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Old 22nd Jan 2021, 8:43 pm   #221
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Default Re: Non-working Commodore PET 3016

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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?
I've been looking into this recently. From what I can gather (by no means definitively) the Western Digital 65C02 is not a drop in replacement as far as the PET 2001/2001N is concerned and I've not yet found reference to a successful implementation. Interestingly if you search on eBay for the MOS 6502 you'll find a few worldwide at quite reasonable prices new.

I agree that there's no need for Colin consider this at this stage.

Alan
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Old 22nd Jan 2021, 9:18 pm   #222
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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.

I’d 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?
Mark - I'll get the frequencies from the 6502. Just so I get it right and don't (yet again) miss any, is this the right set of pins?

2, 7, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 22, 23, 24, 25

Thanks.

Colin.
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Old 22nd Jan 2021, 9:57 pm   #223
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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've moved the earth clip to -ve C62 as suggested. I have also cleaned up the screws, chassis and 'pole' (for want of a better word) down to bare metal where the earth connection meets the motherboard.

Note the switch on the probe on the scope is set to x1. It has a x10 option but I'm not really sure what that does. I was really happy enough to be able to get the scope working at all....

https://drive.google.com/file/d/1jvO...ew?usp=sharing
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Old 22nd Jan 2021, 11:07 pm   #224
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Default Re: Non-working Commodore PET 3016

The second capture of three - from UG5 pin 7 I gather, although the text got displaced a bit - shows a reasonable 1Mhz signal, a bit rounded but it is there. There is a way you can probably get that to look more 'square' but we'll come to that shortly.

Can you now have a look on at least pins 9, 10, and 11 of the CPU to see what you get there, plus any additional pins which Mark suggested??
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Old 23rd Jan 2021, 4:36 am   #225
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Default Re: Non-working Commodore PET 3016

I think pin 37, 39, 3, 2, 7, 9, 10, 11.

Should be enough to confirm 6502 is getting clock, but not running.
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Old 23rd Jan 2021, 11:57 am   #226
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Default Re: Non-working Commodore PET 3016

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I've been looking into this recently. From what I can gather (by no means definitively) the Western Digital 65C02 is not a drop in replacement as far as the PET 2001/2001N is concerned ....

Alan
Should have said Western Design Centre (WDC) of course - sorry, brain wasn't in gear. Data sheet attached for reference.

Alan
Attached Files
File Type: pdf W65C02S.pdf (1.24 MB, 60 views)
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Old 23rd Jan 2021, 12:38 pm   #227
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Default Re: Non-working Commodore PET 3016

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Note the switch on the probe on the scope is set to x1. It has a x10 option but I'm not really sure what that does. I was really happy enough to be able to get the scope working at all....
The x10 option just attenuates the signal more so you can read higher voltages but, more importantly it disrupts the signal you are reading less - e.g. if looking at a crystal oscillator you will likely stop it working on x1 due to the extra capacitance and impedance effect. On x10 it is less likely to have an effect at lower frequencies.

That comes at a cost of sensitivity of course but, usually not a problem with modern scopes and the fact their displays will auto adjust when you select x10 option will help so you don't keep misreading values forgetting to multiply them by 10. It would not hurt to just always use x10 at these signal levels but, make sure you adjust the probe with the reference signal for a nice square wave as in your setup instructions. There is usually a little screw near the BNC connector.
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Old 23rd Jan 2021, 12:56 pm   #228
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Default Re: Non-working Commodore PET 3016

Yes, that was what I was referring to in #224 when I said the waveform could be made to look tidier. The frequency compensation trimmer is sometimes in the body of the probe rather than in the BNC plug. It would also be useful to switch channel 2 off so that channel 1 can use the whole height of the display.

I've been in the loft this morning and unfortunately could not find what I was looking for, a smallish microprocessor PCB which I know for sure has a Rockwell 6502 on it. As it's not where I was absolutely sure it was, I don't think I'll be able to find it in any reasonable time frame. I did find some NOS Z80 CTC (Counter-Timer) ICs though - anyone need one?

Let's see what Colin finds on those signal lines, and hopefully we can say whether the existing 6502 is OK or not.
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Old 23rd Jan 2021, 11:55 pm   #229
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Default Re: Non-working Commodore PET 3016

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I think pin 37, 39, 3, 2, 7, 9, 10, 11.

Should be enough to confirm 6502 is getting clock, but not running.
See link below.

I have had to use the chassis as earth as the earth wire on the probe is not long enough to go to C62 from the 6502.

I have included the frequency that the scope has detected at the bottom of each screenshot.

https://drive.google.com/file/d/1Bdt...ew?usp=sharing
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Old 23rd Jan 2021, 11:56 pm   #230
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Default Re: Non-working Commodore PET 3016

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Note the switch on the probe on the scope is set to x1. It has a x10 option but I'm not really sure what that does. I was really happy enough to be able to get the scope working at all....
The x10 option just attenuates the signal more so you can read higher voltages but, more importantly it disrupts the signal you are reading less - e.g. if looking at a crystal oscillator you will likely stop it working on x1 due to the extra capacitance and impedance effect. On x10 it is less likely to have an effect at lower frequencies.

That comes at a cost of sensitivity of course but, usually not a problem with modern scopes and the fact their displays will auto adjust when you select x10 option will help so you don't keep misreading values forgetting to multiply them by 10. It would not hurt to just always use x10 at these signal levels but, make sure you adjust the probe with the reference signal for a nice square wave as in your setup instructions. There is usually a little screw near the BNC connector.
Apologies but I'm very new to scopes. Do you mean this is a one off exercise to get the square wave or I should do this for every measurement?

Sorry for the dumb question.

Thanks.

Colin.
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Old 24th Jan 2021, 12:00 am   #231
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Default Re: Non-working Commodore PET 3016

Nothing is dumb - just do it once or if you change from x1 to x10 settings as it calibrates the probes. You would also need to do it again if you bought new probes - also try to use the same probes on each port as the trim will be different for each port. This means you match a probe to a port
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Old 24th Jan 2021, 12:06 am   #232
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Default Re: Non-working Commodore PET 3016

Did you notice that on the scope screen there is a setting that allows you to say if the probe is in X1 or X10 switch position - this will mean that the displays read correctly - in your document to ensure we are reading the values correctly can you confirm that option was set by you - if not we will just adjust the values manually when looking.
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Old 24th Jan 2021, 1:06 am   #233
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Default Re: Non-working Commodore PET 3016

I think we had better be specific. If possible Colin, try making all measurements from now on with the following parameters set:

With no signal present, adjust the vertical position of the scope trace line (the 'Y Position') so that it sits on the second faint line counting upwards from the bottom. This will be our 'zero volts'. I don't see an actual 'Y position' control such as there would be on a real scope but I think maybe you can click and drag the yellow '1' tab at the left hand side?

If you can choose between 'DC' / 'AC' / 'GND' for the input mode choose 'DC'.

On the probe: select x10

On the scope 'Front Panel' - select "x10"

Set Channel 1 'Y' sensitivity: 1V / Div. (One Volt per division)

Turn channel 2 off.

With all of the above set, go to UG5 pin 7 where you have a known 1MHz signal and while looking at that, adjust the trimmer on the probe body or BNC plug for the squarest looking signal. Usually, you get a plastic trimming tool with the probe intended for that exact purpose.

The setting of the 'horizontal' control will vary with the frequency of the signal you are trying to look at. For now, just get into the habit of adjusting it up or down until you can see around 5 or 10 cycles (ups and downs) of the signal you are trying to look at.

I understand that the negative leg of C61 isn't always conveniently close to where you want to make measurements. With the power off, use your meter to check for continuity between that component leg and the chassis. If the resistance is less than 1 ohm then it is OK to use the chassis as the scope ground provided the point you choose on the chassis is clean and not painted - the ground lead of the probe must have good electrical contact to circuit 0V.
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Old 24th Jan 2021, 1:27 am   #234
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Default Re: Non-working Commodore PET 3016

Looking back at the readings you collected, especially from pins 7, 9, 10, 11 - although the clock signal is reaching the CPU and passing through it, the CPU seems very inactive. If you can, please just verify (with a meter, with power on) that pin 40, Reset, is still at +5V.

Apart from that, anything else we can go at before we have to suspect the CPU? Mark?
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Old 24th Jan 2021, 1:53 am   #235
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Default Re: Non-working Commodore PET 3016

I guess we had better check for +Ve supply to the CPU since it seems so dead, so Colin, as well as re-checking the voltage on pin 40 (reset) could you also look at the voltage on pin 8 (should be +5V). Although the clock is getting through from one side to the other, that might happen even if the IC was not powered. I'm thinking possible bad socket contacts or missing supply due to damage by damp / corrosion.
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Old 24th Jan 2021, 2:55 am   #236
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Default Re: Non-working Commodore PET 3016

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I guess we had better check for +Ve supply to the CPU since it seems so dead, so Colin, as well as re-checking the voltage on pin 40 (reset) could you also look at the voltage on pin 8 (should be +5V). Although the clock is getting through from one side to the other, that might happen even if the IC was not powered. I'm thinking possible bad socket contacts or missing supply due to damage by damp / corrosion.
As the 6502 is socketed its probably worth removing it from the socket, checking the condition of the pins and then refitting the 6502.

For the scope settings I would suggest starting with 500ns per division and widen out to about 5us per division.

If you don’t see a trace on the scope try adjusting the trigger level, ;keep it on auto might be easiest.

Did the pet have more than one 5v supply? Some of the older systems needed too much current for a single regulator.
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Old 24th Jan 2021, 9:49 am   #237
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Default Re: Non-working Commodore PET 3016

At it happens, there are two +5V supplies, although both were verified as being present at the regulators during early checks. The supply rails are the thickest tracks on the PCB so they are unlikely to have been corroded through, but individual pins could have corroded away where they meet the PCB surface if there has been standing moisture / dew on the PCB.
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Old 24th Jan 2021, 1:54 pm   #238
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Default Re: Non-working Commodore PET 3016

I have some UMC6502A's in my stash, these are NMOS according to the details I can find. Are these likely to work in the PET ?

dc
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Old 24th Jan 2021, 2:27 pm   #239
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Quite possibly Dave, so we'll keep you in mind if that is OK. Still trying to establish whether the original 6502 is doomed or not.
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Old 24th Jan 2021, 2:34 pm   #240
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Default Re: Non-working Commodore PET 3016

No problem, PM me if required.

ATB
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