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[HELP] Commodore PET CBM 3008 3032 Screen Issue

DrAlis

Experienced Member
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Nov 11, 2021
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71
Clipboard01.png

Hello all, I am trying to put a PET back in service and things look great. However I have an issue with horizontal placement on the CRT. Characters at some point stop being placed to the right but at the same location. And then after a while they move right but squished. Above is a screenshot of a screen full of As. Any ideas where to look?
 

Dwight Elvey

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The problem is on your video board. At least that is what it looks like. If you type ABC.... for 40 characters it will be clearer but I feel it looks to be a display issue. As a minimum, I recommend renewing the heat sink grease on the horizontal output transistor.
Dwight
 

daver2

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By video board Dwight means the small board in the monitor as opposed to the video output stage of the large PET logic board.

Just out of interest, do you have access to an oscilloscope to demonstrate that the horizontal drive from the logic board to the monitor board is as expected (i.e. the correct frequency)? This would narrow it down to one board.

Dave
 

DrAlis

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So above is a view of the video board. Have no osci yet but could get one. I am a little shy of working on the board with power on. I was hoping to take out the board and swap some things and put it back. I have to see how to get this suction cup like thing of the tube... Where would I find the output transistor?
 

Gary C

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Its worth twiddling some of the pots but from the drawing http://www.zimmers.net/anonftp/pub/c...01N/321445.gif it looks like the horizontal only has a size adjustment on L704.

Its odd that the beam is obviously not at the left when the video is enabled which suggests something isn't sweeping the beam properly, not really looked at it too long, but is capacitor C715 knackered ?

Might be worth scoping the test points starting with TP 17 to see if it looks like the one shown in http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001N/321447-10.gif
 

Gary C

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But yes, caution is advised.

The green, red and blue cables have a few tingly volts on them, and the one to the rubber cap on the tube has 10,000V on it and it comes via transformer T702 which also has part of the horizontal deflection circuit on it.

Hugo Holden is the guy to root out this problem I think :)
 

DrAlis

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Nov 11, 2021
Messages
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Wow. You guys are amazing... Incredible to me how you guys just whip out the matching schematic and find stuff on it. I google forever to find any odd schematic loosely related to the machine, just to learn that the relevant section isn't even in there. Thanks!

So I got some homework to do now. I found C715 on the schematic Gary provided. So my next task will be to locate a 4,7 uF / 25V on the board, watch some tube safety videos and get that thing out to test.

I watched some videos, but to be safe let me check with you - and I know it is my sole responsibility: So I disconnect everything from power and let it sit for 20min. Then I take a plastic handle screwdriver with the metal part connected to earth and touch all legs of most caps in there and also the anode under the suction cup. Then I am good to get my hands in there. Any obvious flaws in that procedure?

Also, on the nonlinearity: Yes the character start offset to the center, then progression to the right stops and even reverses (!). Then as characters proceed progression to the right picks up again positively in a diminishing fashion... The reverse part is with the extra brightness (doublestroke characters).
 

DrAlis

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Sorry C715 is 6,8uF / 25V - my schematic reading skills are small but maybe growing... grmpf :)
 

daver2

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>>> Incredible to me how you guys just whip out the matching schematic and find stuff on it.

We cheat! We have bookmarked the excellent sites that relate to various machines :).

For anything Commodore, Zimmers is one of the more comprehensive sites available. Thanks Bo.

Dave
 

Gary C

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I feel its a capacitor but while I have quite a bit of hands on experience with CRT circuits, its still a bit, err, by intuition and tests rather than look at circuit and really get to grips with the design.

So the cap I have identified isn't based on 'real circuit investigation' rather a starter for 10.

The zimmers does have the scope traces for each part of the horizontal deflection circuit and checking them for test points 13 to 17 would be worth while, but as I said, the horizontal deflection circuit is also used as an oscillator to drive the HV coil for the CRT's anode and thats High Voltage with a capital H (even for me and I work with 400kV circuits) SO BE CAREFUL

This https://www.eeeguide.com/television-...ction-circuit/ gives a bit of a guide to how this part works (though its not the same circuit at all)
 

Dwight Elvey

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The fact that the display lights up and is about the right size would indicate that the horizontal output is working and the problem is likely in the yoke drive. The capacitor mentioned often causes similar problems of folding, as I recall.
 

daver2

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Note that C715 is a BIPOLAR capacitor (not a normal polarised capacitor) - so watch the specification for any replacement.

Putting a polarised capacitor in this part of the circuit will cause it to fail very quickly...

Dave
 

Dwight Elvey

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Your mileage may very but I was told one can use 2 back to back capacitors of 2 times the value to replace a single unpolarized capacitor.
I've never tried it but it is worth a try if the unpolarized capacitor is hard to find.
Dwight
 

Agent Orange

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Your mileage may very but I was told one can use 2 back to back capacitors of 2 times the value to replace a single unpolarized capacitor.
I've never tried it but it is worth a try if the unpolarized capacitor is hard to find.
Dwight

Capacitors is series add like resistors in parallel and vice-versa.
 

DrAlis

Experienced Member
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Nov 11, 2021
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I got all excited for the magic Hugo... Did he ever show up? I will need some time until I have enough time to get the board out. I also ordered some matching bipolar caps. I don't feel lucky enough to try Dwight's equivalent build. There is also this second PET (other Post) that also seems to have a screen problem (all black). I noticed it has a superdim glow at the yoke only compared to the one with the folded screen. Much dimmer that this one. Should the glow at the cathode be constant regardless of image or does it vary with image signal? I also consider trying to swapping the monitor assembly with the all black one, to get me some insights in both machines. The video PCB on the second one is completely different but I hope the pinout on the cable is the same so I can safely swap. Also has some really weird parts on the PCB in it that I do not recognize: small glass thingies :)
 

DrAlis

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Nov 11, 2021
Messages
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So I got lazy and just placed the two PET next to each other and had the deformed monitor J7 connected to the other PETs J7. So the good news is both PET boards are working. The bad news is both CRT boards and/or yokes are not working to different degrees. Any TV technicians in Bavaria Germany here to stop bye for the rescue? Have beer...
 

Gary C

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Its worth noting that it can take quite a bit of time to get all the help here as there isn't a huge number of people can help and they have real life getting in the way.
The watchword is patience :)
 
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