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2001-8 PET requires some doctoring

S

sirmorris

Member
Joined
Nov 20, 2008
Messages
24
Location
south east, uk
Hi all,

I've just become the proud owner of a 2001-8 with mainboard revision 320008.

When powered up there is a faint horizontal trace on the CRT when the brightness is full-on. The tape drive turns if I hold down play. That's it.

There is a red LED on the main board. Should this indicate system power? It remains off.

I'd love to get this working, I've always wanted a PET..!

When I get home I'll be looking for schematics. If anyone can point me to a service manual I'll be grateful!

Thanks in advance

Charlie
 
sirmorris said:
I'd love to get this working, I've always wanted a PET..!
Click to expand...

With the help of Anders and other PET experts on this forum, we can guide you through the repair process if the CRT turns out to be functional.

Hopefully you have a voltmeter or logic probe and some small knowledge of soldering.
-Dave
 
There's a diagnostic manual here, but it's of limited use without special test gear and software. Unfortunately, the cassette motor turning doesn't say an awful lot as it's almost directly connected to the power supply. I'm not sure how much you know about electronics and fault diagnosis, so forgive me if the following sounds ridiculously simple ;-)

These PETs have a lot of socketed chips, so first thing I'd try would be reseating those and any connectors.

It's worth checking the power supply, too. Just measure the outputs of all the voltage regulators as shown on the schematics Anders linked to. Ensure that power is getting to all the main components.

If that all checks out, you'll want to see if the CPU is running at all and possibly check if the monitor is getting any kind of signal from the main board.
 
Aaah that's exactly what I wanted to hear :) Thanks :)

I'm happy wielding an iron, I have a meter and a logic analyser at the ready.

<salutes>
 
sirmorris said:
I'm happy wielding an iron, I have a meter and a logic analyser at the ready.

<salutes>
Click to expand...

Logic analyzer, good! It helps to have good tools. How many channels do you have? Actually 8 channels will be plenty to troubleshoot the PET.

I once painfully hooked up the complete 6502 address, data, and contol buses to my analyzer. But without the 6502 disassembly option for my analyzer, I was swamped with too much data although it was fun to see all the bus activity. If I was checking out new software where I could set some breakpoints, it may have been a good configuration, but for normal hardware checkout of a broken PET, it is not needed.
 
sirmorris said:
Hi all,
When powered up there is a faint horizontal trace on the CRT when the brightness is full-on. The tape drive turns if I hold down play. That's it.
Charlie
Click to expand...

this looks to me like the CRT board has passed away. This is the board below the CRT neck... I had 3 dying the last months... sometimes it is possible to repair when you replace the big capacitor on this board... I'm also searching for someone who can repair my 3 CRT boards...
 
dave_m said:
Listen to what cosam has to say. He has brought a very sick PET back to life.
Click to expand...
Yep, with a bit of help we managed to get that one going. There was Tezza's before that too, and track18's not long ago (IIRC some nut sent him a replacement ROM half way across the world in exchange for pickled vegetables ;-)

alker said:
this looks to me like the CRT board has passed away. This is the board below the CRT neck... I had 3 dying the last months... sometimes it is possible to repair when you replace the big capacitor on this board... I'm also searching for someone who can repair my 3 CRT boards...
Click to expand...
That could well be it (and is also why I suggested checking for a signal to the monitor). Those boards are quite repairable if you have a scope as there are well-documented test points in the schematics, with diagrams of what the waveforms are meant to look like. If the incoming signal is good, you can see where it goes wrong and figure out what needs replacing.
 
Thanks everyone for input so far - I welcome any detail no matter how small. I'm a newbie at renovating but I dabble with 'putting stuff together'.

I must say I'm looking forward to this :)

I have an 8 channel analyser for later and a meter at the ready. I will be probing the PSU in a little while. If you hear a bang..!
 
OK, so far:

Power checks out.

There is clock, phi2 and what looks like a valid reset pulse - low for about a second after power-on.

There is no activity on /IRQ or /NMI.

R/W looks good for about 180 milliseconds. Then it goes high and stays there, never to come back down. Hmm. Not looking good.

I should have looked at RDY I suppose.. tomorrow perhaps.

I swapped the 6502 with an Acorn Atom that I just happen to have lying here and powered both up - it's not the CPU. The Atom still Atommed and the PET still deaded.

Next I looked at the TTL video signal source in the logic analyser and saw what looked like valid signals. The vsync, hsync and data looked sane. I had a crazy idea to export the video data from the analyser and try to plot it, but life (and the lunch hour) is too short for that kind of fun :)

Wiggling the trim pot next to the main brightness pot produces fleeting flashes of a full-screen image in which petscii characters could be seen - the horizontal timing seems stable.

We definitely have a vertical sweep issue.

I'm borrowing a 10mhz dual trace scope - itself a bit of a vintage item - and will try to get to the test points on the video board.

Is there a secret way to remove the hood or can anyone suggest ways to reach the test points without dying?

This is looking like it's going to be fun :)
 
Good luck! Regarding the 6502, I think it is very uncommon to break. Many people assume if a computer or peripheral doesn't boot it probably is the CPU but based on my very limited experience, so far none of the broken machines I have managed to bring back to life had been due to a broken 6502. Certainly you can damage it with bad voltages, moisture or so, but then I believe there will be a whole array of other things which have gone bad at the same time.
 
sirmorris said:
Is there a secret way to remove the hood or can anyone suggest ways to reach the test points without dying?
Click to expand...

So it looks to be a video board issue after all. You should wait until you get some advice from cosam and others on how to safely proceed. As for me, I am too terrified of high voltage circuits to be of much help. I do know that they will have you check the +12V regulator chip on the video board.
 
sirmorris said:
R/W looks good for about 180 milliseconds. Then it goes high and stays there, never to come back down. Hmm. Not looking good.
Click to expand...
That's not necessarily a bad thing. After initialisation I would image it will just be looping, waiting for something to happen. That probably doesn't involve any writing.

sirmorris said:
Wiggling the trim pot next to the main brightness pot produces fleeting flashes of a full-screen image in which petscii characters could be seen - the horizontal timing seems stable.
Click to expand...
Ah-ha, that there is the vertical size pot, so I think you're close to the culprit. It may even be the pot itself that's had it, or if you're lucky, just a bit dirty. I don't know if you could get a bit of contact cleaner in there somehow. Even a quick squirt of WD40 would probably help.

The PETSCII makes me think that you've probably got another problem besides the video, but even a screen full of garbage would be progress at this point ;-)

dave_m said:
You should wait until you get some advice from cosam and others on how to safely proceed. As for me, I am too terrified of high voltage circuits to be of much help. I do know that they will have you check the +12V regulator chip on the video board.
Click to expand...
Yeah, I made the mistake of overlooking power in the monitor when the main board's power checked out OK. It's probably not the problem in this case, but well worth checking before you end up on a wild goose chase (as I did).

As for working on the monitor safely, I'm probably not the best person to ask ;-) I took off the back panel and just coaxed the board out a bit. You need to remove the whole monitor to get at the four mounting screws underneath. I did however discharge the CRT when moving the 'scope probe to awkward places. I think the bits that are likely to need attention in this case are quite near the edge and much more easily accessible. You can probably just remove the panel and leave the board in place. Just steer clear of the end of the CRT neck. You shouldn't need to be near any of the other potentially dangerous bits.
 
cosam said:
Ah-ha, that there is the vertical size pot, so I think you're close to the culprit. It may even be the pot itself that's had it, or if you're lucky, just a bit dirty. I don't know if you could get a bit of contact cleaner in there somehow. Even a quick squirt of WD40 would probably help.
Click to expand...

I agree. Could be a dry solder joint underneath or the pot itself. Sounds like a good diagnostic clue.

Not out of the woods yet by any means (random PETSCII characters can be caused by a number of things) but you might be able to get video back sometime soon.

Keep at it.

Tez
 
Thanks guys. No sign of the 'scope yet so I'll be having a rest today.

I swooshed the pot with an alcohol based contact cleaner and now it moves really smoothly :) No effect on the problem though.

I'm anticipating a number of problems - getting the video working is a pre-requisite to getting those sorted.

This is one machine which _has_ to come back to life. I've always wanted one.

C
 
Well, if you're looking for something to do, Tezza's dry solder joint theory is worth following up on. You might be able to get at the legs from underneath the monitor to reflow/resolder them.
 
It's been a while..!!

Apologies for the long absence but what with one thing and another...

So the news is that the video is alive! I'd like to say I fixed it but it appeared that the process of removing and replacing the video board after a visual inspection seems to have done the trick. Either that or the video board saw the oscilloscope that I bought and wasn't up for a fight... I'm a bit miffed that I went out and splurged on a nice scope and didn't get to use it on the video board.

Still, at least now I have a scope :D

So I've prodded around the board and the overall feeling is that the address and data lines (especially the data lines) are a bit dirty. I was going to build a single-step circuit but in the end just built a NOP injector. This was intended to show up any addressing issues.

I've scoped the 244s to see whether the bus gating was working and to tell the truth I am no further enlightened :p

Having attached the analyser to the lower 8 bits of the address bus I saw what could be fairly arbitrary address patterns emerging. There was no consistency from sampling session to sampling session.

I've obtained and am in the process of configuring one of Nicolas Welte's PET memory replacement boards. It hasn't helped so far but the board is a new version and it doesn't tally with the online documentation. I'm currently awaiting support.

Should I see some activity on the IRQ line? I read somewhere (or did I dream it?) that the IRQ is needed...

Nice to be back

Charlie
 
Bring us up to date; now that the video's working, what are the symptoms?

Are you sure that NOP injector's working correctly? The address lines at the CPU should definitely show regular patterns in a binary sequence unless the CPU's bad or is being reset.
 
MikeS said:
Bring us up to date; now that the video's working, what are the symptoms?
Click to expand...
Just what I was about to ask, too ;-)

sirmorris said:
Should I see some activity on the IRQ line? I read somewhere (or did I dream it?) that the IRQ is needed...
Click to expand...
IIRC there's no regular interrupt, so IRQ stays high until "something happens". It should pulse when you hit a key, though.
 
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