Non-working Commodore PET 3016

[ScottishColin]

Originally Posted by dave cox

No problem, PM me if required.

ATB

Thanks Dave. Off to test as above right now.

Colin.

[ScottishColin]

Originally Posted by Timbucus

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

Thanks. When I have been measuring frequencies with the scope, the first graph I put into the doc is what you have suggested - a squared off measurement before I start measuring the PET.

[ScottishColin]

Originally Posted by Timbucus

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.

Evening - I have not been setting any 1x or 10x setting on the probe software - just letting the AUTO do its job. I will follow your suggestions in post 233 from now on.

[ScottishColin]

Originally Posted by Mark1960

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.

I have removed the 6502 and checked the pins - they all look OK to me with no sign of corrosion or pitting.

Pin 40 gives me 5.135V

[ScottishColin]

Originally Posted by SiriusHardware

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.

Pin 40 gives me 5.135V
Pin 8 gives me 0.81 V

And since removing the 6502 to check the legs for any signs of corrosion, I'm no longer getting any frequencies from pins that I was getting frequencies from before (pins 3, 37 & 39)....

I was careful and did earth myself whilst removing/checking, but this feels like a step backwards now.

Despondent of Perth.

[ajgriff]

Don't be despondent Colin. The fact that you haven't got 5V on pin 8 means that the the 6502 doesn't have power and it won't do much without it. Could be a number of causes but the most likely is a poor connection between pin and socket (assuming that you measured the voltage at the pin itself).

Alan

[julie_m]

From what I've seen on YouTube, Commodore motherboards are notorious for using really cheap IC sockets that only make contact on one side of the pin. Unseating and reseating a chip from one of these can make things worse, not better.

If you fit turned-pin replacements, it might be worth stacking an extra socket into each one just in case; it's a much messier job to remove one of these from a double-sided board if you damage it.

[Slothie]

With replacing big connectors in frail boards I sometimes use cutters to break up the plastic and desolder the pins one at a time.

[SiriusHardware]

The reading Colin has on pin 8 says there is no power to the CPU power pin, it shouldn't take too much to find out why that is. As others have said (and was mentioned somewhere way back in this thread), PETs have a reputation for bad / unreliable IC sockets.

Colin is now, probably always was quite good at reading circuit diagrams and especially good at making continuity measurements, so the next thing we need him to do is to put the CPU back in the socket and (with power off) begin looking for continuity from each CPU pin to one other thing which that CPU pin should be connected to according to the diagram. Note down any which don't appear to be connected to where they should be.

It might ultimately be necessary to replace all of the sockets, but we'll cross those bridges one at a time.

[ScottishColin]

Thanks all. I did measure the voltage at the pin, so that's a true reading from the 6502's perspective.

I've ordered some 40 pin turned-pin sockets from Cricklewood Electronics in case I need them and I'll get on with chasing the continuity tomorrow.

Thanks all.