• Please review our updated Terms and Rules here

Pet 3032 Vertical Collapse

eight088

Experienced Member
Joined
Oct 31, 2018
Messages
156
Location
Perth, WA Australia
Hey guys

Got a 3032 recently and I'm trying to get it going. Unfortunately it appears to have vertical collapse (single horizontal line in the middle of the screen). I'm trying to work out whats going on with it and need a bit of help. I have taken some signals from J7 on the motherboard, and to me I think the signals look okay so think it might be a problem with the display board. I'm still learning heaps about using my oscilloscope, so would really appreciate any advice.

Cheers :)
 

Attachments

  • video.jpg
    video.jpg
    73.1 KB · Views: 6
  • vertical.jpg
    vertical.jpg
    74.5 KB · Views: 6
  • horizontal.jpg
    horizontal.jpg
    72 KB · Views: 6
The vertical drive signal looks probably ok.

The first thing is to get the schematic for your particular VDU.

There are lots of reasons why the vertical scan can fail. There are commonly (depending on the particular VDU) two or three transistor driver stages, then an output stage power transistor pair . It requires that the circuitry has all good components, that the yoke Vertical coils are good and that the coupling capacitors are good and the circuit is being powered. Vertical scan stages are very similar in their topology to audio amplifiers, with a typical class B output stage biased just out of cross over distortion. One difference being that they have a much lower cut off frequency typically lower than 5 or 10Hz, they need the LF response for a linear sweep.

Since you have a scope it will be fairly easy to follow the signal through the vertical stages in the VDU and find out what has failed.
 
Thanks for the info Hugo. Without pulling the display apart, I decided to try and connect an MDA monitor that I have lying around up to the pet. Had some success, and have confirmed that the VDU is definitely faulty, so will need to inspect that further. I've found the schematics over at zimmers, with a bunch of test points that I can check with my scope, which should make it easier to find the faulty part!

Once I get the VDU working however, I think I'll need more help with the next step... MDA monitor only displays a checker pattern!

Cheers :)
 

Attachments

  • 307432418_518075186795242_5687286489250771470_n.jpg
    307432418_518075186795242_5687286489250771470_n.jpg
    255.1 KB · Views: 14
A checkerboard pattern can mean that an all 1 pattern ($FF) is coming out of the latched screen data bus (LSD bus) into the character generator ROM (F10). See sheet 8 of the schematic for the 2001N PET.

If so, it would have to be a problem in both video RAMs (F7 and F8) or perhaps in the 8 bit latch (F9). There is a signal called TV RAM R/W that I would like to make sure is going high to read out of the video RAMS.
 
Thanks Dave, I'll check TV RAM signal and report back. Heres a picture of the video ram though, and it looks a little suss. Think someone might have worked on it at some stage. Apparently this pet was rescued from the side of the road, after a heap of rain!
 

Attachments

  • video ram.jpg
    video ram.jpg
    481.5 KB · Views: 11
Okay, I have some more wave forms for you to look at. I suspecting the latch at F9, as SD0-7 have activity, where as LSD0-3 seem to be floating, and 4-7 are low. What do you think?
 

Attachments

  • tv ram.jpg
    tv ram.jpg
    125.6 KB · Views: 3
  • sd0.jpg
    sd0.jpg
    143.1 KB · Views: 3
  • lsd0.jpg
    lsd0.jpg
    119.6 KB · Views: 3
Replaced F9, and now getting proper signals on the LSD side... I am now getting a garbled screen, which sort of looks like snow as its scrolling quite fast. At least I'm making some progress!
 

Attachments

  • garbage.jpg
    garbage.jpg
    259.3 KB · Views: 17
I am not sure that connecting an MDA monitor to a PET is going to necessarily work correctly. The PETs 'drive' signals are not the same as synchronising signals.

It is possible that the 'scrolling' is a not 100% synchronised video signal.

Dave
 
A static screen of random data means the PET is not booting up properly. Is that what is happening. You say scrolling?

It is random data yes, and by scrolling I think it might be a sync issue as daver2 has pointed out.

I am not sure that connecting an MDA monitor to a PET is going to necessarily work correctly. The PETs 'drive' signals are not the same as synchronising signals.

It is possible that the 'scrolling' is a not 100% synchronised video signal.

Dave

Cheers Dave, I might put my focus back on the display board, and see if I can pin point the issue there, and then continue diagnosing the mainboard.

Thanks for your help so far guys :)
 
First things first, have you identified the correct schematics for your particular monitor?

If not, check here http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001N/index.html or here http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001/index.html.

Commodore were not known for their consistency - they would just stuff whatever they had into a particular machine to keep them rolling off the production line. Subsequent repairs could also have fitted something non-standard.

The vertical logic is quite simple and should be straight forward to debug. Do you have an oscilloscope though?

The schematics also reference other documents that contain expected oscilloscope traces at various points through the circuitry. Just look for the first point that doesn't match what Commodore indicate...

Dave
 
Last edited:
First things first, have you identified the correct schematics for your particular monitor?

If not, check here http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001N/index.html or here http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001/index.html.

Commodore were not known for their consistency - they would just stuff whatever they had in to a particular machine to keep them rolling off the production line. Subsequent repairs could also have fitted something non-standard.

The vertical logic is quite simple and should be straight forward to debug. Do you have an oscilloscope though?

The schematics also reference other documents that contain expected oscilloscope traces at various points through the circuitry. Just look for the first point that doesn't match what Commodore indicate...

Dave
It looks to me like the scans on the initial post show the vertical drive pulse at 60Hz and the H drive at around 15.7 kHz, so from this we could conclude that his VDU is a 9 inch model. If that is the case, there are essentially only two variants, an early and a late model, of which the late model I think had a width control, and a different layout pcb.

From what I have seen of the 12" PET VDU's, most seem to have used a 20kHz H scan rate and presumably they were the ones fitted with the CRTC controller IC on the computer board.
 
First things first, have you identified the correct schematics for your particular monitor?

If not, check here http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001N/index.html or here http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001/index.html.

Commodore were not known for their consistency - they would just stuff whatever they had into a particular machine to keep them rolling off the production line. Subsequent repairs could also have fitted something non-standard.

The vertical logic is quite simple and should be straight forward to debug. Do you have an oscilloscope though?

The schematics also reference other documents that contain expected oscilloscope traces at various points through the circuitry. Just look for the first point that doesn't match what Commodore indicate...

Dave
Hi Dave

Apologies for the late reply, we have had a long weekend here in Australia and have been busy over the last few days.

Looking at the pcb, it's either 320084 or 320064 (or even 320034?) - I can't quite make out the writing on the PCB. I have found some references to 320064/84 online and seems to be the same as a 2001. I've attached some photos to confirm if I'm looking at the right schematics.

One thing I want to check is if I'm probing around the VDU PCB, is there a maximum time I should leave the computer on for? I'm not overly keen to leave a sharp, bright (even with the brightness turned all the way down) horizontal line on the monitor for even a few moments!

Cheers
Matt

Edit - Yes I do have an oscilloscope :)
 

Attachments

  • vdu pcb2.jpg
    vdu pcb2.jpg
    173.6 KB · Views: 12
  • vdu pcb.jpg
    vdu pcb.jpg
    122.5 KB · Views: 11
  • maybe 320084-02 parts layout.gif
    maybe 320084-02 parts layout.gif
    493.3 KB · Views: 10
  • maybe 320084-02.gif
    maybe 320084-02.gif
    363 KB · Views: 10
>>> Apologies for the late reply, we have had a long weekend here in Australia and have been busy over the last few days.

No problem. We all have lives!

Yes, it looks like the 2001 variant rather than the 2001N variant. There are three (3) different schematics for this type of machine. There are some subtle differences between them as Commodore learnt what worked better! One key capacitor is C22 - but this could have been modified to make one variant into another! Welcome to the world of computer archaeology

Yes, you wouldn't want to leave the electron gun burning away at a line of phosphor... What would @Hugo Holden suggest I wonder?

You certainly want to turn the brightness down - but (as you say) that might not do the trick adequately. I am just wondering about 'bodging' the video signal (via a resistor) so that the video is displayed as black...

I think if you remove one lead of diode CR1 in the video circuitry then this should set the video signal to BLACK by the action of the pull-up resistor to +12V. Don't forget to put it back when you have fixed the vertical circuitry!

Dave
 
Last edited:
One thing I want to check is if I'm probing around the VDU PCB, is there a maximum time I should leave the computer on for? I'm not overly keen to leave a sharp, bright (even with the brightness turned all the way down) horizontal line on the monitor for even a few moments!
Does your VDU have a "Sub-bright" pot? If it has one you could use that to turn down the brightness even more. Or de-focus the beam... ? That would have been my attempt before messing with the circuit.
 
The only other pot (other than brightness) is the vertical size. This monitor also doesn't have a pot for the focus either. Pared right down to the bone!

Now I have mentioned the vertical size pot - your problem isn't associated with this potentiometer going faulty is it?

Dave
 
Just looking at the waveforms in the vertical circuit you have points 2 5 6 14 and 7.

You could always attach your probe to point 6 and then turn the monitor on to see quickly what you get. This is a binary chop and you can move forwards or backwards depending upon what you observe. Incidentally, there are better oscillograph photographs elsewhere...

This shouldn't be overly dangerous to the phosphor if you do it for a short time...

Dave
 
One thing about these 9" VDU's with that particular LOPT, the bias voltage for the brightness control was barely adequate and even when set at min brightness for the usual raster scan (which spreads the beam energy) it was pretty bright.(I modified mine to get the brightness lower). With V scan collapse the horizontal line you will see, even with that control set to min, will be harsh on the phosphor, if it takes too long to find the vertical scan fault.

I would recommend killing the CRT beam until you have fixed the vert scan fault.

There are a few ways you can do this, disconnecting one of the CRT's heater connections will work as that kills the electron emission. But probably the easiest way is to disconnect the H drive pulse, this will simultaneously kill the high voltage supply, kill the EHT and the H scan too. Simply lift one leg of the H drive input coupling capacitor C16 from the pcb. And connect up last thing after the V scan fault is fixed or at least you have some vertical scan. So there will then be no -30V, no +85v and no 10kV EHT and the horizontal driver transistor Q13 and H output transistor Q14 will both be in a non conducting condition. The CRT heater will still be glowing with no other voltages on its other electrodes, this does not harm it. But, the V scan stages do not depend on any of these voltages, only the +12V. This will also make it safer for you to work on too.

Then you can, using the scope, trace the vertical drive signal through the vertical amplifier and output stage, to find out what has gone wrong there. If you posted scope images of those, we could collectively work out what has gone wrong.

Generally, for this is, an open circuit wiper connection on the height control preset does not result in full scan collapse because they have connected the wiper to one side of the preset, but it is worth checking.

(you have probably also noticed by now that the copper trackwork overlay diagram with the component locations, has been left-right reversed or flipped. This copy on Zimmers is the only one I could find, I wonder if that mistake was in the original service manual, I think it must have been and wasn't noticed. There is no easy way to fix that diagram, without a lot of work in photoshop. Also if you want to fix the brightness control range later, to get a lower brightness, there is a simple mod adding a few extra turns around the LOPT core to get the -30v rail up to around -45v which is required to cut-off the beam in the green Amperex GH suffix CRT, but not sure about the white 9VALP4 CRT as I don't have one. Later it is also worth increasing the filter electrolyic cap value on that supply rail to prevent the notorious late turn off spot)
 
Last edited:
Back
Top