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Commodore PET 2001 monitor fixed but no dice

falter

falter

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A while back I discovered that the issue with my chiclet PET, which since the time I'd owned it only produced a screen full of lines or retrace, was the monitor itself. Swapping monitors between this one and a second PET revealed the 2001s motherboard fired up just fine, while the 2001s monitor produced the exact same pattern connected to the other PET. Anyway, the problem shortly got worse.. the monitor's 'field' collapsed to a single line. So I took it to a place with extensive experience repairing all kinds of equipment including CRTs. They replaced some caps and got the retrace back. The repair person says the CRT is okay.

So now I'm lost. The only thing we noticed amiss while experimenting on his counter, was that the pins in the plug for the monitor were rather rusty. He didn't make an effort to clean them, insisting they were working since we were getting something now onscreen, even if it was just retrace. As a side node, he said the retrace was partly a function of having the brightness turned up too high.

What do you think? He suggested bringing the other working PET down so he could compare monitors and see what was going on.
 
The way I read what you are writing (I hope I got it right), it sounds like you aren't getting the video signal to the CRT. I don't know why the service tech wouldn't try to verify that. Getting a raster doesn't mean the CRT unit is working correctly.

Clean those pins, though. You may be right.
 
Yes.. I think you have it. The tech checked and replaced caps, checked various other things and thinks its fine. The 5 or 6 wire connector to the motherboard, if it's not connected the screen is completely black.. if it is connected, you get the raster pattern.
 
OK, so the horizontal and vertical sync are working, but there's no video signal. Clean the pins and try both monitors on both PETs. If that one still doesn't work, something's wrong with the video section of it.
 
Will do and thanks. For my own understanding, what comprises the 'video section' in the monitor? Just so I can direct the technician the next time it goes in for service. I would work on it myself, but the voltage involved makes me nervous, so anything involving a CRT I farm out.
 
I pulled and cleaned the connector wires, but no dice. Same as before: http://vid1381.photobucket.com/albums/ah216/unclefalter/20151020_214300_zps3tsolftr.mp4

I then did the swap between my 4016 and this one.. and the 4016s monitor worked fine with the 2001, but the 2001s monitor did not work with the 4016. So, I've got to find a way to point the repair guy to the right area. He suggested bringing both computers down. The only problem with that is, I'm pretty sure the actual video circuitry in the 4016s monitor is different from the 2001's. I wonder if he'd be able to figure out enough from that.. or what else I could give him to help. This monitor has never worked in the 14 years I've had this computer. Really want to see it light up properly.
 
falter said:
I pulled and cleaned the connector wires, but no dice. Same as before: http://vid1381.photobucket.com/albums/ah216/unclefalter/20151020_214300_zps3tsolftr.mp4

I then did the swap between my 4016 and this one.. and the 4016s monitor worked fine with the 2001, but the 2001s monitor did not work with the 4016. So, I've got to find a way to point the repair guy to the right area. He suggested bringing both computers down. The only problem with that is, I'm pretty sure the actual video circuitry in the 4016s monitor is different from the 2001's. I wonder if he'd be able to figure out enough from that.. or what else I could give him to help. This monitor has never worked in the 14 years I've had this computer. Really want to see it light up properly.
Click to expand...


At least get him the PET 2001 video board schematics. There are three versions. One will match your board.
 
I'm kind of worried about this service tech.

The video section is essentially just the video amplifier, in the case of that monitor.
 
Is there a schematic online? That display is not just lack of video. You have no vertical blanking whatsoever. You shouldn't have that much retrace lines with no video. I also think your CRT tube is ok. Don't be scared of the high voltage. It's probably around 10KV or less and very low current. Stay away from the rubber anode cap on the CRT and you won't get any shock when the display is powered off but don't touch any points on the circuit board either, of course. Power supply caps can hold a charge for a few hours. Visually look around for any wires / connectors loose and with a magnifying glass look for solder cracks in the circuit board or a crack in circuit traces especially around transistors standing up with an aluminum heatsink attached or if there is
a circuit board on the end of the CRT tube. Do you have a voltmeter? With a schematic, I could guide you thru a few DC voltage checks. It should be obvious.

Larry G
 
dave_m said:
It should be fixable by a good TV repairman.
Click to expand...

And a good TV repairman would have tested (and probably fixed) this before he sent it out the door the last time.

But, maybe there just aren't any good TV repairmen anymore. :(
 
Take the PET with the monitor connected and the correct schematic down to the repair man. They should even find various oscilloscope traces in the zimmers documentation for various points within the monitors video and synch paths.

The technician should verify that the monitor is getting the two synch signal (Horizontal and Vertical) and a Video signal from the main board to the monitor (thus proving that the main board is OK). He/she should also check the main d.c. voltage after the voltage regulator on the monitor. From here on it is a 'bog standard monitor' and the technician should have no problem finding and fixing the problem.

By the looks of your video, the brightness is turned way too far up. What you are 'seeing' when the screen is getting white should really be dark. I agree with everyone else that what you appear to be missing (and always have) is the video signal path somewhere within the monitor itself.

If the technician can't find the problem within an hour or so - I would avoid them.

If you are not used to working with high voltage equipment I would advise you to keep your fingers well away from the monitor circuits. Despite what has been said previously - these things can (and do) kill people.

Dave
 
Dang, watching the video again I saw one thing not good. See the black spot in the middle? If that's not crud, it sure looks like phosphor burn on the CRT. Happens with raster collapse mentioned previous and high brightness. It will still be useable but the spot will always be there. Since it's dead center on the raster I'm sure that's what it is.

PS - even with the display powered off, look at the center of the tube with a magnifier. You'll be able to see where the phosphor chipped off.

Larry G
 
I noticed that, too, and maybe I'm seeing it wrong, but I thought I could see a "shadow" in the glass as if it were dirt on the front of the glass.

Let's hope so.
 
Thanks guys. I brought both PETs down along with this schematic: (http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001/video-1.gif (thank god for large format printers)

I thought that had to be the correct one.. the site lists that one and then second and third versions. Hopefully he figures it out. No way am I messing with anything involving a CRT. Just not worth the risk.

The dark spot.. I noticed that but ended up busy with other stuff.. didn't get to investigate what it was, if it was just dirt. I don't recall it being there before.
 
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