Forum Rules and Etiquette

Our mission ...

This forum is part of our mission to promote the preservation of vintage computers through education and outreach. (In real life we also run events and have a museum.) We encourage you to join us, participate, share your knowledge, and enjoy.

This forum has been around in this format for over 15 years. These rules and guidelines help us maintain a healthy and active community, and we moderate the forum to keep things on track. Please familiarize yourself with these rules and guidelines.

Rule 1: Remain civil and respectful

There are several hundred people who actively participate here. People come from all different backgrounds and will have different ways of seeing things. You will not agree with everything you read here. Back-and-forth discussions are fine but do not cross the line into rude or disrespectful behavior.

Conduct yourself as you would at any other place where people come together in person to discuss their hobby. If you wouldn't say something to somebody in person, then you probably should not be writing it here.

This should be obvious but, just in case: profanity, threats, slurs against any group (sexual, racial, gender, etc.) will not be tolerated.

Rule 2: Stay close to the original topic being discussed
  • If you are starting a new thread choose a reasonable sub-forum to start your thread. (If you choose incorrectly don't worry, we can fix that.)
  • If you are responding to a thread, stay on topic - the original poster was trying to achieve something. You can always start a new thread instead of potentially "hijacking" an existing thread.

Rule 3: Contribute something meaningful

To put things in engineering terms, we value a high signal to noise ratio. Coming here should not be a waste of time.
  • This is not a chat room. If you are taking less than 30 seconds to make a post then you are probably doing something wrong. A post should be on topic, clear, and contribute something meaningful to the discussion. If people read your posts and feel that their time as been wasted, they will stop reading your posts. Worse yet, they will stop visiting and we'll lose their experience and contributions.
  • Do not bump threads.
  • Do not "necro-post" unless you are following up to a specific person on a specific thread. And even then, that person may have moved on. Just start a new thread for your related topic.
  • Use the Private Message system for posts that are targeted at a specific person.

Rule 4: "PM Sent!" messages (or, how to use the Private Message system)

This forum has a private message feature that we want people to use for messages that are not of general interest to other members.

In short, if you are going to reply to a thread and that reply is targeted to a specific individual and not of interest to anybody else (either now or in the future) then send a private message instead.

Here are some obvious examples of when you should not reply to a thread and use the PM system instead:
  • "PM Sent!": Do not tell the rest of us that you sent a PM ... the forum software will tell the other person that they have a PM waiting.
  • "How much is shipping to ....": This is a very specific and directed question that is not of interest to anybody else.

Why do we have this policy? Sending a "PM Sent!" type message basically wastes everybody else's time by making them having to scroll past a post in a thread that looks to be updated, when the update is not meaningful. And the person you are sending the PM to will be notified by the forum software that they have a message waiting for them. Look up at the top near the right edge where it says 'Notifications' ... if you have a PM waiting, it will tell you there.

Rule 5: Copyright and other legal issues

We are here to discuss vintage computing, so discussing software, books, and other intellectual property that is on-topic is fine. We don't want people using these forums to discuss or enable copyright violations or other things that are against the law; whether you agree with the law or not is irrelevant. Do not use our resources for something that is legally or morally questionable.

Our discussions here generally fall under "fair use." Telling people how to pirate a software title is an example of something that is not allowable here.

Reporting problematic posts

If you see spam, a wildly off-topic post, or something abusive or illegal please report the thread by clicking on the "Report Post" icon. (It looks like an exclamation point in a triangle and it is available under every post.) This send a notification to all of the moderators, so somebody will see it and deal with it.

If you are unsure you may consider sending a private message to a moderator instead.

New user moderation

New users are directly moderated so that we can weed spammers out early. This means that for your first 10 posts you will have some delay before they are seen. We understand this can be disruptive to the flow of conversation and we try to keep up with our new user moderation duties to avoid undue inconvenience. Please do not make duplicate posts, extra posts to bump your post count, or ask the moderators to expedite this process; 10 moderated posts will go by quickly.

New users also have a smaller personal message inbox limit and are rate limited when sending PMs to other users.

Other suggestions
  • Use Google, books, or other definitive sources. There is a lot of information out there.
  • Don't make people guess at what you are trying to say; we are not mind readers. Be clear and concise.
  • Spelling and grammar are not rated, but they do make a post easier to read.
See more
See less

Commodore PET 2001 flyback dead transformer.

  • Filter
  • Time
  • Show
Clear All
new posts

    Commodore PET 2001 flyback dead transformer.

    Dear All,
    I am not a technician and I hope that I have found the right English translation for the part that is dead in my PET 2001.
    Some photos to be sure.
    Can somebody help me to find a spare part?
    Thank you very much
    Attached Files

    Your English is perfect!

    I have not had much luck in finding one for you either. I will have another look - there are alternative part numbers that can be used. EDIT: Sorry, not getting any hits at the moment.

    What part number is written on the VDU PCB (Printed Circuit Board) that the transformer came out of? It should be something like 320032, 320033 etc. There were three (3) monitors made for this machine over time...

    Just out of interest - how have you determined that it is this component that is faulty? It may not be.

    Last edited by daver2; August 27, 2021, 12:37 AM.


      the computer is not with me, at home of a friend that helps me.
      He sent me the attached pictures, are they with the required P/N? They are different than the ones you wrote in your email...
      Attached Files


        Hello Dave, do I have any hope?


          I did have a look, but didn’t find anything.

          The CDU-B79-CMD 9 is the 2001N 321445 monitor.

          I’ll have another look shortly for you.



            In the 1970's and later, it became popular to build the EHT rectifier into the body of the line output transformer. It simplified the insulation requirements. In small monochrome TV's or VDU's there is usually just the one EHT rectifier in series with the EHT lead. More often than not, when the transformer fails, it is not the actual transformer windings shorting or going open circuit, is is the EHT rectifier shorting out. This loads the transformer's internal EHT winding during scan time with the capacitance of the CRT bulb, and the transformer appears defective. You can check this by trying to measure any resistance on the EHT winding connection and the EHT cable, it pays to use a VTVM, not a DVM for the test. Still it is always worth adding the new rectifier as a test.

            In any case, often, all that needs to be done to effect a repair, is to cut the EHT cable in the middle of its length and insert another rectifier, these are easy to get on ebay. For a 12" monochrome monitor usually a 15 to 20kV rated part is fine. It needs multiple layers of insulation over the join, usually 12 to layers of heat shrink sleeve is enough, or you can use a section of acrylic tube about 10mm or 1/2 inch internal diameter and about 3 inches long and fill it with non-acid cure silicone rubber over the added rectifier and solder joins.

            These sorts of rectifiers work:


            The best EHT rectifiers in the world are made by VMI (Voltage multipliers inc)

            Last edited by Hugo Holden; September 13, 2021, 01:27 PM.


              FWIW, the PET prototype and a few of the very early production units used flyback transformers taken from Zenith portable televisions.
              During development they used parts from a Zenith TV for the wooden prototype and later bought a bunch of these TVs retail for parts to use in the pilot production run.
              Unfortunately I don't know what specific model was used and of course the PCB layout changed, so one of those probably wouldn't fit in the footprint anyway, even if it's electrically compatible.

              Also, FWIW, I've never seen one of the flyback transformers fail. Usually it's just a bad solder connection on the board.
              I've seen tubes wear out, resistors burn out, deflection drivers and voltage regulators fail, but never seen a bad flyback.


                Originally posted by daver2 View Post
                I did have a look, but didn’t find anything.

                The CDU-B79-CMD 9 is the 2001N 321445 monitor.

                I’ll have another look shortly for you.

                Hello... a bit of luck...?


                  As Hutch mentioned its uncommon for them to go. How are you certain yours is dead. There has been a ton of bad info going around the internet and youtube videos on how flybacks fail left and right, but this is far from true. When it comes to crt troubleshooting, I never suspect the flyback.. I would suspect the crt itself first.

                  Now i only own two PET's. Both 2001-8 models but those flybacks dont look anything like the flybacks in my systems. The ones i have look very siilar to the flybacks in the ADM 3/A terminal. the ones attached in the photos look almost identical to the ones found in the Apple monitor II model A2M2010.
                  Last edited by VERAULT; December 13, 2021, 05:56 AM.


                    For what is worth, I can add that also in my limited experience of about a dozen 9" 2001/2001N monitor repairs I've never seen a bad flyback. It would be interesting to know how it was tested.
                    In other CRT monitor repairs I've found one bad flyback in a DEC VT-340 and on a couple of arcade vector monitors.
                    Commodore issued different versions of the 9" monitors (at least 4 that I have seen) and they have different flyback transformer models (probably only pinout varies on a few of them).

                    Frank IZ8DWF


                      From post #2...

                      >>> Just out of interest - how have you determined that it is this component that is faulty? It may not be.

                      The question still stands...

                      How have you (or your friend) determined that the LOPT is faulty?



                        You can do a "Ringing Test" on the Flyback Transformer and make the correct decision on it's state.

                        Here is a link to that type testing.

                        If you don't have the equipment, find a local HAM Operator's Club in your area, and ask for a Technician that can help you.
                        Lot's of older HAM Operators are still around.

                        What is a "Gimmick?"

                        If you need a short-term solution, you might want to consider using a Gimmick.

                        A short tutorial here.


                        1. Got a schematic? A "gimmick" is usually just a couple of piece of hookup wire (solid core), not connected to one another, but twisted together. They form a very small value capacitor, whose value can be varied by adjusting the number of twists. Gimmicks are/were used by hams as part of the neutralizing circuit in an RF power amplifier, usually connected between plate and grid to inject a small amount of negative feedback. Gimmicks were also used in old receiver designs where you wanted to lightly couple a local oscillator into a mixer stage.

               might be of help with a Schematic similar to this one:

                        Larry -- KA0DMJ
                        Last edited by ldkraemer; December 14, 2021, 06:15 AM.


                          Originally posted by daver2 View Post
                          From post #2...

                          >>> Just out of interest - how have you determined that it is this component that is faulty? It may not be.

                          The question still stands...

                          How have you (or your friend) determined that the LOPT is faulty?

                          Hello. I have asked and he sent me the attached two photos.
                          I completely do not know what they are and what they are used for... I hope you are much more skilled than me!

                          He wrote me that the "yellow" tool reports the FT is NOK, while the white one reports it is OK.
                          He also wrote that the yellow can reach much higher voltages while the white one can only test with some [mA].
                          My friend idea is that there is a loss of insulation internally, so not repairable.

                          Attached Files


                            The EHT on all of the three (3) monitor boards for the Commodore PET 2001 machines only (!) operates at some 10 or 11 kV - so not sure why your friend has a reading of 24.5 kV. Is that what your friend is testing it with? If so, he may have damaged them by testing them...

                            Either that, or they are breaking down because they were never designed to operate at that Voltage. Is your friend testing them (erroneously) as though they are a TV CRT perhaps?

                            If your friend is referring to internal insulation breakdown - at what voltage is it breaking down at? If the answer is 24.5 kV - then there is nothing wrong with the transformer...



                              The 24.5 kV on the box of the tool. In the photo the tool is off. I confirm that my friend told me that the tested voltage is around 10 kV.