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New PETs!

Moonferret

Moonferret

Experienced Member
Joined
Jun 20, 2003
Messages
489
Location
Northwest (UK)
Hi Folks!

Just managed to pick up a pair of Commodore 3032's. Both are in fairly good condition but both are suffering from the same problems. Screens are just showing random characters and the displays are also quite jittery.

The good news is both seem to be responding to keyboard input so I'm guessing the fault lies with the display circuitry. Chips are soldered so will have a go at fault finding with the old piggyback method first.

Any suggestions with regard to the jittery / wobbly display? (I'm assuming this is going to be down to something on the video board rather than the CPU board)

I've attached some piccys :)
DSC02485.jpgDSC02487.jpgDSC02488.jpg
 
This is a common issue. There are a lot of threads on this forum related to similar problems. I'd take a look through them first and then see where you think you are. You can start replacing RAM, chips, etc. Shot gun it or get out the meters and start checking voltages and oscilloscope patterns. Some times you get lucky and all you need to replace is a socketed chip. Because you have to of the same systems, you could if you wanted to start swapping the socketed chips to see if you can make one working system that way.
Bill
 
Bill, Thanks for the reply. Already started reading through the posts :)

I think my aim should be to get one CPU board working correctly first before starting on the display wobble. Unfortunately, only the larger chips are socketed. RAM & Logic is all soldered in. Do have some spare ROM sets though (I believe the original owner upgraded the machines from BASIC 2.0 to V4)

Should keep me busy for the next couple of weeks anyway :)
 
Nothing new but your logic and steps do sound good so far. The CPU clears the display and prior to that it should be random (but repeating) garbage. One of your systems looks like a video chip or character rom error since it's at least reading the basic rom and printing what it thinks should be there.
 
Moonferret said:
Chips are soldered so will have a go at fault finding with the old piggyback method first.
Click to expand...

Yes, most likely one or both of the video RAM chips.

Any suggestions with regard to the jittery / wobbly display? (I'm assuming this is going to be down to something on the video board rather than the CPU board)
Click to expand...
Yes, check the +12V regulator on the video board, the 7812 chip (IC901). If you see ripple/noise on the DC voltage, replace it.
-Dave
 
Well done on the PETS,

Yes, I'm with dave. If you can see characters changing in response to key presses, then the video RAM chips are prime suspects.

Like their four-legged counterparts, PETS seem to need a lot of TLC to stay healthy!
 
dave_m said:
Yes, very possibly but I would think if the caps were bad, the display would really jump around a lot.
Click to expand...

They could just be drying up and losing capacity...I like to measure caps when I remove them, and it's not uncommon to see capacitors from "mostly working" boards that only have 10-25% of their rated capacity!
 
Measure the 12v output of the 7812. If the DC level is more than 0.1-0.2v less than 12v and the AC level is more than 0.1v then it likely has weak caps. The input voltage should be at least 15v DC if the AC is less than about 0.5v, and needs more DC as the AC increases so that the minimum DC is always 14.5v - 15v or more.
 
Thanks for all the good advice guys! I have good news to report. I've managed to get one of the boards working!! :) By piggybacking chips, I found that one of the 2114 video RAM chips was duff. I've desoldered it and popped a replacement in (In a socket!) and it all seems ok. Keyboard needs a good clean next as only around 50% of the keys are responding.

Will try and get the other board working next. Working on the video board looks like it might be a challenge. Any suggestions on the best way to get at it?
 
Agreed, probably a bad capacitor (or bad ground connection somewhere?) causing the jiggle, and a problem in the video circuitry. Check for low AC voltage where there should be pure DC.

The one on the right looks pretty straightforward, just a stuck 1 bit in the upper four, probably IC F7.

Note that space = 0, E = U, etc.; see:

http://en.wikipedia.org/wiki/PETSCII

Edit: Oops, didn't see page 2, that you've already solved the easy one. Good work.
 
Moonferret said:
...Will try and get the other board working next. Working on the video board looks like it might be a challenge. Any suggestions on the best way to get at it?
Click to expand...
Know anybody with an oscilloscope (or have one yourself)? I'd check the video signals for noise or ripple first if possible to make sure the problem is actually in the display; not the most convenient to work on if it is, as you've already discovered.
 
Ok, quick update. Second board is now working!! Carried on with piggybacking chips. First the video RAM, no change at all there. Next, after reading previous threads, I had a go at the main RAM. No change at all with the display when piggybacking on to any of the chips in the second row but on certain chips in the first row, I'd occasionally get some characters changing on the screen.

Didn't have my scope to hand but I thought it was worth a go anyway. De-soldered the first row of RAM and replaced with a new set in sockets.

Upon power up, I was at first, getting a screen of garbage which then changed to a screen full of zeros and then what looked liked the welcome message. This was exactly what the first board was doing. Ahh!! :)

Swapped out the 2114 in location F7 (Spot on there Mike!) and it fired up. Hey Presto! It works!! :)

I've cleaned the keyboards so both machines now can be used. Tested both by loading some programs from tape (Machines came with 30+ PETSOFT tapes) and both seem to be fine.

On to the displays...Mike, I do have a scope somewhere which I'm going to dig out later but I think I'm going to have to do a bit more reading as I don't really know what I should be looking for in terms of the output.
 
Tez, you were right about them needing a lot of care and attention. After my last post, both started playing up again. Machine 1 was only coming up with a screen full of random characters. Tracked this down to duff capacitors on the reset circuit causing the 555 not to trigger. Now fixed and working again.

Second machine is a bit more tricky. It had the same issue with the reset circuit which was easy enough to fix. However, now it is still displaying a screen full of random characters with the exception of small areas which seem ok. Eg. upon power up I can see the start of the welcome banner '*** C' then gibberish. I can make out part of the 'READY.' but its not where it should be. System does respond to key presses though.

I was almost starting to think that the repairs were going to easily :)

Good news though. The original owner contacted me and has managed to dig up a pair of 8250lp drives units. Anybody got a spare IEEE488 cable?
 
Yes, I have a box full of PET to IEEE-488 cables. I estimate the total costs to be around £15-20 including shipping and PayPal fees. Send me a PM if you are interested.
 
Moonferret said:
Tez, you were right about them needing a lot of care and attention. After my last post, both started playing up again. Machine 1 was only coming up with a screen full of random characters. Tracked this down to duff capacitors on the reset circuit causing the 555 not to trigger. Now fixed and working again.

Second machine is a bit more tricky. It had the same issue with the reset circuit which was easy enough to fix.........
Click to expand...

Now this is interesting. I had just that kind of problem myself but with an Apple II+ clone, not a PET. I thought it would be a rare issue but maybe it's not that uncommon.

Good info for our diagnostic knowledge base.
 
Anders, Thanks for the offer of the cables. I've sent you a PM

Tez, It was a post on this forum that prompted me to look at the reset circuit. Knowing the issues caused by old capacitors, I'd imagine it could be a fairly common issue on these machines.
 
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