Timex Sinclair 1000 repair

[JKnightandKARR]

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.

[Chris Savage]

-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.

using 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.

[JKnightandKARR]

-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.

using 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

[granz]

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.

As far as 121GW, NO idea what you mean other then this: https://eevblog.store/products/121gw-multimeter

That's the meter that you use to measure lightning - 1.21 jigawatts.   

[Chris Savage]

That's the meter that you use to measure lightning - 1.21 jigawatts.   

Thank you, granz for getting it!

[JKnightandKARR]

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.

As far as 121GW, NO idea what you mean other then this: https://eevblog.store/products/121gw-multimeter

That's the meter that you use to measure lightning - 1.21 jigawatts.   

That's the meter that you use to measure lightning - 1.21 jigawatts.   

Thank you, granz for getting it!

Got ya. Am tired n feeling blah tonight..

[JKnightandKARR]

Awwww man..... my BNC to BNC cable broke... I can't connect my analog scope to the freq counter...

[Chris Savage]

Awwww man..... my BNC to BNC cable broke... I can't connect my analog scope to the freq counter...

What are you trying to do?

[JKnightandKARR]

Awwww 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

[Chris Savage]

Had 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?

[JKnightandKARR]

Awwww 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

Had 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.

[JKnightandKARR]

https://jknightandkarr.altervista.org/Computers/VISTA_Newsletter_v3_n5_OCR.pdf
Another schematic

[Chris Savage]

Another 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.

[JKnightandKARR]

Another 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!

[JKnightandKARR]

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.