Wobbly PET

[Matty3008]

Hi All

I've been lucky enough to inherit a PET CBM 3008 in excellent cosmetic condition and working (sort of...read on) fine.

Everything works great in terms of loading programs etc. however at all times there is a very slight wobble in the characters on the screen (less than 1mm).

Just yesterday after having the system on for about 20 minutes things got a whole lot worse and now the video dances around like crazy. Seems like a CRT problem as all the characters are still correct and the machine still loads etc, just the screen s crazy.

I post a (slightly jerky but you get the idea) video below to give you an idea. When left off for a while it's ok again for another 20 minutes then goes crazy again. Sounds like an overheating issue of some sort.

Does anyone have any idea how this can be fixed? It's a shame that apart from this the machine is working great!

 

[Drea]

I've seen something similar happen when one (can't remember which) of the adjustment screws inside the monitor are set wrong on the 8000 series, but I don't know if this model have them..but maybe worth trying...

 

[carlsson]

It could be bad capacitors: http://www.vintage-computer.com/vcforum/showthread.php?t=26008

I seem to recall someone else asking the same question recently, but that was the only reference I could find.

 

[Matty3008]

Thanks drea/carlsson

Do you think this could be an adjustment issue if the wobble only happens once the machine has been on for a while? In the first instance there is a tiny bit of wobble which is fine, but after 20 minutes it goes haywire and is unreadable.

I suspect overheating of one kind or another, but I can't seem to find anything overheating on the main board. I can't get into the monitor assembly at the moment without attacking it with a screwdriver.

Any other suggestions?

Matt

 

[Drea]

Thanks drea/carlsson

Do you think this could be an adjustment issue if the wobble only happens once the machine has been on for a while? In the first instance there is a tiny bit of wobble which is fine, but after 20 minutes it goes haywire and is unreadable.

I suspect overheating of one kind or another, but I can't seem to find anything overheating on the main board. I can't get into the monitor assembly at the moment without attacking it with a screwdriver.

Any other suggestions?

Matt

Most likely not an adjustment issue then, but i'm no expert (at all)!

Here's a pic of the screws, they are the small metal ones http://tinypic.com/view.php?pic=rucoyt&s=5

Shouldn't take more then a few minutes to try, just remember the original position, so you can set them back

 

[dave_m]

I suspect overheating of one kind or another, but I can't seem to find anything overheating on the main board. I can't get into the monitor assembly at the moment without attacking it with a screwdriver.

There is a +12V DC regulator on the monitor board that you might want to check. See schematics on zimmer. If it is noisy, it may need to be replaced. Be care about high voltages in the CRT area.

 

[Matty3008]

great. thank drea/dave_m i'll look into these and report back after the weekend.
matt

 

[crock]

great. thank drea/dave_m i'll look into these and report back after the weekend.matt

I had exactly the same symptoms and fixed it by replacing the electrolytic caps on the monitor board. I bought enough to re-cap 10x boards for about ~$50 from digikey, so about $5 per monitor. The effort was not too great, about 2 hours for disassembly, rework and reassembly. Dissassembly was a bit awkward as the wires are soldered between the board and deflection yoke so getting the tube and board out of the case was fiddly. The other minor problem was finding ones the same physical size. The design improvements have meant that simlar rated caps from 1977 to 2012 are physically smaller, so in some cases I replaced them with ones of somewhat higher voltage just to get ones with similar pin sizing. I think I particularly had problems with the 10uf bipolar cap.

Rob

 

[dabone]

On my 4016 and the 8032 I just fixed, the yolk assembly has plugs to the mainboard.
(2 connectors, 2 wires each, for vertical and horz.)


Later,
dabone

 

[Matty3008]

thanks dabone - what are you saying? that i should try reseating the plugs?

 

[crock]

thanks dabone - what are you saying? that i should try reseating the plugs?

I think dabone was replying to my post. rob

 

[Matty3008]

I've decided I don't have the skills to fix this so have listed it on ebay.

Does anyone know if the 3008 is particularly rare? I can't find any for sale on ebay over the last 12 months. 3016s yes, but 3008 none (apart from an orange one recently)!

Matt