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Commodore PET 2001 flyback dead transformer.

Flavio

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Jul 8, 2021
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49
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
 

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daver2

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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.

Dave
 
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Flavio

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Hello,
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...
Flavio
 

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daver2

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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.

Dave
 

Hugo Holden

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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:

https://www.ebay.com/itm/35361866049...Cclp%3A2334524

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


https://www.ebay.com/itm/193724945025?hash=item2d1ae81281:g:2igAAOSwl4Fflsv3
 
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Hutch

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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.
 

VERAULT

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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.
 
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iz8dwf

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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
 

daver2

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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?

Dave
 

ldkraemer

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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. https://drive.google.com/file/d/0B8j...ZvttaJ1QzuRmZQ

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?"

https://www.vcfed.org/forum/forum/ge...or-replacement
If you need a short-term solution, you might want to consider using a Gimmick.

A short tutorial here. https://www.youtube.com/watch?v=v1wWL6TGWOE

https://www.vcfed.org/forum/forum/ge...n-track/page15

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.


http://www.zimmers.net/ might be of help with a Schematic similar to this one:
http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001N/321445.gif


Larry -- KA0DMJ
 
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Flavio

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Messages
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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?

Dave

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.
 

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daver2

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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...

Dave
 

Flavio

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(Untitled)

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.
 

iz8dwf

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Sorry, but I strongly suspect an erroneous testing on that transformer. I wonder what the symptoms were when it was still installed in its circuit.
Frank
 

DrAlis

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Nov 11, 2021
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Could you give statistics on what parts were broken with your nine repairs. Minuszerodegrees has these for IBM PC repairs and I found them super useful.
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
 

iz8dwf

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Well, sorry to say this, but I believe that most of the time, any "repair statistics" on electronic device is silly. Unless of course we can identify some particularly problematic components, like for example the (in)famous RIFA X2 capacitors.
There's only one way to troubleshoot and repair electronic circuits: Understanding electronics and the circuit we are working on.
However, going by memory, two of them had a bad smoothing electrolytic after the bridge rectifier, a couple of them had some rusted (and failed) 1N914-like small diodes. One had a bad 2N2369 on the video amplifier chain that tested good with a multimeter, but was not a transistor anymore (no amplification).
One had a failed LM7812 (rare issue indeed, but it happens). Most of the others had for sure only some bad solder joints. On most of them, it was a good idea to clean the vertical size trimmer.
HTH
Frank IZ8DWF
 

DrAlis

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Thanks. And I disagree on the usefulness. Defect probabilities are essential to good care of doctors and electronics repair alike! I just had a dead 7812 and having seen your list I might have found it quicker. And you give good examples too. The Tantals, the Rifas, W1D floppy belts, etc. They make repair accessible and easier. I'll actually check some of your items in my pig pets. Merry Xmax.
 

Hugo Holden

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I mentioned in the previous post about flyback transformers, especially transistor types that are potted, they are quite different to those for tube TV's where the windings were not well insulated and unreliable. In the transistor flyback transformer case, generally the windings are very very reliable, it is the integral EHT rectifier that is the weak point with the penchant for failure. That as I noted can often be repaired simply by adding another one.

A flyback transformer is dead easy to test out of circuit, all that is required is a signal generator to apply a small diagnostic test voltage of one volt, 50kHz to the primary and then check the secondary windings are producing the correct voltage with the scope. These ratios are generally evident from those found on the schematic. If there is a winding short (rare) the input impedance will be very low, it will then significantly load the generator and the correct secondary voltages will not be present.

The integral EHT rectifier as I mentioned is better checked with a VTVM for any leakage. It can read open, but in those cases it is still usually normal as some types have a very high forward voltage drop and the voltage source in a meter is not enough to get them into conduction. It is leakage that you are on the lookout for with that test.

I have never seen such nonsense on the internet, especially on you tube about CRT VDU's and flyback transformers. It might be a generational thing, as CRT TV's and VDU's started to fade away in modern production. Younger technicians did not get the formal training on them and are poorly read up on the operating principles, which old school TV technicians once knew about, very well. So you get all sorts of false assumptions and conclusions which are very misleading for those seeking help.

Keeping failure records for certain types or brands of TV or VDU had some utility in the repair industry, as it could save some time and make for a quick repair. However it cannot be relied on and repair tech's who needed those, rather than using first principle methods, would from time to time get caught with an infrequently occurring fault and end up stumped on the repair. The better method is to approach each fault as though it is the first time you have seen it and work through it in a logical manner with the correct test instruments, typically meters and the scope for a VDU, or you might end up with your foot in a pothole.
 
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