin 16, yielded no results, so I used the 121GW, which has a Hz function, Pin 16, a signal, and 5v DC voltage.
-Input signal wire on mod = nothing, 5v DC and no Hz.
-Input side of D9 = said previous measurments.
-Output side of D9 = 5v DC and no Hz.
Needless to say, I didn't check anything on R32. Going by that there's a wire on the UK side, I might try the signal inpit wire on that pin, once I locate it..
As for D9 since the freq vanished there and even though theres 5v out, could the diode be the issue or should I just locate the UK straight from Pin 16 and use that?? Obviously the US side is supposed to have a freq of somekind going into the RF box, and my scope shows NOTHING other then the 5v increase. Suggestions? I have seen mods I can buy and have concidered them.
Quote from: JKnightandKARR on Jan 02, 2026, 01:44 PM-Output side of D9 = 5v DC and no Hz.
Needless to say, I didn't check anything on R32. Going by that there's a wire on the UK side, I might try the signal inpit wire on that pin, once I locate it..
As for D9 since the freq vanished there and even though theres 5v out, could the diode be the issue or should I just locate the UK straight from Pin 16 and use that?? Obviously the US side is supposed to have a freq of somekind going into the RF box, and my scope shows NOTHING other then the 5v increase. Suggestions? I have seen mods I can buy and have concidered them.
Are there any electrolytic caps inside this unit.
Anything over 15 years old, the first thing I do (before
ever powering it up) is replace any electrolytic caps. They can leak or dry up and then they no longer work as they're supposed to.
Quote from: JKnightandKARR on Jan 02, 2026, 01:44 PMusing my EEVBlog 121GW DMM and I have the folowing results:
I can't be the
only one to see 121GW and immediately see the connection, am I?!? Please tell me that relates to what I
think it relates to.
Quote from: Chris Savage on Jan 02, 2026, 08:00 PMQuote from: JKnightandKARR on Jan 02, 2026, 01:44 PM-Output side of D9 = 5v DC and no Hz.
Needless to say, I didn't check anything on R32. Going by that there's a wire on the UK side, I might try the signal inpit wire on that pin, once I locate it..
As for D9 since the freq vanished there and even though theres 5v out, could the diode be the issue or should I just locate the UK straight from Pin 16 and use that?? Obviously the US side is supposed to have a freq of somekind going into the RF box, and my scope shows NOTHING other then the 5v increase. Suggestions? I have seen mods I can buy and have concidered them.
Are there any electrolytic caps inside this unit. Anything over 15 years old, the first thing I do (before ever powering it up) is replace any electrolytic caps. They can leak or dry up and then they no longer work as they're supposed to.
Quote from: JKnightandKARR on Jan 02, 2026, 01:44 PMusing my EEVBlog 121GW DMM and I have the folowing results:
I can't be the only one to see 121GW and immediately see the connection, am I?!? Please tell me that relates to what I think it relates to.
It's got some caps, but none between the ULA and my composite mod. I probed the Z80 and the ULA and they seam to be doing something.
As far as 121GW, NO idea what you mean other then this: https://eevblog.store/products/121gw-multimeter (https://eevblog.store/products/121gw-multimeter)
Quote from: Chris Savage on Jan 02, 2026, 08:00 PMI can't be the only one to see 121GW and immediately see the connection, am I?!? Please tell me that relates to what I think it relates to.
Quote from: JKnightandKARR on Jan 02, 2026, 08:10 PMAs far as 121GW, NO idea what you mean other then this: https://eevblog.store/products/121gw-multimeter (https://eevblog.store/products/121gw-multimeter)
That's the meter that you use to measure lightning - 1.21 jigawatts. ;D :P
Quote from: granz on Jan 02, 2026, 09:34 PMThat's the meter that you use to measure lightning - 1.21 jigawatts. ;D :P
Thank you, granz for getting it!
Quote from: granz on Jan 02, 2026, 09:34 PMQuote from: Chris Savage on Jan 02, 2026, 08:00 PMI can't be the only one to see 121GW and immediately see the connection, am I?!? Please tell me that relates to what I think it relates to.
Quote from: JKnightandKARR on Jan 02, 2026, 08:10 PMAs far as 121GW, NO idea what you mean other then this: https://eevblog.store/products/121gw-multimeter (https://eevblog.store/products/121gw-multimeter)
That's the meter that you use to measure lightning - 1.21 jigawatts. ;D :P
Quote from: Chris Savage on Jan 02, 2026, 09:37 PMQuote from: granz on Jan 02, 2026, 09:34 PMThat's the meter that you use to measure lightning - 1.21 jigawatts. ;D :P
Thank you, granz for getting it!
Got ya. Am tired n feeling blah tonight..
Awwww man..... my BNC to BNC cable broke... I can't connect my analog scope to the freq counter...
Quote from: JKnightandKARR on Jan 03, 2026, 03:28 PMAwwww man..... my BNC to BNC cable broke... I can't connect my analog scope to the freq counter...
What are you trying to do?
Quote from: Chris Savage on Jan 05, 2026, 10:01 AMQuote from: JKnightandKARR on Jan 03, 2026, 03:28 PMAwwww man..... my BNC to BNC cable broke... I can't connect my analog scope to the freq counter...
What are you trying to do?
Had the Freq counter hooked to my analog scope, but end broken
Quote from: JKnightandKARR on Jan 05, 2026, 10:46 AMHad the Freq counter hooked to my analog scope, but end broken
I apologize...what I meant was...why did you need to connect the scope to the freqency counter?
Quote from: Chris Savage on Jan 05, 2026, 10:01 AMQuote from: JKnightandKARR on Jan 03, 2026, 03:28 PMAwwww man..... my BNC to BNC cable broke... I can't connect my analog scope to the freq counter...
What are you trying to do?
Had the Freq counter hooked to my analog scope, but end broken
Quote from: Chris Savage on Jan 05, 2026, 11:34 AMQuote from: JKnightandKARR on Jan 05, 2026, 10:46 AMHad the Freq counter hooked to my analog scope, but end broken
I apologize...what I meant was...why did you need to connect the scope to the freqency counter?
I hooked it up to get reding freq from the analog scope easier to read.
Well I hooked both up to a CRT n nada from both, so guess replace the regulators and caps to start with. Also have seen ULA or whatever they are called, that use modern parts, probly PLA, and go from there.
https://jknightandkarr.altervista.org/Computers/VISTA_Newsletter_v3_n5_OCR.pdf (https://jknightandkarr.altervista.org/Computers/VISTA_Newsletter_v3_n5_OCR.pdf)
Another schematic
Quote from: JKnightandKARR on Jan 07, 2026, 01:45 PMAnother schematic
I wanted to go over what I covered on the phone so when you reply with your findings, people aren't confused. I previously mentioned replacing ALL electrolytic caps in the system. But in today's conversation I mentioned the following next steps to confirm functionality:
- Verify DC voltage at the Z80 CPU (PIN 11) using your multimeter on DC voltage. Should be ~5.0VDC and then switch to AC to measure any ripple voltage, looking for excessive noise.
- Verify the clock pulse at PIN 6 on the Z80 CPU. By using a digital scope you can see the waveform and measure the frequency at the same time, which should be ~3.25MHz.
- Verify the output of the /M1 line on the Z80 CPU (PIN 27). By using a digital scope you can see the waveform and measure the frequency. The /M1 line should have activity if the Z80 CPU is executing instructions.
Many digital scopes have a USB port which can be used for screenshots of what you're measuring. If you can do this for the clock and /M1 signals, please post them for analysis.
Quote from: Chris Savage on Jan 07, 2026, 02:35 PMQuote from: JKnightandKARR on Jan 07, 2026, 01:45 PMAnother schematic
I wanted to go over what I covered on the phone so when you reply with your findings, people aren't confused. I previously mentioned replacing ALL electrolytic caps in the system. But in today's conversation I mentioned the following next steps to confirm functionality:
- Verify DC voltage at the Z80 CPU (PIN 11) using your multimeter on DC voltage. Should be ~5.0VDC and then switch to AC to measure any ripple voltage, looking for excessive noise.
- Verify the clock pulse at PIN 6 on the Z80 CPU. By using a digital scope you can see the waveform and measure the frequency at the same time, which should be ~3.25MHz.
- Verify the output of the /M1 line on the Z80 CPU (PIN 27). By using a digital scope you can see the waveform and measure the frequency. The /M1 line should have activity if the Z80 CPU is executing instructions.
Many digital scopes have a USB port which can be used for screenshots of what you're measuring. If you can do this for the clock and /M1 signals, please post them for analysis.
Thanks Chris, will do!
https://github.com/ikjordan/picozx81?tab=readme-ov-file (https://github.com/ikjordan/picozx81?tab=readme-ov-file)
I did find this, which does involves multiple platforms one of which is the Raspberry Pi Pico. If I make one of these, might try n add the expansion port.
(https://jknightandkarr.altervista.org/Computers/PowerSupply.jpg)
https://jknightandkarr.altervista.org/Computers/PowerSupply.jpg (https://jknightandkarr.altervista.org/Computers/PowerSupply.jpg)
Power Supply at the jack.
(https://jknightandkarr.altervista.org/Computers/5v_card.jpg)
https://jknightandkarr.altervista.org/Computers/5v_card.jpg (https://jknightandkarr.altervista.org/Computers/5v_card.jpg)
5V @ the card edge connector. Verified, 5v
(https://jknightandkarr.altervista.org/Computers/9v_A.jpg)
https://jknightandkarr.altervista.org/Computers/9v_A.jpg (https://jknightandkarr.altervista.org/Computers/9v_A.jpg)
(https://jknightandkarr.altervista.org/Computers/9V_B.jpg)
https://jknightandkarr.altervista.org/Computers/9V_B.jpg (https://jknightandkarr.altervista.org/Computers/9V_B.jpg)
Couple of shots of the 9V at the card edge connector. Kinda messy..
(https://jknightandkarr.altervista.org/Computers/M1.jpg)
https://jknightandkarr.altervista.org/Computers/M1.jpg (https://jknightandkarr.altervista.org/Computers/M1.jpg)
M1 at the card edge connector. Freq = 810 Khz.
(https://jknightandkarr.altervista.org/Computers/Pin6.jpg)
https://jknightandkarr.altervista.org/Computers/Pin6.jpg (https://jknightandkarr.altervista.org/Computers/Pin6.jpg)
Pin 6. 3.25 Mhz
(https://jknightandkarr.altervista.org/Computers/Pin11.jpg)
https://jknightandkarr.altervista.org/Computers/Pin11.jpg (https://jknightandkarr.altervista.org/Computers/Pin11.jpg)
(https://jknightandkarr.altervista.org/Computers/PIN11_scope.jpg)
https://jknightandkarr.altervista.org/Computers/PIN11_scope.jpg (https://jknightandkarr.altervista.org/Computers/PIN11_scope.jpg)
Pin 11 DMM Vs Scope.... UNKNOWN why meter voltage is diff from the Scope measure, which was 5v..... DMM showed no AC voltage.
(https://jknightandkarr.altervista.org/Computers/7805.jpg)
https://jknightandkarr.altervista.org/Computers/7805.jpg (https://jknightandkarr.altervista.org/Computers/7805.jpg)
Temp at the voltage regulator
(https://jknightandkarr.altervista.org/Computers/Z80.jpg)
https://jknightandkarr.altervista.org/Computers/Z80.jpg (https://jknightandkarr.altervista.org/Computers/Z80.jpg)
Z80 Temp
Well, I mean your 5V, Φ and /M1 signals look good. So it would seem the Z80 is running.
Quote from: Chris Savage on Jan 09, 2026, 09:11 AMWell, I mean your 5V, Φ and /M1 signals look good. So it would seem the Z80 is running.
Yeah, but the Pin 11 reading dmm vs scope confuses me.... maybe retest later with diff meter.. on the heat thing. I think I'll upgrade that 7805 to the upgraded part... burnt myself last night.
Quote from: JKnightandKARR on Jan 09, 2026, 12:50 PMYeah, but the Pin 11 reading dmm vs scope confuses me.... maybe retest later with diff meter.. on the heat thing. I think I'll upgrade that 7805 to the upgraded part... burnt myself last night.
As an electrician who has worked with high voltages, there's a principle called, "LIVE-DEAD-LIVE" testing. It's a way to confirm that a circuit is shut down and safe to work on. What if you were working on 480VAC and your meter measured no voltage? You might work on the circuit and it may actually be live, but perhaps your meter had a blown fuse or bad probe / lead. Using a proving unit, you prove that the meter can detect 480VAC, then you check the circuit under test, then you check the proving unit again to confirm the meter is working. Safety first.
But at lower voltages, you can use this same principle to confirm that the meter is working properly. In your case you weren't detecting voltage at the CPU pin, but the question is, could you detect voltage using that same meter from other points? One of my power supplies has lugs with multiple voltages at the outputs (3.3V, 5V, 9V, 12V) and I will often confirm voltages at these posts before testing a circuit.
Quote from: Chris Savage on Jan 09, 2026, 03:45 PMQuote from: JKnightandKARR on Jan 09, 2026, 12:50 PMYeah, but the Pin 11 reading dmm vs scope confuses me.... maybe retest later with diff meter.. on the heat thing. I think I'll upgrade that 7805 to the upgraded part... burnt myself last night.
As an electrician who has worked with high voltages, there's a principle called, "LIVE-DEAD-LIVE" testing. It's a way to confirm that a circuit is shut down and safe to work on. What if you were working on 480VAC and your meter measured no voltage? You might work on the circuit and it may actually be live, but perhaps your meter had a blown fuse or bad probe / lead. Using a proving unit, you prove that the meter can detect 480VAC, then you check the circuit under test, then you check the proving unit again to confirm the meter is working. Safety first.
But at lower voltages, you can use this same principle to confirm that the meter is working properly. In your case you weren't detecting voltage at the CPU pin, but the question is, could you detect voltage using that same meter from other points? One of my power supplies has lugs with multiple voltages at the outputs (3.3V, 5V, 9V, 12V) and I will often confirm voltages at these posts before testing a circuit.
The meter is fine along with probes. I probed multiple spots n got readings from them, just not much from there, and that was after the DC and AC measurments. Infact, pin 11 came before pin 6.
Quote from: JKnightandKARR on Jan 09, 2026, 07:31 PMThe meter is fine along with probes. I probed multiple spots n got readings from them, just not much from there, and that was after the DC and AC measurments. Infact, pin 11 came before pin 6.
I have three types of probes for my multimeters (not counting temp, current, etc).
(https://mm.digikey.com/Volume0/opasdata/d220001/medias/images/662/TL75.jpg?hidebanner=true)
The standard probes that come with it, which are great for AC wall outlets, fuse boxes and automotive use. These don't work well for probing electronics component leads because they're bulky and not very "pointy".
(https://mm.digikey.com/Volume0/opasdata/d220001/medias/images/847/MFG_TL175.jpg?hidebanner=true)
I also have CAT III / IV probes, which are for high voltages and also have a sheath that can cover the probes to prevent a short.
But the one I use for probing electronics are pin probes.
(https://mm.digikey.com/Volume0/opasdata/d220001/medias/images/4843/MFG_501_6275.jpg?hidebanner=true)
These pin probes tend to make a better connection to leads on electronics components, having a needle-like tip that can even penetrate solder and insulation.
Quote from: Chris Savage on Jan 09, 2026, 10:18 PMQuote from: JKnightandKARR on Jan 09, 2026, 07:31 PMThe meter is fine along with probes. I probed multiple spots n got readings from them, just not much from there, and that was after the DC and AC measurments. Infact, pin 11 came before pin 6.
I have three types of probes for my multimeters (not counting temp, current, etc).
(https://mm.digikey.com/Volume0/opasdata/d220001/medias/images/662/TL75.jpg?hidebanner=true)
The standard probes that come with it, which are great for AC wall outlets, fuse boxes and automotive use. These don't work well for probing electronics component leads because they're bulky and not very "pointy".
(https://mm.digikey.com/Volume0/opasdata/d220001/medias/images/847/MFG_TL175.jpg?hidebanner=true)
I also have CAT III / IV probes, which are for high voltages and also have a sheath that can cover the probes to prevent a short.
But the one I use for probing electronics are pin probes.
(https://mm.digikey.com/Volume0/opasdata/d220001/medias/images/4843/MFG_501_6275.jpg?hidebanner=true)
These pin probes tend to make a better connection to leads on electronics components, having a needle-like tip that can even penetrate solder and insulation.
https://eevblog.store/products/brymen-bl21s2-t4sc-gold-plated-silicone-test-probes (https://eevblog.store/products/brymen-bl21s2-t4sc-gold-plated-silicone-test-probes)
This is what i got along with a similar pair that has coveres like yours do that cover all but the tips. I'll remeasure with all 3 of my meters to see what happens.
Quote from: JKnightandKARR on Jan 09, 2026, 10:35 PMI'll remeasure with all 3 of my meters to see what happens.
Did the other meters give you different readings?
While we're on the subject, what's the scope view of the video output?
Quote from: Chris Savage on Jan 11, 2026, 09:55 PMQuote from: JKnightandKARR on Jan 09, 2026, 10:35 PMI'll remeasure with all 3 of my meters to see what happens.
Did the other meters give you different readings?
While we're on the subject, what's the scope view of the video output?
I haven't had the time to mess with it, work been sucky n I'm too tired to care about anything at the moment... but will do it soon.
Quote from: JKnightandKARR on Jan 11, 2026, 11:31 PMI haven't had the time to mess with it, work been sucky n I'm too tired to care about anything at the moment... but will do it soon.
No problem. I understand. It's just that, while we are exploring alternative options for you on the Z80 front, I would like to see you get this classic (vintage) computer working, if possible.
Also, related to this: Brian (AKA @MicroNut) has (had?) a ZX-81 remake. He brought it to a retro-computing show that I put on at our local library. If I remember right, he had a digital "cassette tape" system that connected to the Sinclair's tape interface. Maybe he can give some information about his system.
Quote from: granz on Jan 12, 2026, 03:55 PMAlso, related to this: Brian (AKA @MicroNut) has (had?) a ZX-81 remake.
Define: remake?
Quote from: Chris Savage on Jan 12, 2026, 04:44 PMQuote from: granz on Jan 12, 2026, 03:55 PMAlso, related to this: Brian (AKA @MicroNut) has (had?) a ZX-81 remake.
Define: remake?
i've been doing research n you can get parts to make all new systems.
Quote from: JKnightandKARR on Jan 12, 2026, 06:51 PMi've been doing research n you can get parts to make all new systems.
If you're talking about the Minstrel Issue 3 Sinclair ZX81-Compatible Microcomputer Kit (https://www.hackster.io/news/tynemouth-software-launches-minstrel-issue-3-sinclair-zx81-compatible-microcomputer-kit-627bb602863b), I've posted about these before, but didn't think it was an exact replacement, but rather a clone using some updated parts.
(https://hackster.imgix.net/uploads/attachments/1026338/image_U5I2XJDrgB.png)
I was going to order one of these last year, but the the tariff thing ruined that.
Quote from: Chris Savage on Jan 12, 2026, 08:00 PMQuote from: JKnightandKARR on Jan 12, 2026, 06:51 PMi've been doing research n you can get parts to make all new systems.
If you're talking about the Minstrel Issue 3 Sinclair ZX81-Compatible Microcomputer Kit (https://www.hackster.io/news/tynemouth-software-launches-minstrel-issue-3-sinclair-zx81-compatible-microcomputer-kit-627bb602863b), I've posted about these before, but didn't think it was an exact replacement, but rather a clone using some updated parts.
(https://hackster.imgix.net/uploads/attachments/1026338/image_U5I2XJDrgB.png)
I was going to order one of these last year, but the the tariff thing ruined that.
One i know of was ZX81 +38 or something like that
Quote from: JKnightandKARR on Jan 12, 2026, 09:58 PMOne i know of was ZX81 +38 or something like that
I was going to order this one (https://www.tindie.com/products/tynemouth/minstrel-3-with-keyboard-z80-based-zx81-kit/), but as you can see, shipping to the U.S.
starts at $50.00!!!
Quote from: Chris Savage on Jan 12, 2026, 10:52 PMQuote from: JKnightandKARR on Jan 12, 2026, 09:58 PMOne i know of was ZX81 +38 or something like that
I was going to order this one (https://www.tindie.com/products/tynemouth/minstrel-3-with-keyboard-z80-based-zx81-kit/), but as you can see, shipping to the U.S. starts at $50.00!!!
Order that with the sockets for the chips! Only an additional $50.... lol kinda $$$$$$ can buy an original for 1/4 cost
Quote from: Chris Savage on Jan 12, 2026, 10:52 PMQuote from: JKnightandKARR on Jan 12, 2026, 09:58 PMOne i know of was ZX81 +38 or something like that
I was going to order this one (https://www.tindie.com/products/tynemouth/minstrel-3-with-keyboard-z80-based-zx81-kit/), but as you can see, shipping to the U.S. starts at $50.00!!!
https://github.com/mahjongg2 (https://github.com/mahjongg2)
I found this, idk if this looks interesting to you or not, beats $250... lol
Edit: Here's what looks like original PCB of the ZX-81
https://github.com/Bluescreen2001/ZX81-PCB/tree/main/ZX81_keyboard (https://github.com/Bluescreen2001/ZX81-PCB/tree/main/ZX81_keyboard)
Quote from: Chris Savage on Jan 12, 2026, 04:44 PMQuote from: granz on Jan 12, 2026, 03:55 PMAlso, related to this: Brian (AKA @MicroNut) has (had?) a ZX-81 remake.
Define: remake?
A modern remake of the old ZX-81. There are several (one of them is here: https://www.hackup.net/2022/01/building-a-new-zx81-computer/ (https://www.hackup.net/2022/01/building-a-new-zx81-computer/).) I don't remember which one Brian had, as that show was many years ago. That is why I tagged Brian, I was hoping that he could pitch in.
Quote from: JKnightandKARR on Jan 12, 2026, 11:51 PMOrder that with the sockets for the chips! Only an additional $50.... lol kinda $$$$$$ can buy an original for 1/4 cost
Yeah, I did think the $50.00 for extra sockets was a bit much. Of course, the modernized versions don't require an old TV to get video.
Quote from: granz on Jan 13, 2026, 06:57 AMA modern remake of the old ZX-81. There are several (one of them is here:
I don't think tagging works on SMF. :-\ That PCB looks like the one JKnightandKARR (https://savagechats.com/index.php?action=profile;u=4) linked. Mahjongg?
The remake that Granz was mentioning that I had was a DE0-Nano FPGA board with a ZX81 softcore called ZXGate I found on the internet. The "cassette" was an Arduino Due which had a TFT screen and a SD card reader. Sadly I lost the code for the Arduino but I also made a "cassette" with a Propeller 1, SD card reader and a serial 16x2 LCD. I no longer have that code either. :(
The cassette remake is just reading an SD card and then bit banging pulses based on each bit of each byte. Once the timing is determined and using a 3.3V output you can feed it into the "mic" jack. It wouldn't be too hard to replicate it again and I found that it is more reliable than the actual cassette system.
I also have 2 TS1000s, a US ZX81 and the ZXMore kit (can also do ZX80) that is no longer available.
Thanks Brian, I was pretty sure (>99%) that you had a TS-1000 clone, but I forgot that it was an FPGA system. I also think that I have a DE-0 system somewhere, that I inherited when Peter moved out.
By the way - did you get an email alert from my tagging you, or did you just happen to log in and see the posts?
Quote from: Chris Savage on Jan 13, 2026, 10:22 AMQuote from: JKnightandKARR on Jan 12, 2026, 11:51 PMOrder that with the sockets for the chips! Only an additional $50.... lol kinda $$$$$$ can buy an original for 1/4 cost
Yeah, I did think the $50.00 for extra sockets was a bit much. Of course, the modernized versions don't require an old TV to get video.
Just alittle bit... lol
Quote from: MicroNut on Jan 13, 2026, 03:15 PMThe remake that Granz was mentioning that I had was a DE0-Nano FPGA board with a ZX81 softcore called ZXGate I found on the internet. The "cassette" was an Arduino Due which had a TFT screen and a SD card reader. Sadly I lost the code for the Arduino but I also made a "cassette" with a Propeller 1, SD card reader and a serial 16x2 LCD. I no longer have that code either. :(
The cassette remake is just reading an SD card and then bit banging pulses based on each bit of each byte. Once the timing is determined and using a 3.3V output you can feed it into the "mic" jack. It wouldn't be too hard to replicate it again and I found that it is more reliable than the actual cassette system.
I also have 2 TS1000s, a US ZX81 and the ZXMore kit (can also do ZX80) that is no longer available.
That'd be VERY handy! I'd like one if one can be recreated.
Quote from: Chris Savage on Jan 13, 2026, 10:25 AMQuote from: granz on Jan 13, 2026, 06:57 AMA modern remake of the old ZX-81. There are several (one of them is here:
I don't think tagging works on SMF. :-\ That PCB looks like the one JKnightandKARR (https://savagechats.com/index.php?action=profile;u=4) linked. Mahjongg?
Did you see the files I posted? I think original PCB designs, as well as new designs as you already saw, for the ZX=81 or Timex Sinclair 1000, seem that the 2 systems, even though technically the same, are somewhat different, at least as far as the RF boxes are concerned.
Granz, no I didn't see the alert.
JKKnightandCARR - sadly I lost all my ZX81 FPGA and Cassette source code when the external hard drive I had it on went bad. I do have the ZX-gate source though which is good since it is no longer on the net.
I'm thinking of redoing it. I know the ZX-Gate used a PS2 Keyboard and VGA output. When I used the DE0-Nano I tapped into the video output before the vga conversion and had to use a transistor driver to get the composite output. I now have a DE10-Lite which has a built in VGA and Arduino header with double the space of the Nano so I will do it on that instead. Once that is done it'll be easy to port it over to the Nano
I'll start a thread on the Projects category when I get it going so I won't hijack this thread.
Quote from: MicroNut on Jan 13, 2026, 11:00 PMGranz, no I didn't see the alert.
I've looked into mods for SMF that can do this, but the few I see don't look like they've been updated in years.
EDIT: Ironically, the manual says they're supported. I need to check on permissions. I will look into this now.
@Chris Savage I think I gotta place to start. Posted on the Vintage Computer Forum and was told to check Reset (Pin 26) which is active low, so 0v is expected. at the card edge connector, I am getting 5v, so here's the place to start I think.
Quote from: JKnightandKARR on Jan 22, 2026, 10:05 PM@Chris Savage
I think I gotta place to start. Posted on the Vintage Computer Forum and was told to check Reset (Pin 26) which is active low, so 0v is expected. at the card edge connector, I am getting 5v, so here's the place to start I think.
Reset is, as you mentioned, an active low signal. That means that when it is reading 0V, the CPU is in reset condition. When
Reset is +5V, then the CPU is
not in reset, and should be running properly.
Quote from: granz on Jan 22, 2026, 11:08 PMQuote from: JKnightandKARR on Jan 22, 2026, 10:05 PM@Chris Savage
I think I gotta place to start. Posted on the Vintage Computer Forum and was told to check Reset (Pin 26) which is active low, so 0v is expected. at the card edge connector, I am getting 5v, so here's the place to start I think.
Reset is, as you mentioned, an active low signal. That means that when it is reading 0V, the CPU is in reset condition. When Reset is +5V, then the CPU is not in reset, and should be running properly.
Ok, the VCF forums said it should be low. Probly replace all caps n go from there...
Quote from: JKnightandKARR on Jan 23, 2026, 01:12 AMOk, the VCF forums said it should be low. Probly replace all caps n go from there...
The VCF forums said the /RESET line should be LOW normally? That is incorrect. It should only be LOW for like a millisecond at power-up. After that it should remain HIGH. And it must be, since you already confirmed that the /M1 line was changing states, which indicates the Z80 is executing instructions. It couldn't do that if /RESET was LOW.
Quote from: Chris Savage on Jan 23, 2026, 09:11 AMQuote from: JKnightandKARR on Jan 23, 2026, 01:12 AMOk, the VCF forums said it should be low. Probly replace all caps n go from there...
The VCF forums said the /RESET line should be LOW normally? That is incorrect. It should only be LOW for like a millisecond at power-up. After that it should remain HIGH. And it must be, since you already confirmed that the /M1 line was changing states, which indicates the Z80 is executing instructions. It couldn't do that if /RESET was LOW.
Ok, then that's normal then. Rechecked n I missread the info..
(https://jknightandkarr.altervista.org/Computers/20260128_112338.jpg)
https://jknightandkarr.altervista.org/Computers/20260128_112338.jpg (https://jknightandkarr.altervista.org/Computers/20260128_112338.jpg)
Well, Hooked up my Rigol DS1194Z's Logic Analyzer up to D0-D3, saw no need to try all 8 data lines. Anyhow, the Z80 is clearly doing something, so thinking of picking up one of the more modern replacements, such as this: https://www.tindie.com/products/charlieingley/vla81-zx81-ula-replacement/ (https://www.tindie.com/products/charlieingley/vla81-zx81-ula-replacement/) since from my searching, the originals are prone to failure, so that's what I have so far.
So it appears that the issue is either in the video output, or the video input of your display.
Before you spend $33, plus S/H, have you tried swapping the ULA from the other TS1000? That may help (unless the other one is also dead - although it shouldn't cost you more than a bit of time to try it.)
Quote from: Chris Savage on Jan 28, 2026, 11:59 AMSo it appears that the issue is either in the video output, or the video input of your display.
My TV is fine, I been using the inputs for my Wii. I'll hook up my scope to pin 16 of the ULA n see what I get now that I have it figured out, shouldn't be my composite mod, iI replaced my transistor with one I tested with my LCR meter.
Quote from: granz on Jan 28, 2026, 12:59 PMBefore you spend $33, plus S/H, have you tried swapping the ULA from the other TS1000? That may help (unless the other one is also dead - although it shouldn't cost you more than a bit of time to try it.)
Can't hurt, however it didn't work either when I hooked up to. CRT tv. As I said above, probly hook up my scope to the ULA to see what I get.
Quote from: JKnightandKARR on Jan 28, 2026, 01:09 PMCan't hurt, however it didn't work either when I hooked up to. CRT tv.
Yeah, I was meaning that you could swap the ULAs and then try both computers; it is possible that one ULA, and the other TS-1000 may work. That would be a quick, easy way to test that.
Quote from: granz on Jan 28, 2026, 01:14 PMQuote from: JKnightandKARR on Jan 28, 2026, 01:09 PMCan't hurt, however it didn't work either when I hooked up to. CRT tv.
Yeah, I was meaning that you could swap the ULAs and then try both computers; it is possible that one ULA, and the other TS-1000 may work. That would be a quick, easy way to test that.
Yup.
(https://jknightandkarr.altervista.org/Computers/Ch2_ULA_Pin16.jpg)
https://jknightandkarr.altervista.org/Computers/Ch2_ULA_Pin16.jpg (https://jknightandkarr.altervista.org/Computers/Ch2_ULA_Pin16.jpg)
Not much here.
(https://jknightandkarr.altervista.org/Computers/Ch3_ULA_Pin16.jpg)
https://jknightandkarr.altervista.org/Computers/Ch3_ULA_Pin16.jpg (https://jknightandkarr.altervista.org/Computers/Ch3_ULA_Pin16.jpg)
WE HAVE A SIGNAL!!
(https://jknightandkarr.altervista.org/Computers/After_Trans.jpg)
https://jknightandkarr.altervista.org/Computers/After_Trans.jpg (https://jknightandkarr.altervista.org/Computers/After_Trans.jpg)
This is the signal after the transistor.
(https://jknightandkarr.altervista.org/Computers/RCA_Out.jpg)
https://jknightandkarr.altervista.org/Computers/RCA_Out.jpg (https://jknightandkarr.altervista.org/Computers/RCA_Out.jpg)
Output on the jack. Pretty much the same.
If I am guessing, which I am, I'd say my issue is the composite mod... I followed the directions, so either my NPN is fried, even though my cheap LCR meter says "We're Good" or something wrong with the design and it fried when turned it on... IDK... Guess buy a composite mod kit n go from there.
https://web.archive.org/web/20250123231729/https://www.bytedelight.com/?page_id=3560 (https://web.archive.org/web/20250123231729/https://www.bytedelight.com/?page_id=3560)
This is the circuit I used for the composite mod btw.
Well...... Pretty sure I figured out my problem, on one of the ZX-81s anyhow... Circuit above is the one I used. Now...... PROVIDING..... I could pay attention to what I was doing, I'd wire it as in the link.....
E-Output
B- Signal
C- 5V+
it MIGHT work..... Me:
E-Signal
B- 5v
C- Output
Need to pay more attention and not work on stuff when tired..... new transistor and will try again..... I think Bill Engval said it best...... HERE'S YOUR SIGN........
Well, it seems you may have figured it out. Let us know how it goes.
Quote from: Chris Savage on Feb 06, 2026, 09:38 PMWell, it seems you may have figured it out. Let us know how it goes.
Just guessing, but if the transistor was wired right, probly would have worked...
(https://jknightandkarr.altervista.org/Computers/screen.jpeg)https://jknightandkarr.altervista.org/Computers/screen.jpeg (https://jknightandkarr.altervista.org/Computers/screen.jpeg)
It's a start!!
Congrats, Joe! Hopefully you will get the rest worked out quickly.
Looks like, "POKE" and then some scribble. But, as
@granz said, it's a start. At least you know now that it's alive.
Quote from: granz on Feb 08, 2026, 08:52 PMCongrats, Joe! Hopefully you will get the rest worked out quickly.
Thanks. I'll probly replace that mod with one that'll work better.
Quote from: Chris Savage on Feb 08, 2026, 09:43 PMLooks like, "POKE" and then some scribble. But, as @granz said, it's a start. At least you know now that it's alive.
Yeah. The mod 1) didn't have power, and the 33 ohm resistor was killing the signal, so maybe it's not strong enough??
Quote from: JKnightandKARR on Feb 08, 2026, 09:56 PMYeah. The mod 1) didn't have power, and the 33 ohm resistor was killing the signal, so maybe it's not strong enough??
(https://blog.codesurfer.dev/content/images/size/w1000/2021/12/Screen-Shot-2021-12-28-at-1.51.12-PM.png)
So, I looked around on the internet and there is a lot of varied schematics, and even in the ones like yours, much variance in the values (https://blog.codesurfer.dev/zx81-timex-sinclair-1000-composite-video-mod/).
Many people who have taken this task on have expressed issues that they've had finding the right values (http://www.breakintoprogram.co.uk/computers/composite-video-modification-for-the-zx81) to get the signal they want. It seems there may also be a little bit of difference in the various displays and the signals they require.
As to your original question
and schematic. I was concerned when I saw the 100Ω resistor, since at 5V (minus the transistor voltage drop), you're looking at nearly 200mW of power consumption, which is the max of the 2N3904. Normally you wouldn't choose a transistor that is at the max of your electrical specs.
Quote from: Chris Savage on Feb 08, 2026, 10:57 PMQuote from: JKnightandKARR on Feb 08, 2026, 09:56 PMYeah. The mod 1) didn't have power, and the 33 ohm resistor was killing the signal, so maybe it's not strong enough??
(https://blog.codesurfer.dev/content/images/size/w1000/2021/12/Screen-Shot-2021-12-28-at-1.51.12-PM.png)
So, I looked around on the internet and there is a lot of varied schematics, and even in the ones like yours, much variance in the values (https://blog.codesurfer.dev/zx81-timex-sinclair-1000-composite-video-mod/).
Many people who have taken this task on have expressed issues that they've had finding the right values (http://www.breakintoprogram.co.uk/computers/composite-video-modification-for-the-zx81) to get the signal they want. It seems there may also be a little bit of difference in the various displays and the signals they require.
As to your original question and schematic. I was concerned when I saw the 100Ω resistor, since at 5V (minus the transistor voltage drop), you're looking at nearly 200mW of power consumption, which is the max of the 2N3904. Normally you wouldn't choose a transistor that is at the max of your electrical specs.
That circuit, better you think? I see 1 issue with mine with that documentation, is the ULA 2C184E vs ULA 2C210E. The ULA 2C210E has the back porch signal, the other doesn't, and mine is the 2C184E, so NO backporch signal. This might be part of the problem, according to the documentation.
EDIT: I found this http://zx.zigg.net/misc-projects/ZX81_Video_Conditioning.pdf?ref=blog.codesurfer.dev (http://zx.zigg.net/misc-projects/ZX81_Video_Conditioning.pdf?ref=blog.codesurfer.dev) This is what I need ain't it?
Once you get your TS working, you will want some books for it. I had several books, but these are my favorites:
(https://www.timexsinclair.com/wp-content/uploads/2020/04/The-ZX81-Companion-191x300.jpg)
http://[url="https://dn720002.ca.archive.org/0/items/the-zx-81-companion/The%20ZX81%20Companion_text.pdf"
This has a disassembly of the ZX-81's ROM in the back. I don't really remember much else than that, but I used mine so much that it got pretty dog-eared.
(https://www.timexsinclair.com/wp-content/uploads/2020/04/BytingTS1000-L-203x300.jpg)
https://www.amazon.com/Byteing-Deeper-into-Timex-Sinclair/dp/0471898880 (https://www.amazon.com/Byteing-Deeper-into-Timex-Sinclair/dp/0471898880) $16.73 + $4.49 S/H (Could not find this as a PDF.)
(https://www.technophile.one/wp-content/uploads/2022/01/Sinclair-ZX81-Manual-1.jpg)
https://www.historybit.it/wp-content/uploads/2018/09/ZX81_BasicProgramming.pdf (https://www.historybit.it/wp-content/uploads/2018/09/ZX81_BasicProgramming.pdf)
I was always captivated by the cover picture of the programming manual. It still grabs my attention.
(https://technonew.speccy.org/album/ref_card/images/ref_card_1_2_3_jpg.jpg)
https://www.scribd.com/document/323515982/zx80-81-refcard (https://www.scribd.com/document/323515982/zx80-81-refcard)
This was another well-used document from my ZX-81 days.
I don't remember having this (although I probably did get it with my kit,) but it may help:
https://k1.spdns.de/Vintage/Sinclair/80/Sinclair%20ZX81/Service%20and%20Assembly%20Manual/ZX81%20Service%20and%20Assembly%20Manual%20%28scans%29.pdf
Hope these help.
@granz Those books are definitely great. The reference card was well used by me. In my moves I lost it though. :( A couple of years ago Bean (on the Parallax forums) was selling his ZX81 books and the reference card was one of them. So I bought it and some other books. Some of them are the ones you mentioned.
Quote from: MicroNut on Feb 09, 2026, 09:36 AM@granz Those books are definitely great. The reference card was well used by me. In my moves I lost it though. :( A couple of years ago Bean (on the Parallax forums) was selling his ZX81 books and the reference card was one of them. So I bought it and some other books. Some of them are the ones you mentioned.
I had that reference card in my books, and carried it pretty much everywhere; that and
The ZX81 Companion. I actually had many more books about the ZX81, but those listed above were my favorites.
Quote from: granz on Feb 09, 2026, 10:33 AMQuote from: MicroNut on Feb 09, 2026, 09:36 AM@granz Those books are definitely great. The reference card was well used by me. In my moves I lost it though. :( A couple of years ago Bean (on the Parallax forums) was selling his ZX81 books and the reference card was one of them. So I bought it and some other books. Some of them are the ones you mentioned.
I had that reference card in my books, and carried it pretty much everywhere; that and The ZX81 Companion. I actually had many more books about the ZX81, but those listed above were my favorites.
I'll check those out, thanks. IK do have the manual and one for games I got with it. OK, I pulled the ULA from both of them, and did a swap. For ease I am calling the one I bought Mine/My, and the one Granz/His sent me as Granz. Anyhow, My ULA is the old ver so, no backporch signal, Granz has the newer ULA with the backporch signal. After the swap, Mine no longer has a picture and the input looses and regains the signal, so maybe the newer ULA is bad, so Will do a swap back, and go from there.
Quote from: JKnightandKARR on Feb 09, 2026, 11:45 AMAfter the swap, Mine no longer has a picture and the input looses and regains the signal, so maybe the newer ULA is bad, so Will do a swap back, and go from there.
Yep, sounds like the ULA in the system that I sent is no good. Like I said, I have/had no idea of the status of that computer.
Good troubleshooting work, Joe.
Quote from: granz on Feb 09, 2026, 11:48 AMQuote from: JKnightandKARR on Feb 09, 2026, 11:45 AMAfter the swap, Mine no longer has a picture and the input looses and regains the signal, so maybe the newer ULA is bad, so Will do a swap back, and go from there.
Yep, sounds like the ULA in the system that I sent is no good. Like I said, I have/had no idea of the status of that computer.
Good troubleshooting work, Joe.
Going to recheck the sockets and make sure it is seated correctly, also was GOING to try the one you sent since it has a working ULA on my other TV, though my VCR, but figures.... my VCR broke yesterday...
Quote from: MicroNut on Jan 13, 2026, 03:15 PMThe remake that Granz was mentioning that I had was a DE0-Nano FPGA board with a ZX81 softcore called ZXGate I found on the internet. The "cassette" was an Arduino Due which had a TFT screen and a SD card reader. Sadly I lost the code for the Arduino but I also made a "cassette" with a Propeller 1, SD card reader and a serial 16x2 LCD. I no longer have that code either. :(
The cassette remake is just reading an SD card and then bit banging pulses based on each bit of each byte. Once the timing is determined and using a 3.3V output you can feed it into the "mic" jack. It wouldn't be too hard to replicate it again and I found that it is more reliable than the actual cassette system.
I also have 2 TS1000s, a US ZX81 and the ZXMore kit (can also do ZX80) that is no longer available.
Just curious, but happen to know a good source of parts? I need a ULA w/backporch and preferably a composite mod with adjustable settings for image for each of my systems. I found one, but NO shipping to USA available...
Quote from: JKnightandKARR on Feb 26, 2026, 08:56 PMJust curious, but happen to know a good source of parts? I need a ULA w/backporch and preferably a composite mod with adjustable settings for image for each of my systems. I found one, but NO shipping to USA available...
At a glance I found the following...
Sinclair ZX81 / ZX80 / Timex 1000 - Composite Video with Backporch - Kit (https://ebay.us/m/NfRjBY)
Like the C=64, sometimes you have to buy bad units that are sold for parts. Doesn't guarantee you'll get the parts you need, but here are two examples.
Timex Sinclair 1000 Personal Computer (PARTS/NOT WORKING) -Not Tested- AS-IS- (https://ebay.us/m/O464Kf)
Timex Sinclair 1000 Personal Computer & 1016 RAM, UNTESTED For Parts (https://ebay.us/m/19idA5)
SellMyRetro in the UK sells a lot of ZX81 related items. They have VLA81 (a drop in modern replacement for the ULA) for sale https://www.sellmyretro.com/offer/details/vla81--_-sinclair-zx81-ula-replacement--_-new-63982 (https://www.sellmyretro.com/offer/details/vla81--_-sinclair-zx81-ula-replacement--_-new-63982).
Also I have been on a ZX81 forum with a wealth of info and friendly people ZX Sinclair World (https://www.sinclairzxworld.com/index.php)
With that seller being in New York, the shipping is as low as a very reasonable $5.
I found some useful stuff on Ebay, just need the $$. Thanks all.