Will a PET 12" CRT work on an originally 9" 3032/4032 model?

[fachat]

Subject says it all basically. I have a broken 4032 (9" model, no CRT) and I think the screen is broken. It does not heat up and nothing is shown. The CPU however is working (to some degree at least, I've scoped the pins, and the screen output also looks all black except for a very small part - which could be the READY prompt).

When I started it the first time, it did not start up. I measured the transformer and it did not give much voltage (around 1-2V AC) ... until suddenly a sizzling noise appeared, and a little smoke was coming from under the transformer :-(

I took it all apart but could not see anything, so I tried powering on the transformer only (without board and monitor) and got about 16V AC. Then I put the board back in and it seemed to work as well (as I said scoped the signals).
Only when I put on the monitor, it just stays black, the tube does not heat up (I think - I cannot see any glowing light through the back as I remembered it should be)

As it is, I have a surplus 12" CRT, and looking at the connectors, they look very much alike (GND, AC power supply, std. PET video connector). Looking at the schematics the only thing I see is that the 12" model has the video signal inverted (5V is white here, 0V is black, on the 9" it's the other way round).

Has anyone tried that before?

Alternatively I have another broken 3032 that I could salvage (but I also wanted to restaurate that), and maybe even try to fix the CRT.

Thanks for any help
André

 

[carlsson]

As for testing, aren't you the same André Fachat who gave me instructions on how to build a video adapter to get video onto an external composite monitor? Admittingly I got rather poor results, but it could be my choice of components, soldering skills or that I didn't have an capable enough monitor. I'm just thinking out loud possible alternative ways than connect a 12" tube to a 9" PET, whether it is designed to work or not.

 

[fachat]

As for testing, aren't you the same André Fachat who gave me instructions on how to build a video adapter to get video onto an external composite monitor? Admittingly I got rather poor results, but it could be my choice of components, soldering skills or that I didn't have an capable enough monitor. I'm just thinking out loud possible alternative ways than connect a 12" tube to a 9" PET, whether it is designed to work or not.

Yes that's me

As for my problem. In the non-CRTC machines the video timing cannot easily be reprogrammed by modifying the editor ROM and putting other CRTC register values there. So using a composite monitor would only work if it accepts the non-CRTC-PET video timing. So if I have a an extra PET monitor, why not use it...

But in the meantime I had a closer look at the CRT schematics for the 9" and 12" machines, and the timing required by these two monitors is considerably different. So I decided to salvage the monitor from my broken 9" 3032, and voilà, working display.

Unfortunately not everything is working yet. The display on the screen is stable, but the characters are all missing the bit 2 (value 4). So "COMMODORE" becomes "CKIIK@KRA"... But what's more is that it only shows "88" bytes free and also behaves somewhat strangely (e.g. the boot prompt is all in a single line). Keyboard input is basically ok (only bit 2 is missing on screen), "Return" as well as "crsr right" do not work, so no simple testing.

With a scope I measured D2 on UE8 (the video RAM data bus driver) and actually confirmed that on writes, D2 is always zero already (trigger on pin 19 = write enable, measure pin 13 or 14 = BD2, then press "4" to get a value with D2 set written to screen -> D2 is always zero!)

So my next assumption is that UE9 could be broken. It buffers the CPU data bus to RAM (DRAM and Video). This could be the single problem for the display and the strange behaviour.
I'll measure that one when I got a new fuse, I accidentally blew one... (I wanted to test the 3032 board - it did not work, did not even get 5V VCC. So I started measuring the 12V AC supply, and accidentally shorted something. I was quick removing the VOM probes, but the fuse was faster. This board does not have any protective color coat on it!) Unfortunately the 3032 has a different fuse (physical, not electrical) size.

Anyway, I hope to get the 4032 fixed relatively easily, but the 3032 is getting more and more difficult to repair... (CRT monitor, power regulators...)

André

 

[fachat]

As for testing, aren't you the same André Fachat who gave me instructions on how to build a video adapter to get video onto an external composite monitor?

Actually I only collected other schematics from the net. My own boards are derived from them and work on my composite monitor. No guarantee that it works on other monitors!

Edit: what were the symptoms when you tried them? You might need to fiddle with the CRTC timing parameters to get a stable display.

 

[carlsson]

The symptoms were rolling picture, looking like multiple frames on screen. I used 3032 and 4032 boards, so possible some were non-CRTC and would complicate timing. I tried some 8032 boards too but got total garbage.

 

[fachat]

The symptoms were rolling picture, looking like multiple frames on screen. I used 3032 and 4032 boards, so possible some were non-CRTC and would complicate timing. I tried some 8032 boards too but got total garbage.

The rolling picture is a symptom of bad timing. If you use a CRTC model, try to adapt the CRTC timing parameters. Non-CRTC is bad luck.
I should probably add a note on my web site....

André

 

[carlsson]

Actually I have parted out most of my loose boards so only a few remain. Those boards are missing so many big chips that they only are good for spares anyway, so I no longer have any pressing need for a monitor interface.