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Old 31st Jan 2021, 8:00 pm   #421
ScottishColin
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Default Re: Non-working Commodore PET 3016

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Originally Posted by SiriusHardware View Post
On consideration, and following my own advice of 'always try everything you can before spending more money', let's do that-

-Reinstate UB3 pin 16 connection
-Remove UD6 through UD9, remembering where each one goes
-Power up and see if the output of UB3 pin 16 is distressed / is not distressed.

If it is not distressed, replace each of UD6 to UD9, checking the output of UB3 pin 16 after each reinsertion.

If it starts off not distressed and becomes distressed when you have one or more of UD6 to UD9 in, try to work out if it is just one of those ICs which causes the effect or if it is related to the number of ICs you have inserted.

Edit, Ah yes, I missed where BA10 and several other lines circle round and go up to the top left of sheet 1.
I just want to check something before I start. I have reinstated UB3/16 and depending on timing, I capture differing waves. Attached is a doc showing this. Previously when this happens (and it happens on other pins), I have only been showing you the squared off capture.

Is this normal behaviour? If so, should I just capture a screenshot when I get squared waves going forwards?

Thanks.

https://drive.google.com/file/d/1v8i...ew?usp=sharing
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Old 31st Jan 2021, 8:16 pm   #422
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Default Re: Non-working Commodore PET 3016

It sounds (and looks) like you are getting a really bad connection to UB3 pin 16 most of the time which is odd because if you have cut and repaired that connection you should have bright solder to put the probe tip on now.

It may be that the connection between the inside of the IC and pin 16 is physically intermittent.

With UB3 pin 16 still reinstated try monitoring the same signal on pin 4 of UB2 (a 7425) and, while monitoring that signal put a bit of crush pressure on UB3 to see if the signal on UB4 pin 2 comes and goes.

Also worth trying: Use your other probe - I know they are new but it is always possible that the probe itself could be faulty (If it was, it would be equally come-and-go on every point you tried to get a signal on).

Also check the BNC plugs at the scope end of the probe leads are properly twist-locked in place and are not about to fall out.

Edit: Sorry, I see you feel you are having this problem with all measurements on all pins - in that case definitely check the scope probe plugs are secure and try your other probe.
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Old 31st Jan 2021, 10:54 pm   #423
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Default Re: Non-working Commodore PET 3016

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Originally Posted by SiriusHardware View Post
It sounds (and looks) like you are getting a really bad connection to UB3 pin 16 most of the time which is odd because if you have cut and repaired that connection you should have bright solder to put the probe tip on now.

It may be that the connection between the inside of the IC and pin 16 is physically intermittent.

With UB3 pin 16 still reinstated try monitoring the same signal on pin 4 of UB2 (a 7425) and, while monitoring that signal put a bit of crush pressure on UB3 to see if the signal on UB4 pin 2 comes and goes.

Also worth trying: Use your other probe - I know they are new but it is always possible that the probe itself could be faulty (If it was, it would be equally come-and-go on every point you tried to get a signal on).

Also check the BNC plugs at the scope end of the probe leads are properly twist-locked in place and are not about to fall out.

Edit: Sorry, I see you feel you are having this problem with all measurements on all pins - in that case definitely check the scope probe plugs are secure and try your other probe.
Hi. It happens with both probes, both of which are secured. I tried various earth points and it made no difference to the signals on UB2 pin 4 whilst pressing UB3 - they came and went as per

I guess it may be a limitation of the scope which isn't very helpful.

I have ordered two replacement ICs and several 20 pin sockets so I guess that's next - replace UB3 and see where that takes us.

Thanks again for your help.

Colin.
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Old 31st Jan 2021, 11:07 pm   #424
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Default Re: Non-working Commodore PET 3016

That shouldn't be happening with your scope.

The middle trace from your last posted results, was that with UB3 pin 16 reconnected? Because it actually looked normal, or as normal as the other ones do, in that trace. Maybe you already had UD6 to UD9 out at that point?

One thing which might help is something called a fibre (some spell it 'fiber') pen - it's a cross between a pen and a very stubby small brush and is used for cleaning / polishing small areas of metal, such as IC pins and possibly PCB pads as well. I sometimes use one for cleaning tarnished IC pins. All the IC pins and pads on that machine look quite heaviliy oxidised, so maybe using one of those would help you to get a clean contact to take measurements from.
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Old 31st Jan 2021, 11:13 pm   #425
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Default Re: Non-working Commodore PET 3016

The ROMs were still in when I took the trace and UB3 /16 was reconnected.

I have a fibre pen - useful for my model railways. I'll use that tomorrow and see if it helps.
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Old 1st Feb 2021, 12:26 am   #426
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Default Re: Non-working Commodore PET 3016

With regard to the 74ls244 buffer outputs not pulling the output down to ground, were you grounding the probe at pin 10 of the 74ls244, or using some other nearby ground?

If that was using some other nearby ground for the scope, while using that same ground, try putting the scope on pin 10 of each 74ls244. You should get a flat line at 0v. This would just make sure there is not a weak ground connection in the pcb which might be possible if the ground is connected by through hole plated holes that are not good quality.
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Old 1st Feb 2021, 12:50 am   #427
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Default Re: Non-working Commodore PET 3016

Would that not also affect the perceived swing on the address line waveforms which come out of the 6502? Those waveforms do seem to have a healthy 0V level.

As Colin was observing both waveforms at once using the two input channels of the scope, there must have been a common / shared scope ground for both inputs?

Edit: Of course I'm assuming here that the two scope input channels share a common ground at the scope end. They might not, but I suspect they do.

Last edited by SiriusHardware; 1st Feb 2021 at 1:03 am.
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Old 1st Feb 2021, 4:36 am   #428
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Default Re: Non-working Commodore PET 3016

I was thinking of the possibility that the 74ls244 are not well connected to 0v, but the scope ground was on a good ground. This seems the only possibility other than the 74ls244 themselves.
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Old 1st Feb 2021, 8:53 am   #429
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Default Re: Non-working Commodore PET 3016

Well, Colin can check for good continuity between pin 10 of each 74LS244 and his known good ground to check that out, and he can also measure the voltage on those 74LS244 pins relative to his known good 0V, although he may already have started to remove the original ICs.

With the PCB so obviously having been damp at some point, corroded VIAs certainly remain a possibility.
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Old 1st Feb 2021, 1:44 pm   #430
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Default Re: Non-working Commodore PET 3016

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With regard to the 74ls244 buffer outputs not pulling the output down to ground, were you grounding the probe at pin 10 of the 74ls244, or using some other nearby ground?

If that was using some other nearby ground for the scope, while using that same ground, try putting the scope on pin 10 of each 74ls244. You should get a flat line at 0v. This would just make sure there is not a weak ground connection in the pcb which might be possible if the ground is connected by through hole plated holes that are not good quality.
Hi Mark - I was using Pin 8 of UG4 for both probes as I have a handy resistor soldered to it for a previous experiment.

Colin.
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Old 1st Feb 2021, 1:48 pm   #431
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Default Re: Non-working Commodore PET 3016

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Well, Colin can check for good continuity between pin 10 of each 74LS244 and his known good ground to check that out, and he can also measure the voltage on those 74LS244 pins relative to his known good 0V, although he may already have started to remove the original ICs.

With the PCB so obviously having been damp at some point, corroded VIAs certainly remain a possibility.
I checked Pin 10 of UB3/UC3 to UG4/8 - good continuity with both with a reading of 0.3
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Old 1st Feb 2021, 1:49 pm   #432
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Default Re: Non-working Commodore PET 3016

(UG4 is an unpopulated IC position, so Colin uses the 0V pad of that as a 'local' 0V, as it is right next to UG5 which we took an interest in earlier).

However Colin, make sure when using that as a GND that you have the GND clip / lead connected to the end of the resistor which is soldered to UG4 0V, not to the end which is sticking up in the air.

Continuity sounds good, but Mark was right, it was worth checking.
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Old 1st Feb 2021, 4:15 pm   #433
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Default Re: Non-working Commodore PET 3016

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(UG4 is an unpopulated IC position, so Colin uses the 0V pad of that as a 'local' 0V, as it is right next to UG5 which we took an interest in earlier).

However Colin, make sure when using that as a GND that you have the GND clip / lead connected to the end of the resistor which is soldered to UG4 0V, not to the end which is sticking up in the air.

Continuity sounds good, but Mark was right, it was worth checking.
Confirmed that I had it at the pad end of the resistor.

fyi I have a spare 6502 here now while I await the other ICs. If there's anything I can safely test with that while I wait, I'll get on with it, otherwise we're waiting for the new 74LS244s/sockets.

Colin.
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Old 1st Feb 2021, 4:32 pm   #434
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Default Re: Non-working Commodore PET 3016

At the moment we think the 6502 is probably OK thanks to your pioneering work at the weekend, so we'll just take a breather until those part arrive.
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Old 2nd Feb 2021, 5:34 pm   #435
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Default Re: Non-working Commodore PET 3016

OK - desoldered and resoldered and new ICs.

Where shall I start testing?

Colin.
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Old 2nd Feb 2021, 5:50 pm   #436
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Default Re: Non-working Commodore PET 3016

First with the data bus of the 6502 still isolated from the board, do a comparison of the input and output of the 74ls244 again. You should now see the output square wave closer to 0v now and close to 5v on all the buffer outputs.

If you have the correct output on the buffers, then reconnect the data bus from the 6502 to the board. Then check if you now still have activity on the address outputs from the 6502.
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Old 2nd Feb 2021, 6:07 pm   #437
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Default Re: Non-working Commodore PET 3016

In other words, repeat the measurements from #355 again, now that you have new buffer ICs fitted.
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Old 2nd Feb 2021, 6:49 pm   #438
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Default Re: Non-working Commodore PET 3016

I'm going to test continuity from all UB3/UC3 pins first to make sure my soldering was up to scratch, then I'll go back to post 355.
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Old 3rd Feb 2021, 11:15 am   #439
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Default Re: Non-working Commodore PET 3016

Morning. Continuity all fine to the right pins.

I need my third hand for the tests in post 355, so while I was waiting I removed ROM chips to see what signal I can get from pin 9.

I have discovered that I can get signals from pin9 of the 6502 (with all socket pins connected) if UD7 is removed and UD9/8/6 are inserted. I attach the scope output as the waves aren't uniform in a time sense - please could you look and let me know what you think.

I'll get on with post 355 when I can.

https://drive.google.com/file/d/1yBu...ew?usp=sharing
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Old 3rd Feb 2021, 11:52 am   #440
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Default Re: Non-working Commodore PET 3016

So with all 6502 pins reinstated but UD7 removed, the CPU runs? That's very interesting in itself. It will still be useful to see if the output swing from the new buffers looks any better, so please do carry out that check, as and when you have enough hands to go around.

Your captures from the low address pins on the 6502 look fine, we would expect a bit of semi-randomness when the CPU is trying to execute code. The only way to get steady waveforms on those lines is to rig a NOP test as discussed earlier, but I don't think we need to do that just yet.
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