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SuperPET 64k RAM Expansion DIMM Troubleshooting

Will let Dave do the driving as his rom but have you checked all socketed chips are making good connections and also the power wires to the molex connector are firmly in place and not making intermittent contact?
 
Right Andy, it does seem to me like a bad power connection somewhere. I've check pin 1&5, 8&9 on J8 from the top since I had the proper voltage info but need to check the others as well. Also thinking I need to pull the system board and check the transformer and line in connections also? There are only five socketed chips and I have removed/replaced three of them already.
The behavior of blank screen/bright center dot/Basic ready prompt randomly when powering on is what has me perplexed.

I guess after buying this machine 40 years ago it's time for some fun and games ;)
 
There is never a dull moment with PET’s …. On mine the wire to the molex was flaky and had to reseat the wires. It could be a dry joint or cracked track also so difficult to track down.
 
I am thinking that pulling the system board and examining it for breaks etc is the best line of attack at this point unless there are other checks that can be made beforehand. Pressure applied to the board while it presents a Basic prompt will hang the machine.

I have a Tynemouth ROM/RAM replacement board that I have installed to troubleshoot but it hasn’t made a difference - no improvement in startup reliability.
 
A bad CPU socket will cause things like this. You will need patience and persistence to find this problem. Where are you applying pressure? Can you refine the pressure test or perhaps use some freeze spray to localize the problem.
 
Dave, yes I believe it may be the cpu socket because it seems sensitive to both pressure and temperature. Initially it seemed to be anywhere on the board but after hitting it with the cold air it seems the likely culprit. I guess I will attempt to reflow the solder joints of the socket. I have done some soldering in the past but am nervous about taking the soldering iron to this virgin board. :(

Any words of wisdom before I jump in?
 
It is unlikely to be the soldering on the socket, but one (or more) of the socket pins holding the IC in place.

Is the IC socket made out of white plastic? If so, these are particularly problematic...

Dave
 
miraco said:
Any words of wisdom before I jump in?
Click to expand...

To remove a 40 pin socket. It may be best to carefully cut the socket out with diagonal cutters leaving just the pins soldered in place. Then you may remove the pins one at a time with minimum heat into the board. You can delay between pin removals to assure not heat buildup.
 
daver2 said:
It is unlikely to be the soldering on the socket, but one (or more) of the socket pins holding the IC in place.

Is the IC socket made out of white plastic? If so, these are particularly problematic...
Click to expand...

I agree, while it would not hurt to reflow the joints, it will probably be a worn to bent contact in the socket.
 
Yep a white socket :confused:

So what are the odds that the socket is bad, > 50% ?

Sure wish I had a board to test this on first!
 
If you are not sure of your desoldering capability you can purchase some cheap veroboard and 40 pin sockets from ePay (or a local electronics shop if you have one). Solder the sockets into the veroboard and then try to remove them and solder some replacements back in place without damaging the veroboard.

Either that, or buy some ‘scrap’ PCBs from ePay (containing 40 pin ICs) and try the same trick.

Dave
 
Thanks Daver2, before I bite off the socket replacement I am doing a bit more testing to feel confident that it is the source of the troubles. I have swapped out the 6502 with a couple of others that I have in the parts box. The one I currently have installed is up and running your pet tester - currently at pass 000023 (35) which is further than it has gotten previously before the machine freezes.

Also, how many passes do you typically perform when troubleshooting memory? Does running Pet Tester from a 2532 make any difference or would a 2716 be preferable?
 
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If you have a white socket for the 6502 then these are usually easier to replace as you can simply lever the white plastic up and off the pins. Then reflow the soldered pins with new solder, then desolder each pin (sounds odd I know but reflowing with new then desoldering makes the process much cleaner and quicker). Intermittent issues like this does point to a bad contact. With the dram, it usually just fail.
 
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