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PET 9-inch display help needed

B

Bockmeister

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Nov 14, 2025
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Has anyone ever seen this failure mode on a PET 9-inch display before? Before re-capping the display, it was immediately displaying the raster in a vertically squished area in the middle of the screen, but you could just make out the output.
After re-capping, as long as the power has been off for a while, it will display normally for about 5 seconds, then the green slowly fills the screen from top to bottom.
I do have an oscilloscope and the PDF of the schematic, but I was hoping someone might have an annotated diagram of the PCB with the TP's labeled.
BTW, the only cap that I haven't replaced is C28, the 10uF that from what I've read, seems like most people don't replace. Any insight would be greatly appreciated.
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Welcome to VCFED (as this is your first post).

There are multiple versions of the 9" PET monitor. We need to identify which one you actually have.

Can you either post a link to the schematic you are using, or post a photograph of the component side of the monitor PCB.

Are there any numbers on the PCB, to help identify it?

One thing that is going wrong is the blanking (or lack of)...

Dave
 
Your 10uF cap is fine, as the H scan is good.

It appears there is a problem with the relative heater to cathode voltage of the CRT, this controls the beam current & brightness. Either the CRT grid is not negative enough, or the CRT cathode is not positive enough when the fault occurs.

Also it looks like your video signal got inverted too, suggesting the location of the fault is in the video-amp and CRT cathode circuitry and the cathode voltage dropped too low..

The power suppply to the video output stage and to the CRT's cathode is zener regulated , check that first. Also check the negative voltage from the brightness control pot (clean the pot too)
 
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There are three (3) designs for the PET 2001 9" monitors.

Two of these designs generate a +85V boost supply, and the third generates a +95V boost supply.

All three designs generate a -30V supply for the brightness control.

Measure these two supply rails first.

As Hugo has stated, the boost supply (+85V or +95V) is zener regulated down to +33V fir the video circuitry to drive the CRT cathode. Measure this supply also.

If you don't want to poke around on a live monitor (and I don't) you can solder insulated wires onto the points you wish to measure (obviously with the power OFF) and bring the wires out of the monitor to a 'chocolate block' connector. Don't forget to bring out the 0V rail. This permits you to measure things in perfect safety with the power ON.

Dave
 
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daver2 said:
Welcome to VCFED (as this is your first post).

There are multiple versions of the 9" PET monitor. We need to identify which one you actually have.

Can you either post a link to the schematic you are using, or post a photograph of the component side of the monitor PCB.

Are there any numbers on the PCB, to help identify it?

One thing that is going wrong is the blanking (or lack of)...

Dave
Click to expand...
Thanks for the warm response! Here's the PCB. The only schematic I have is the one from the 4016/4032 Service Manual PDF and it's labeled 320032. It's important to note that at power-up, it does display correctly for about 5 seconds.
 

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Thanks.

It will be something that is time or temperature related.

Measuring the two (2) Voltage rails as I suggested above, plus the output from the main voltage regulator, will start to narrow the problem down.

Dave
 
The +12v rail coming out of the 7812 is only 7.5v, so I suspect either it's warming up really, really fast due to rotted out heat-sink compound, or the 7812 is bad. The -30v rail is at -26v and the +85v boost is at 67. I just gave the 7812 a shot of component chiller and it was happy for a moment. Now I have to deal with drilling out the rivet holding the 7812 to the heatsink and PCB.
 
This PET 4016 is in much better shape after replacing the 12 volt regulator. I've ordered a complete set of quality capacitors from Console5 Sales, so my next step is to replace the Amzn (Bojack, etc) caps that I had on-hand with name-brand ones and then I'll address the linearity situation. Many thanks to Dave and Hugo for their comments that helped me confirm I was looking at a correct schematic and voltage specs.
 
Bockmeister said:
The +12v rail coming out of the 7812 is only 7.5v, so I suspect either it's warming up really, really fast due to rotted out heat-sink compound, or the 7812 is bad. The -30v rail is at -26v and the +85v boost is at 67. I just gave the 7812 a shot of component chiller and it was happy for a moment. Now I have to deal with drilling out the rivet holding the 7812 to the heatsink and PCB.
Click to expand...
They did push the 7812 to close to its max thermal dissipation in this application and it relies on good thermal coupling to the heatsink, or it will shut down. When they were riveted in from new, the rivet expanded the copper tab of the regulator, distorting its flat surface. Over the years the thermal paste dried out and became powdery. The thermal resistance of the tab to the heatsink goes way too high. Because of that distortion, it pays to fit a new regulator IC and fresh thermal compound and do it up firmly with a screw/nut & spring washer. Watch out, there are a lot of 7812's out there (especially from the major suppliers) with a thin metal tab, avoid those like the plague.
 
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