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.

[ScottishColin]

OK - results attached in a spreadsheet.

Summary is that all pins pass continuity tests to their selected destinations when checked from the underneath of the board, but I only get partial success from the top of the board.

All tests carried out with power off and the motherboard removed from the PET.

[SiriusHardware]

I don't have anything 'officey' on this PC (may be able to view it at work tomorrow) but it sounds as though you found that some of the connections between the IC pins and the places they should be connected to are missing. If so, that's a significant breakthrough.

I know you've ordered sockets, do you feel reasonably confident about fitting one? The technique is essentially similar to UG5, in so far as you cut the IC socket at least into two halves by cutting the horizontal spars which join the top end, the middle and the bottom so that instead of trying to desolder one 40-pin socket you have two 20 pin items to desolder. That's still a lot of pins so you then, if possible, cut each plastic side of the socket into smaller and smaller pieces as well. The potential for damage to the pads and tracks on the top side through cutter slips and skids is very high though, so be ultra careful.

The aim is to end up with nothing left on the PCB except 40 separate pins, melt the solder on each in turn and withdraw them from the top side with tweezers, then clear all the holes ready for the new socket.

Which brings us to another subject: What sort of soldering iron do you have? When you removed UG5 you did pretty well, but what damage there was was caused by heat being applied to some of the pads for too long. Pads which don't have tracks going to them are especially prone to this sort of damage because there is nowhere for the heat to run away to.

One thing which would help would be a temperature - controlled soldering iron if you don't already have one - if not maybe you can borrow one from somewhere?

[Timbucus]

I assume you were touching the pins of the socket - in which case there is poor continuity - that could just be the socket pins tarnished or a dry joint - if you have any deoxit then a spray, leave a short while and put the chip back in may help. Before that though I would resolder all pins until they flow - especially the ones that are poor - if you have to remove the socket that is not the end of the world as the fresh flux will simplify the solder suck. If both of those fail then a socket swap is probably in your future unless others can think of anything else.

[ScottishColin]

Originally Posted by SiriusHardware

I don't have anything 'officey' on this PC (may be able to view it at work tomorrow) but it sounds as though you found that some of the connections between the IC pins and the places they should be connected to are missing. If so, that's a significant breakthrough.

I know you've ordered sockets, do you feel reasonably confident about fitting one? The technique is essentially similar to UG5, in so far as you cut the IC socket at least into two halves by cutting the horizontal spars which join the top end, the middle and the bottom so that instead of trying to desolder one 40-pin socket you have two 20 pin items to desolder. That's still a lot of pins so you then, if possible, cut each plastic side of the socket into smaller and smaller pieces as well. The potential for damage to the pads and tracks on the top side through cutter slips and skids is very high though, so be ultra careful.

The aim is to end up with nothing left on the PCB except 40 separate pins, melt the solder on each in turn and withdraw them from the top side with tweezers, then clear all the holes ready for the new socket.

Which brings us to another subject: What sort of soldering iron do you have? When you removed UG5 you did pretty well, but what damage there was was caused by heat being applied to some of the pads for too long. Pads which don't have tracks going to them are especially prone to this sort of damage because there is nowhere for the heat to run away to.

One thing which would help would be a temperature - controlled soldering iron if you don't already have one - if not maybe you can borrow one from somewhere?

My soldering iron is temperature controlled and I can test the temp that I dial it to with a heat gun to make sure.

What temperature would you reccomend?

Colin.

[SiriusHardware]

I can't really answer that question because for most of my working life I didn't have access to a temperature controlled iron, so I got very adept at judging how long to apply an iron to any given joint or surface for and 'controlling' the temperature of the joint that way. Others here will have a better answer for you. Manual pump desoldering tends to need slightly higher temperatures than soldering because you need the solder to stay 'liquid' for long enough for the sucker to get it.

Most importantly, avoid lead-free solder. Only use lead-tin solder on something like this because the job will be delicate enough without the higher temperatures needed for unleaded solder.

[Mark1960]

For tin lead solder I use about 270c to 280c, but my temperature controlled iron is cheap and not calibrated. Lower temp is sometimes ok for soldering lighter joints but sometimes needs the heat applied for longer which can be just as damaging as higher temperatures for a shorter time. Try to experiment but stay below 300c.

[ScottishColin]

Originally Posted by Mark1960

For tin lead solder I use about 270c to 280c, but my temperature controlled iron is cheap and not calibrated. Lower temp is sometimes ok for soldering lighter joints but sometimes needs the heat applied for longer which can be just as damaging as higher temperatures for a shorter time. Try to experiment but stay below 300c.

Great - thanks. I have ordered some tin lead solder too.

[Refugee]

That spread sheet is certainly showing as duff socket.
I usually cut the cross bars to split the socket and then bend the halves back and forward three or four times to leave just the tips of the pins in the holes.
I then fit a rubber sleeve to the sucker tip to improve the seal over the joint and clean the pump out after each hole. The tips of the pins will jam the sucker if you don't get them out before doing the next hole.
This reduces the number of times you need to heat the pads.

[SiriusHardware]

It's only fair to warn you that Lead/Tin solder is deemed a hazardous substance due to the lead content - try not to rub it into your skin and definitely don't go mistaking it for liquorice.

However it is much, much better solder than any currently available substitute so the benefit outweighs the risk for occasional use IMO and for boards like this which were only designed with the working temperature for lead-tin solder in mind, it is essential.

[McMurdo]

Unseating and reseating a chip from one of these can make things worse

A habit they carried onto the commodore 64.
2 of the 3 roms in my '64 had failed and when I replaced them the computer became workable but erratic. The cheap sockets were to blame, and as noted, were single-side contact only. These deserve turned pin sockets...cheap enough these days!