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PET with flashing/warping screen

T

tavus

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Aug 3, 2025
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Howdy folks- new to vintage computing and brand new to PETs. Found this one at a garage sale and have been super excited to get it working. However, seem to be having issues with the monitor. When powered on, it seems to 'flash' or warp. There is no potential knob adjustment for horizontal hold (or any knob, other than for brightness), which was my first thought. I do see that the screen shows that the PET has booted up to CLI though- which is great. Any thoughts on what might be occurring?

For reference on tools and skillset, I have an oscilloscope and multimeter available, but mostly have worked on simpler/hobbyist electronics. Excited to learn though.

Here's the video :
View attachment IMG_5929 2.mov
 
Looks like there is serious hum in the monitor's power supply. You should be getting a clean 12V DC regulated on the output of the 7812 regulator. First port of call would be to check C1 (4,700 uF) inside the monitor.
 
There are no H HOLD or V HOLD controls on the monitor as the design does not contain H or V oscillators.

The main logic board generates H DRIVE and V DRIVE signals that 'kick' the monitor circuits. As a result, the interface between the monitor and logic board is much simpler, as is the monitor circuitry. The down side is that a fault on the logic board can generate the wrong DRIVE signal(s) to the monitor and destroy it...

First, disconnect the monitor from the logic board. The connector should be identified as J7 on the logic board. This will prevent damage to the monitor.

Use your oscilloscope to check J7 pin 3 (this is the V DRIVE signal. It is a TTL level (5 Volts) and should be approximately 50 or 60 Hz (depending upon where you live)).

Also check J7 pin 5 (this is the H DRIVE signal. It is a TTL level (5 Volts) and should be approximately 15 kHz).

If these signals are correct and stable, the fault is definitely in the monitor circuitry.

With J7 still disconnected, proceed as per post #2 and check the 12V DC supply of the monitor.

Note that there are at least three (3) different variants of the monitor. Check which one you have...

Dave
 
daver2 said:
There are no H HOLD or V HOLD controls on the monitor as the design does not contain H or V oscillators.

The main logic board generates H DRIVE and V DRIVE signals that 'kick' the monitor circuits. As a result, the interface between the monitor and logic board is much simpler, as is the monitor circuitry. The down side is that a fault on the logic board can generate the wrong DRIVE signal(s) to the monitor and destroy it...

First, disconnect the monitor from the logic board. The connector should be identified as J7 on the logic board. This will prevent damage to the monitor.

Use your oscilloscope to check J7 pin 3 (this is the V DRIVE signal. It is a TTL level (5 Volts) and should be approximately 50 or 60 Hz (depending upon where you live)).

Also check J7 pin 5 (this is the H DRIVE signal. It is a TTL level (5 Volts) and should be approximately 15 kHz).

If these signals are correct and stable, the fault is definitely in the monitor circuitry.

With J7 still disconnected, proceed as per post #2 and check the 12V DC supply of the monitor.

Note that there are at least three (3) different variants of the monitor. Check which one you have...

Dave
Click to expand...
Thanks Dave and A4000Bear, will go through these steps and report back! Excited to be a part of VCFed
 
Glad to have you...

If you have a message from BASIC - you are already 90% of the way there! It is amazing how many of these machines have logic faults...

Note that there are three (3) flavours of this machine (assuming no bright spark has updated it inside). Check which flavour you have.

Some machines have unobtanium 6550 RAMs and 6540 ROMs...

Dave
 
A4000Bear said:
Looks like there is serious hum in the monitor's power supply. You should be getting a clean 12V DC regulated on the output of the 7812 regulator. First port of call would be to check C1 (4,700 uF) inside the monitor.
Click to expand...
I agree.
In particular if the PET runs at 60Hz and the mains frequency is 50Hz, or the other way around, you'd get a picture that flickers about 10 times per second (the difference between 50 and 60).

if you are lucky you might simply have a bad smoothing capacitor in the monitors power supply.

Good to know: The monitor is fed directly from it's own winding on the transformer. Thus any power related problem will be within the monitor.

Also if you don't feel up to fixing the monitor (for now), you could disconnect the power from the transformer, and use the video output signals on the user port with an external monitor. A bit of a weird use case, but still.
 
daver2 said:
There are no H HOLD or V HOLD controls on the monitor as the design does not contain H or V oscillators.

The main logic board generates H DRIVE and V DRIVE signals that 'kick' the monitor circuits. As a result, the interface between the monitor and logic board is much simpler, as is the monitor circuitry. The down side is that a fault on the logic board can generate the wrong DRIVE signal(s) to the monitor and destroy it...

First, disconnect the monitor from the logic board. The connector should be identified as J7 on the logic board. This will prevent damage to the monitor.

Use your oscilloscope to check J7 pin 3 (this is the V DRIVE signal. It is a TTL level (5 Volts) and should be approximately 50 or 60 Hz (depending upon where you live)).

Also check J7 pin 5 (this is the H DRIVE signal. It is a TTL level (5 Volts) and should be approximately 15 kHz).

If these signals are correct and stable, the fault is definitely in the monitor circuitry.

With J7 still disconnected, proceed as per post #2 and check the 12V DC supply of the monitor.

Note that there are at least three (3) different variants of the monitor. Check which one you have...

Dave
Click to expand...
Getting 60Hz on pin 3 and 15.6kHz on pin 5, will move on testing the power supply!
 
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