My CBM 8032 is sick again... visible raster scan on CRT

[powerlot]

Hi community

As the title says, my PET is sick again (my vintage electronic stuff seems to break down faster than I can fix it). Of course this happened when I wanted to show it off to visitors...

Before I poke around I thought I'd ask for some tips where to look? It still chirps on start up but nothing moves on the screen, instead it shows this:



Turning the brightness knob doesn't do much except changing the brightness a bit. As it still chirps, I guess the fault is in the CRT circuitry?

Thanks for your help

 

[daver2]

Sorry to hear that. Some machines do have a few more failures as they get the 'bad components' out of their system!

Remind me, do you have an oscilloscope or logic probe?

If you do, the first thing is to check the signals from the logic board to the monitor (H DRIVE, V DRIVE and VIDEO) for signs of activity'.

It does look like the monitor circuitry to be honest - but rather than guess, let's approach the problem logically by the 'divide and conquer' method...

It could be that the V DRIVE and HDRIVE signals are present, but not the VIDEO signal - so the monitor is displaying what it is told to do (i.e. a totally black screen). You must have V DRIVE and H DRIVE signals for the raster and high voltages to be present.

Dave

 

[Hugo Holden]

When a CRT's brightness is set too high (or too high for any other reason) slanted horizontal axis retrace lines occur, that are brighter than the background raster.

In many more sophisticated VDU's these are blanked out by applying a voltage pulse to the grid, or cathode, of the CRT during vertical retrace, but in the PET VDU's they don't do this.

So if the background brightness is elevated to the extent that the scanning lines are seen, so will these retrace lines be seen, but looking brighter.

There seems to be a paucity of video data on your raster. If the video signal from the PET had become abnormal, it could raise the CRT's video drive voltage up, illuminating the CRT like this, and showing no or little video data.

The best initial move is to put the scope on it, check the video input to the VDU and the V drive pulse. The fault could possibly be outside the VDU, so that needs to be excluded before jumping into the VDU for more tests.

One thing to check (and I think I have read somewhere some time ago that anther VDU like yours produced this effect) if there are dry solder joins on the pcb where the connector was fitted:

I found the link:

Commodore PET 8032 Repair and Restoration

This is an old post, preserved for reference. The products and services mentioned within are no longer available. Oh dear, here we go again....

blog.tynemouthsoftware.co.uk

Make sure to exercise the brightness control. But, usually if this is faulty, with an open wiper for most VDU's this causes the beam to go off, but it can be the other way around in some designs, I would have to check your VDU's schematic.

 

[ScottishColin]

I get this effect sometimes when the VDU connector on the motherboard fails. A quick wiggle of the connector fixes it.

I need to do something about that.....

Colin.

 

[powerlot]

Wow, quick replies!

Remind me, do you have an oscilloscope or logic probe?

If you do, the first thing is to check the signals from the logic board to the monitor (H DRIVE, V DRIVE and VIDEO) for signs of activity'.

I have a scope, the signals are present.

One thing to check (and I think I have read somewhere some time ago that anther VDU like yours produced this effect) if there are dry solder joins on the pcb where the connector was fitted:

I get this effect sometimes when the VDU connector on the motherboard fails. A quick wiggle of the connector fixes it.

Bingo... after verifying that the video signals are ok, I started wiggling on the connector and then tugging on the wire and it looks like the problem is on the analog video PCB connector (the one on the motherboard side appears to be in order).

I'll reflow the solder joints and apply the usual maintenance for connectors once the capacitors and CRT have discharged.

Thanks a lot, I'm glad it was such a trivial issue

 

[daver2]

>>> Wow, quick replies!

We aim to please ! I suppose it also helps to be in a similar time zone...

A Quick fix also on your part!

Dave

 

[Hugo Holden]

Wow, quick replies!


I have a scope, the signals are present.




Bingo... after verifying that the video signals are ok, I started wiggling on the connector and then tugging on the wire and it looks like the problem is on the analog video PCB connector (the one on the motherboard side appears to be in order).

I'll reflow the solder joints and apply the usual maintenance for connectors once the capacitors and CRT have discharged.

Thanks a lot, I'm glad it was such a trivial issue

The PET 9" VDU also has this problem.

The solder quality was somewhat brittle and they acquire a 360 degree ring fracture around the pins of the connector on the VDU board. I have two 9" Pet VDU's, one is a spare. One of them had the problem, causing a failure, when I inspected the other, under magnification, the process was starting, so I ended up re-soldering the pins on both VDU boards.