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Help with PET 3016. Locks up after 3 seconds.

D

Dazza

New Member
Joined
Apr 1, 2012
Messages
7
Hi,
I've done a bit of googleing and came across this forum. I've spent a while reading and trying to understand some of the posts but nothing seems to resemble my problem. I've been having a bit of a clear out when I came across our old Pet 3016, dragged it out plugged it in and it boots as it should, screen displays
### Commodore basic ###
15359 Bytes free
Ready

The cursor flashes as it should do, then after about 3 seconds I get a few random characters and it locks up with no response from the keyboard. screen display stays on.
I've reseated the socketed IC's and checked them all for 5v DC all seems normal.
I'd love to get this going again but I'll need help so any pointers, help or direction to further reading would be greatly appreciated. I have a very basic knowledge of these machines from years ago when it was in use at work! so go gentle.

TIA
Darren.
 
Sounds like it might be something heating up? Check all the voltages at the RAM chips (+5, -5, +12); you might also try blowing across the board with a fan to see if that makes a difference.
 
Partly damaged 6520 PIA (there are two of those, identical but for different uses), or if it is the 6522 VIA that handles keyboard input? Does the cursor still flash, or does it freeze after the random characters appear?
 
The cursor flashes for 3-5 seconds after the ready prompt, then a burst of random chars, then lock-up. I'm unable to type anything whilst its flashing.

Thank-you
 
As it's a 3016, I am assuming the dynamic 2001 board. In which case the KB is handled by the PIA at C7. Try swapping it with the other PIA at C6 (which controls the IEEE port.) Don't do this with a disk attached.

edit: In fact, before you do this, just try unplugging the KB and try again. It's possible there may be a short on the KB itself.
 
Wow, thank-you for your responses!

Un-plugging the keyboard made no difference, one of the 1st things I tried. It has been stored for a while so I though the KB could be a problem, but its the same with it unplugged.
I intend to dismantle the KB and give it a dusting/ clean out

I'll try the PIA swap when I get back home...
 
Swapped PIA in C6 & C7 round, no change.

Will be checking all RAM voltages next and then a continuity check of all IC legs <=> PCB to check there are no dry joints. They do look in good condition though.
 
It may or may not be related but I had an issue in an Apple II+ once where the machine worked for the first few seconds and then froze up. Turned out to be a capacitor on the power up generator circuit.

Might not be the same but the PET does have the same kind of CPU? Check the RESET line on the CPU (Pin 40). If it's low there is a problem on that circuit somewhere. If it's anything like the Apple, it should only be low for a brief instant on a reset.

Tez
 
Thanks for your reply Tez, I am tending towards a faulty socket as in your 'two faulty PET motherboards' blog. I have had the 1st screen shot at the top of the page a couple of times also.

Out with the Fluke tonight me thinks.
 
CPU pin 40 is always high so it's not resetting, all IC pins to PCB joints check out ok. All IC voltages present.
 
I had a similar problem with the reset circuit on a PET 2001. I confirmed that it was the issue by manually tying pin 40 to ground for about a second, then releasing - and it came back to life. Mine turned out to be a bad capacitor connected to the 555 timer, might want to check in that area.
 
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