Hello all,

I recently acquired a commodore pet 8096 upgraded from a 8032, 8050 duel disk drive and a 8023P printer.
None of any of the above components work, they all have problems and I was wondering if any of you could help me.
Firstly the PET, it chirps on startup but the monitor doesn't switch on and as a consequence nothing is displayed. From time to time I get a green light on the screen but this then drifts off the screen after 5 mins. The disk drive switches on and the lights come on but they don't change colour to green. Also, drive 1's green light flickers sometimes and I have only got the centre LED to go to green two times. This tells me it is probably a fault with the processor at UN1. The centre LED also went to orange once or twice?
The printer does absolutely nothing. I checked inside the printer and one red wire was cut so I fixed that and the transformer 'crackled' on startup, so not sure how to fix that.
I know this probably isn't the right place to post this since this forum is all about the 6502 processor, but I read a repair guide on here so worth a try.
Any help would be much appreciated.
Thanks in advance

Welcome!

I think you need one of two things, or even both
- general purpose troubleshooting guidance for a computer system
- specific troubleshooting and repair guide for a PET

Do you have electronic tools? Do you have spare parts to swap in and out? Could you build up any kind of test board?

Edit: here's a guide from André, (aka fachat here on the forum): http://www.6502.org/users/andre/petindex/repairs.html

Hello,

I think I have the tools to repair the damage but I think the CRT is at fault, this means I would have to work with high voltage circuitry and I don't think I have the necessary skills. I have your basic stuff like a voltmeter, solder station etc...
I don't have any spare parts, unfortunately if I did I would have already replaced the CPU and video ram and roms (according to another forum).
I had a look at that repair guide and too be honest it doesn't make much sense to me. All it seems to tell me is that most of the chips in the PET and disk drive are faulty which Im sure isn't the case since sometimes it does pass the selftest on startup.
Do you know anyway that I can test the CRT? As I said one time I got it to display a green line so not sure if that means anything?

Thanks,

If you have a logic probe, mutimeter with frequency counter, or a scope, then you could have a look at the video timing circuits - there can be no picture on the screen unless the horizontal and vertical sync signals are there. And if they are not there, the tube might be fine but unable to display anything.

Before that even, check the power rails and the power pins of each chip: everything should be near 5 volts (relative to the ground pins, which you should also check)

If you have a continuity tester on your multimeter that can help find open circuits and short circuits.

If chips are unseated or have bent or broken pins, you can determine that by carefully removing and replacing each one after inspection.

Hello,

I thought about reseating all the chips that are socketed however the memory expansion board gets in the way and unlike others I've seen the memory board has the CPU on it and you can only power the machine on through the expansion board (probably not the right way of putting it). What I mean is you cannot bypass it and seat the CPU on the main motherboard. I don't have a logic probe so can't do that.

I think ill try and reseat the chips since the tube actually does glow when the unit is switched on as I checked. Is there an easy way of removing chips? I tried to do it on a amstrad computer but ending up destroying the socket since it was such a tight fit.


Thanks again,
What would be really useful for me is a list/picture of all the chips and there locations on the board so that I can check them e.g. video chips and ROM chips.

To remove a chip, use a small flat blade and lever up one end very slightly, then the other end, then back again and keep alternating until it is out. Keep the chip parallel to the motherboard - if one end flips out, you'll bend some pins. They don't like to be bent many times, but you can usually straighten them once.

Touch the chassis often to be static safe.

Edit, for the boards there are some photos and repair notes at
http://www.primrosebank.net/computers/p ... rs_ram.htm

ICs and CRTs don't go bad just from age and sitting dormant for decades. Capacitors do though. I expect that a CRT, like most any tube, has a couple of failure modes, one being that over thousands of hours of use, the cathode's emissivity slowly degrades (which would just make the beam gradually dimmer), and the other is a short from cathode to heater, from the insulation breaking down. The latter is more spontaneous, but it would happen with use, probably not storage. ICs can get damaged with static or exceeding the other things in the "absolute maximum ratings." I've heard of crystals frequently being damaged by video gamers slapping the side of the computer hard in their excitement (or possibly frustration).

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

The classic failure in storage is capacitors: both electrolytic and tantalum ones can die, and need to be replaced. Also battery leakage is an issue in some computers - probably not this one.

Hello,

Sorry I meant the capacitors. I don't think it's to do with its emissivity since I did get it to show a single solid line which was very bright, but now it does absolutely nothing.

I reseated all the chips I could on the disk drive and that did nothing as well, I am yet to do the same for the PET. Although the centre LED did go green twice so not sure about that. Also, when I left it for about a week the PET did actually switch the CRT on. I heard static coming from behind the CRT and that's when the line appeared.

If it needs capacitors replacing then I think I'll just have to let it go to someone who is capable of fixing it since I dont think I am.

Thanks,

Best to be safe around high voltages! I had a broken vectrex - still do - don't want to get close to the HT.

Hello,

I don't usually get involved with high voltage circuitry as I know how dangerous it can be. I did get a shock from the 8023P printer which actually I didn't feel at the time.

I have attached a picture of the stage I am at with the disk drive, it is worth noting that this stage is very intermittent and works only when you turn the drive off and immediately turn it back on.

Any thoughts?

Thanks,

Bad caps!