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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 21st Jan 2021, 10:18 pm   #201
ScottishColin
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Default Re: Non-working Commodore PET 3016

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Originally Posted by SiriusHardware View Post
Mark, are you still having difficulty viewing the circuit daigrams online or was that just on one particular device? If you can't view them at all I can try grabbing them and emailing them to you.

This is where I wish my Sybex 'Programming the 6502' wasn't buried in the garage. It's probably been nibbled away to nothing by Silverfish by now.

I'll have a look to see where the input to RDY comes from on this system.
So according to my poor schematic reading, input to Pin2 on the 6502 comes from Pin23 J4.

http://www.zimmers.net/anonftp/pub/c...N/320349-1.gif

Is that right? Not sure it can be as there's nothing plugged into J4.
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Old 21st Jan 2021, 10:25 pm   #202
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Default Re: Non-working Commodore PET 3016

#196 looks like an original nmos 6502 to me.

RDY held high and the 1MHz clock on pin 37, 39 and 3.

Reset on pin 40 should be low for a short time and then stays high.

Then you should have activity on SYNC pin 7 and on the address lines.

I think its still a good idea to check each of those pins again using the scope.

Another thought is that even if the 6502 is not running, there should probably still be some output visible on the display, but would show garbage. Again that probably needs someone with experience with the pet to confirm.
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Old 21st Jan 2021, 10:26 pm   #203
SiriusHardware
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Default Re: Non-working Commodore PET 3016

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input to Pin2 on the 6502 comes from Pin23 J4
Yes, I find the same. The RDY line goes to pin 23 of the memory expansion connector J4. It does go to one other thing, though, a pull up resistor which should keep it pulled up to +5V(ish) if nothing is pulling it low.

The resistor in question is R14, 1K, can you just visually inspect that resistor and also (with power off), use your old meter to measure the resistance of that resistor in-circuit? Also check that you have continuity (zero ohms) between the 6502 pin 2 and one end of that resistor?
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Old 21st Jan 2021, 10:28 pm   #204
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Default Re: Non-working Commodore PET 3016

fyi I have a copy of "The PET revealed" by Nick Hampshire which has all the circuit diagrams in Appendix A. I should have looked at that before...
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Old 21st Jan 2021, 10:33 pm   #205
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Default Re: Non-working Commodore PET 3016

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Originally Posted by SiriusHardware View Post
Mark, are you still having difficulty viewing the circuit daigrams online or was that just on one particular device? If you can't view them at all I can try grabbing them and emailing them to you.

This is where I wish my Sybex 'Programming the 6502' wasn't buried in the garage. It's probably been nibbled away to nothing by Silverfish by now.

I'll have a look to see where the input to RDY comes from on this system.
Difficulty looking in the zip file is just an ipad issue. But my laptop is being used for work and I use a different username to keep work separate from hobbies and personal stuff, other wise i get distracted and donít do any work. Just checking in on the ipad when i take a break.
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Old 21st Jan 2021, 10:46 pm   #206
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Default Re: Non-working Commodore PET 3016

Do I take it that RDY is an input only? It just has one 1K pullup resistor and the rest of the track heads off to one of the connections on the expansion connector and apparently no-where else.

The track between R14 and the CPU pin 2 is quite long and on the top side so it will be worth looking to see if it, or the connection between CPU pin 2 and that track, are open circuit.

Next thing would be to see if pin 2 has a low resistance to 0V with power off and / or whether the voltage on that socket pin comes up to +5V with the CPU removed.

SYNC goes nowhere except to the expansion connector.
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Old 22nd Jan 2021, 2:19 am   #207
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Default Re: Non-working Commodore PET 3016

Ready is similar to nwait on the 8060 or z80, but only extends memory read cycles on the original nmos 6502. Colin verified its held high so shouldn’t be stopping the 6502 from running.

Checking sync output was just another possible check if the 6502 was doing something, should be high during instruction fetch, but Colin shows no activity and stays high.

Reset is high, so shouldn’t be stopping the 6502 from running. It would be good to verify that it is low for a short time after power up, just in case the 6502 is in some unknown state. Would also be worth checking the circuit that drives reset, see if there is an easy way to force reset low without risk of damage by grounding the output of the gate driving reset.

Seems colin verified clock input and output from the 6502, so it should be running and accessing memory but seems like it isn’t so starting to suspect the 6502.

Still worth looking over the signals with a scope to confirm what we think the multimeter is telling us.

Might still be worth looking at the video timing circuit to see if there are any problems there too.
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Old 22nd Jan 2021, 9:01 am   #208
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Default Re: Non-working Commodore PET 3016

I must have been tired last night, read the SYNC and RDY voltages the wrong way around so Colin, ignore #203 and #206.

Agree with Mark that if you can use your scope to verify that there is / is not activity on at least pins 9, 10, 11 of the CPU we can go forward from there. If you had a spare CPU we would ask you just to try that in, but I don't think you do so we'll proceed by other means.
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Old 22nd Jan 2021, 10:52 am   #209
ScottishColin
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Default Re: Non-working Commodore PET 3016

Whilst I get on with the testing (which may take me a day or two to get used to the scope) it seems it might be a good idea to buy a spare 6052 to rule that out.

I've googled but there seem to be many 6502 variants. Do either of you know somewhere that sells the 'right' 6502?

Thanks.
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Old 22nd Jan 2021, 10:58 am   #210
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Default Re: Non-working Commodore PET 3016

I have a saying, 'Always try the easy / cheap thing first'. If you can wait for a day or two until you are a bit more familiar with your scope, you may be able to rule out the need to buy another 6502 and save some money. If you just like the idea of having a spare, by all means go ahead anyway.
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Old 22nd Jan 2021, 12:53 pm   #211
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Default Re: Non-working Commodore PET 3016

post #203
Continuity confirmed between the left end of R14 and Pin2 of the 6502

Resistance measured with the old meter set at 2K - 0.945
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Old 22nd Jan 2021, 1:02 pm   #212
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Default Re: Non-working Commodore PET 3016

That's useful to have confirmed but I did say earlier that it was not necessary to make the measurements in #203 and #206 - a bit of brain-fade on my part, I thought you had measured a low voltage on RDY, not the high voltage you actually measured.

Focus now on using your scope to look first at the 1MHz output on UG5 pin 7 (which we are sure is there, so it's a good practice target) and then move to 6502 pins 9, 10, 11 (at least) to see if you have any activity on those pins.
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Old 22nd Jan 2021, 3:02 pm   #213
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Default Re: Non-working Commodore PET 3016

OK - link to the first scope tests attached. I've put them into a doc so you can see the screenshots.

I have set the scope to Auto each time to try to get the best view of the waveforms. I guess I need to know whether they look good or awful - they're certainly not square.

I can get no reading at all from pins 9,10 and 11 of the 6502. I used the resistors we pinned to UG4 for each of the tests. Was that right for the 6502 tests?

https://drive.google.com/file/d/1H9z...ew?usp=sharing

Thanks.
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Old 22nd Jan 2021, 3:39 pm   #214
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Default Re: Non-working Commodore PET 3016

For scope tests you need to to have the earth side (croc clip) of the scope probe lead to a known 0V point. The resistor lead going to UG4 pin 8 pad will be OK for this but don't let the croc lead clip short to anything else (like the leg of the other resistor).

The 2.5V centre-point you made with those resistors was for the benefit of the 'frequency' feature on your new meter only. For all other voltage and scope measurements use a known 0V point as ground.

I'll take a look at the images when I get home.
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Old 22nd Jan 2021, 3:52 pm   #215
ScottishColin
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Default Re: Non-working Commodore PET 3016

Understood. I have disconnected the resistors from each other and connected the clip to the pin 8 side of UG4 (hope that makes sense) as per your pst.

Please use this link:

https://drive.google.com/file/d/1Q77...ew?usp=sharing
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Old 22nd Jan 2021, 4:24 pm   #216
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Default Re: Non-working Commodore PET 3016

I know little about PETs but now a 'scope is available is it worth verifying that the three video signals are present (or not) on the monitor's board? If Colin is uncomfortable (understandably) about live measurements near the CRT (10kV!) this could be done at the socket end of the connections to the motherboard followed by wiring continuity checks with the PET switched off. Just a thought.

Personally I'd also clean all the plugs and sockets connecting the monitor especially knowing that the machine has lain dormant for a lengthy period. After all the main fault symptom is 'no display'.

Forgive me if I've missed something but this has developed into a rather long thread.

Alan
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Old 22nd Jan 2021, 5:49 pm   #217
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Default Re: Non-working Commodore PET 3016

We've had some setbacks mainly due to an errant meter which refused to read a signal which was actually there, so we are right back at the start now but this time equipped with a scope.

What we are pursuing at the moment is just to find out whether the CPU is active or not because if it is not then there will be no point in looking at video signals.

As it happens the monitor on these is so primitive that it does not have its own line oscillator stage, so as such it relies on a stream of sync pulses being output by the mainboard to excite the line output stage in the monitor.

You are therefore quite right to suggest that a first line of attack would be to see if the video and sync signals are there and then if they are, move on to the monitor and if not, fall back to the motherboard. We did try that quite early on but then got mired down by problems with measurements of frequencies in particular, as Colin did not then have any other method of detecting / looking at pulse waveforms.

Now that Colin has a scope, we should be able to make better progress.

The other thing to note is that, as far as I know this has never worked in Colin's ownership, so it's not like it was put away working or previously seen working and now doesn't work - so literally anything is possible with this. Throw some obvious damp / moisture damage into the mix, and we have an interesting challenge.
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Old 22nd Jan 2021, 6:01 pm   #218
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Default Re: Non-working Commodore PET 3016

Right, so looking at Colin's link in #215 I would say that in the second and third captures the scope does not have a good enough GND connection to a known 0V point on the mainboard.

The waveforms on the computer board should ideally look more like the one in the first trace which you took from the calibration / test waveform output on the scope.

If the flying earth lead on your scope probe is long enough try clipping it to the lead on the right hand (-Ve) end of the big capacitor which has a cable tie around it.
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Old 22nd Jan 2021, 6:41 pm   #219
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Default Re: Non-working Commodore PET 3016

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Right, so looking at Colin's link in #215 I would say that in the second and third captures the scope does not have a good enough GND connection to a known 0V point on the mainboard.

The waveforms on the computer board should ideally look more like the one in the first trace which you took from the calibration / test waveform output on the scope.

If the flying earth lead on your scope probe is long enough try clipping it to the lead on the right hand (-Ve) end of the big capacitor which has a cable tie around it.
I think Colin was using the resistor divider for his ground clip in #215, but from his following posts I think he knows now to use a 0v connection for the ground clip.

It would be good to get a set of scope traces for the 6502 pins that we discussed, to confirm the frequency measurements indicating the 6502 is not working, and also some practice for Colin with the scope ready for the next part of the investigation. Putting the scope captures in a doc file is a good system as it allows a brief description of what was being measure. Maybe use ctrl-print screen so only the scope window is captured.

I think Colin should stay well out of the CRT High voltages, but checking at the output of the main board is probably a good next step.

Iíd recommend trying to find as many faults a possible before trying to buy more spares, I think there is more than just a faulty 6502.

Does anyone have any recommendations on a source for nmos 6502 or any confirmation that the later 65C02 or other variants could be used in this version of the PET?
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Old 22nd Jan 2021, 7:12 pm   #220
SiriusHardware
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Default Re: Non-working Commodore PET 3016

I'm going to delve into the loft tomorrow for other reasons but my chip museum is also there and I'll have a look to see if I have an original 6502 up there.

We all agree that there is no reason at this stage for Colin to be delving into the monitor.
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