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 Post subject: Oscilloscope telequipment D66 short circuit winding
PostPosted: Apr Mon 20, 2020 2:41 am 
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Joined: Apr Thu 16, 2020 5:32 pm
Posts: 14
Hello,
I have an oscilloscope telequimment D66 its problem is the transformer, short circuit in the secondary winding as I can repair it thank you.


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 Post subject: Re: Oscilloscope telequipment D66 short circuit winding
PostPosted: Apr Mon 20, 2020 3:11 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 16624
Location: Albuquerque, NM 87123
I would like to see how you repair a shorted secondary.


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 Post subject: Re: Oscilloscope telequipment D66 short circuit winding
PostPosted: Apr Mon 20, 2020 6:05 am 
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Joined: Jan Tue 10, 2012 8:39 am
Posts: 1232
A shorted winding is rare. How do you know?

When a transformer fails, sometimes it can be repaired depending on where the problem is. Often it's at the terminals, which are accessible. However if it's buried inside layers of copper it's a different story.


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 Post subject: Re: Oscilloscope telequipment D66 short circuit winding
PostPosted: Apr Mon 20, 2020 5:36 pm 
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Joined: Nov Wed 30, 2016 7:35 pm
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Location: Sunbury, Ohio 43074
I tend to concur. Shorted windings are extremely rare, and hard to detect. Is the transformer overheating?

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Preserving the hist. of electronics, one boat anchor at a time! :)
https://www.bbtvtestequipment.com


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 Post subject: Re: Oscilloscope telequipment D66 short circuit winding
PostPosted: Apr Mon 20, 2020 7:09 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 26020
Location: Detroit, MI USA
Actually, that's a very well known failure on some solid state scopes. One of the secondary windings, typically the bias winding or the CRT filament winding shorts to the core which is of course grounded. When that happens you either can't control the brightness if it's the CRT bias winding, or there is no trace if it's the CRT filament winding.

I've seen this happen on Leader, Lectrotech, and Telequipment models from the 1970's so it probably happens on others as well. Sometimes depending on which winding is shorted, you can completely disconnect the wires to it and install a separate small transformer to take it's place.

Note that since the cathode in scopes runs at a relatively high negative voltage, your replacement transformer has to be rated with insulation to handle that voltage and then some if you don't want it to fail as well.

If any other winding is shorted to the core, or if the turns within any winding are shorted, obviously a replacement power transformer will be needed.

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Dennis

Experience is what you gain when the results aren't what you were expecting.


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 Post subject: Re: Oscilloscope telequipment D66 short circuit winding
PostPosted: Apr Mon 20, 2020 8:26 pm 
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Joined: Apr Thu 16, 2020 5:32 pm
Posts: 14
Hello
there is a temperature rise at the transformer and voltage drop at the 450 v voltage which short circuit I think no solution. the only solution is to change the transformer and I didn't find it.


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 Post subject: Re: Oscilloscope telequipment D66 short circuit winding
PostPosted: Apr Mon 20, 2020 8:43 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Albuquerque, NM 87123
Are you testing the transformer with no loads on it?


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 Post subject: Re: Oscilloscope telequipment D66 short circuit winding
PostPosted: Apr Mon 20, 2020 9:12 pm 
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Joined: Apr Thu 16, 2020 5:32 pm
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yes johnnysan, I tested the transformer without load it heats up


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 Post subject: Re: Oscilloscope telequipment D66 short circuit winding
PostPosted: Apr Mon 20, 2020 9:46 pm 
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Joined: Jan Tue 10, 2012 8:39 am
Posts: 1232
A short to the core may not be a problem because the laminations are insulated from one another, if it's the common EI construction. If it's a toroid, similar. So a core isn't really conductive.

But if the transformer is heating, there is no cure but to remove the short. One way is to run it until it overheats and the shorted wires melt. Once that happens, the transformer will run cool again but one output will be missing. At that point you can proceed with a small auxiliary transformer to supply the missing power.


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 Post subject: Re: Oscilloscope telequipment D66 short circuit winding
PostPosted: Apr Mon 20, 2020 10:19 pm 
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Joined: Apr Thu 16, 2020 5:32 pm
Posts: 14
hello bob91343
how I can locate the problem of the short circuit, if it was in primary or secondary,


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 Post subject: Re: Oscilloscope telequipment D66 short circuit winding
PostPosted: Apr Mon 20, 2020 10:32 pm 
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Joined: Jan Tue 10, 2012 8:39 am
Posts: 1232
I don't know but if you can run it so the shorted winding burns out, you are in business.

A very precise careful measurement of the inductance and Q of each winding could theoretically pinpoint the defective winding but that involves disconnection of all windings and an inductance bridge (unless the bridge has very small exciting signal to avoid making the rectifier diodes conduct). The winding with the lowest Q will be the shorted one. You may or may not be able to get in there to fix it, depending on its location. That's why I suggest trying to burn it out.

Some transformers are heavily potted, while others are not. If yours is the former, you can't repair it.


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 Post subject: Re: Oscilloscope telequipment D66 short circuit winding
PostPosted: Apr Mon 20, 2020 10:42 pm 
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Joined: Apr Thu 16, 2020 5:32 pm
Posts: 14
ok how i will measure the inductance and Q and what are the threshold values for deducing in winding is faulty,thanks


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 Post subject: Re: Oscilloscope telequipment D66 short circuit winding
PostPosted: Apr Tue 21, 2020 12:27 am 
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Joined: Jan Tue 10, 2012 8:39 am
Posts: 1232
Measure inductance and Q with an impedance bridge, preferably at a frequency near 60 Hz. Carefully record the Q of each winding and see if one is appreciably lower than the rest. That would be the shorted winding.

The lower the stimulus the better, as that may result in less coupling between windings. There is no specific threshold, so the comparison method would be the way to evaluate.

I don't pretend to be an expert here. I am just applying common sense to the problem in hopes that a solution will happen.

The assumption is that the mutual inductance among the various windings is low enough that one can extract the individual Qs and see a difference. One method to evaluate my idea is to intentionally short a winding of a known good transformer and see what numbers you get. Of course the shorted winding will show zero resistance and zero inductance but measuring the others may yield useful criteria, such as how close physically the windings are to one another.


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 Post subject: Re: Oscilloscope telequipment D66 short circuit winding
PostPosted: Apr Tue 21, 2020 12:56 am 
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Joined: Apr Thu 16, 2020 5:32 pm
Posts: 14
I have an LCR meter is the measurement of the inductance of the windings reveals the short circuit winding


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 Post subject: Re: Oscilloscope telequipment D66 short circuit winding
PostPosted: Apr Tue 21, 2020 1:03 am 
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Joined: Jan Tue 10, 2012 8:39 am
Posts: 1232
I don't know anything about your LCR meter so I can't comment. However, a bridge is the best way to go, I think.

How does your meter measure the unknown? Does it measure Q? At what frequency?


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 Post subject: Re: Oscilloscope telequipment D66 short circuit winding
PostPosted: Apr Wed 22, 2020 10:43 pm 
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Joined: Jan Tue 10, 2012 8:39 am
Posts: 1232
An impedance bridge is a very useful piece of equipment for these measurements. They are high end lab devices which are not cheap; however, they are available used at reasonable prices. Do not attempt these measurements with modern hand held LCR meters, as they are usually not bridges.

I have two good ones, both made by General Radio and both old. One is model 1650-A and the other is model 1658. You measure on the inductance range and see what value it yields for Q. Compare each winding; the lowest Q is probably the shorted winding.


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 Post subject: Re: Oscilloscope telequipment D66 short circuit winding
PostPosted: Apr Thu 23, 2020 1:44 pm 
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Joined: Feb Sat 11, 2012 12:44 am
Posts: 281
Location: Islip, Long Island, NY
Here is an interesting video of oscilloscope transformer rewind.

https://www.youtube.com/watch?v=ZVSbt5q0KF8&t=232s

If you've seen this before its worth another look.

Jerry
W2JI


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