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PET 2001 screen jitters

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    PET 2001 screen jitters

    My trusty old Commodore PET 2001 works pretty well, but the screen sometimes jitters. (Eventually it calms down.) Any suggestions?

    Any chance of posting a video somewhere to show us what you mean?



      This video clip on YouTube is only ten seconds, but it'll show you the problem.


        What's the part number of the monitor's PCB?
        Assuming it's not newer than 320033, check CR8, CR12, C9 and R20 (clean R20 first). In general, inspect the video board solder joints.
        Frank IZ8DWF





            The part number of the PCB is 320008. I couldn't find anything marked CR8, CR12, C9, or R20. Could you refer me to a diagram or photo? The inside was rather dusty since it probably hasn't been cleaned since I purchased it in 1978, so I blew on everything with a can of compressed air. That seems to have helped.

            I also noticed that the plug from the motherboard to the monitor was not seated firmly, so I pushed it down.

            Last edited by morykwas; May 4, 2021, 04:05 PM.


              The PCB (and components) referred to are on the MONITOR PCB not the main logic board PCB.

              Look at the bottom of the page at for files called video-n. There are multiple versions of the monitor used for the PET 2001.

              The problematic area is likely to be the vertical drive circuitry.

              Beware though, there are very high voltages in here - both when switched on and for a period of time afterwards (as the capacitors and tube hold their charge).



                Thanks for the information -- and the warning! I'm afraid this might be beyond my ability level.

                Back in the olden days, there used to be TV repair shops. Is there anything like that now? I don't want to spend a lot of money, but it would be nice to get this fixed.


                  You underestimate yourself...

                  With the power OFF - and the PET disconnected from the mains for a day or so...

                  First of all - identify what PCB you have in the monitor and report back.

                  You should be also able to identify the preset resistor R20 (the vertical size control). Note down where it is currently set to and use a screwdriver to adjust first fully anti-clockwise and then fully clockwise and then set it back to roughly where it was originally.

                  Box the PET/monitor back up and retest.

                  See what happens.



                    Okay, Dave, it's noon on Wednesday, I've unplugged the power, and I'll wait until Thursday afternoon to try it. I will let you know how it goes...


                      Have a beer, or three, while you wait !



                        Looking at... ... if you look at the vertical circuit (you have a vertical shimmy) you will see that the vertical is very dependent on the 12V rail; so if you have wobble on the 12V rail then you might get wobble. You could look at the 12V and cast an eye at C3 (and to a lesser extent C1). Within the vertical circuit itself you have C13 and then after that it gets a bit more complicated.

                        As far as electrocuting yourself is concerned the voltage to avoid is the 10kV and a glass tube is a great insulator and can hold charge for a long time (days). You can either try and avoid getting near the anode cap (and the flyback aka LOPT) or discharge it... a la...

                        A wire attached to an INSULATED screwdriver (which you poke under the anode cap) with the other end of the wire at chassis ground should dissipate any residual charge in the tube.


                          Whoever initially suggested discharging CRT's for typical VDU maintenance, unfortunately got it wrong. Ignore the youtube videos suggesting this should be routine.

                          The total amount of charge stored in the CRT bulb is not enough to harm you, it gives less of a zap than a typical farmer's electric fence. The only risks are if you take off the anode cap (EHT cap), and get a zap, you could withdraw your hand rapidly and cut it on a sharp object. Or you could, if carrying a non discharged CRT across the room, get a zap and drop it on the floor.

                          In general VDU servicing there is no need at all to go under the insulated anode cap, unless you want to test the EHT with a probe, which you slip under there with the set running and generating EHT, or , with the set off, you need to remove the cap to replace the LOPT or the CRT.

                          Unless the insulation of the cap and EHT wire is grossly degraded, you won't get a shock from it, so simply leave it alone. You are more likely to get a zap going under the cap to attempt a discharge when your earth clip falls off.

                          Also, when you do have to remove a CRT and/or Lopt, and want to discharge the CRT first, do via a resistor to a solid ground point, a 100k to 1M 2W type is fine. This limits the peak discharge current (that you get with a direct short) which can actually be in the hundreds of Amps vicinity, otherwise in some cases can cause damage to the internal Aquadag connection to the metal anode connection on the inside of the CRT bulb. An even better method that I use is simply to slip the EHT probe tip under the cap with the set turned off and the resistor in the probe, typically 100M Ohm, gently discharges the CRT.

                          The other option, prior to removing the anode cap, if you are shy about the discharge, just wait a few days and the charge mostly dissipates, this is because the semiconductor EHT rectifiers do not have zero reverse leakage. This was not the case with tube EHT rectifiers though.
                          Last edited by Hugo Holden; May 5, 2021, 08:00 PM.


                            Okay, I held my breath and opened the back of the PET monitor -- so far, so good -- but I didn't find any markings that would identify R20, CR8, CR12, C1, C3, C9, C13, or anything else. Nor could I find the model number of the PCB. Am I looking in the right place?




                              Can you take lots of photos... I have never actually looked closely at my monitor yet and this is interesting to me. Thanks!

                              The higher res the better so we can zoom