Testing a power transformer - Printable Version
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Testing a power transformer - georgetownjohn - 11-13-2024
Hello Forum Members,
As I prepare to start working on my 37-640, the first thing I want to do is see if the power transformer is ok. Since I am a novice to all this, can someone recommend/explain the way to do this since all I have is a Trtiplett 630 meter, signal generator and frequency counter. Thanks--John
RE: Testing a power transformer - Chas - 11-13-2024
How to dynamically test a radio power transformer:
Remove or disconnect the rectifier. Check filament and pilot circuits for short in particular pilot lamp sockets.
Power the radio, any transformer defect will be seen in moments and up to 30 minutes later. Crackling noises, heating, smoke are unrecoverable failures. For transformers with rectifier on top of the transformer any indication that the rectifier socket is failing make an inspection to be sure there is no short before testing.
If the transformer is relatively cool after 30 minutes it is fine.
GL
Chas
RE: Testing a power transformer - georgetownjohn - 11-13-2024
Thankyou for the reply but since the chassis is in such bad shape as well as the power cord, I don't dare try to turn it on especially without a variac and being a novice at this. I am hoping that I could do some type of resistance tests on the transformer without powering up the chassis.--John
RE: Testing a power transformer - Chas - 11-13-2024
Suit yourself. A failure is a failure, stepping the voltage up will not matter and neither does applying full voltage to prove robustness. So long as the rectifier is out and no shorts in filaments/pilots.
None the less,
GL
Chas
RE: Testing a power transformer - morzh - 11-13-2024
OK.
If the chassis is in bad shape:
1. Cut the power cord. Better yet, just unsolder it from where it goes to, and solder a temporary one, whatever it is. I never check for short/open, but you might: use a DMM on Ohms, put across the power plug.
Note: do not hold with both hands: you might receive a mild, but unpleasant jolt, even from DMM.
See if the number shown is reasonable: many tens to a couple hundreds of Ohms.
2. As Chas suggested, remove the rectifier. And all other tubes as well.
3. Insert your power plug to your Variac (turned OFF) output. I never suggest using "dim wit tester", it is a bad way to test things.
4. Set your Variac to Zero.
5. Insert your DMM (meter) leads into the Rectifier tube socket's pins that are the High-Volt pins (slim ones).
6. Turn on your variac, and gradually increase the voltage. Notice the DMM showing the value. If it shows nothing, see if your connection is OK. If it is, unplug everything and check both the In and Out windings for short/open. If you see the voltage rising and nothing smoking, proceed to 110VAC; your DMM voltage now should be about 600VAC. If it is: turn the Variac OFF (keep the 110V setting).
7. do the same procedure but now put the leads into the fat pins contacts. After the Variac turn-on, You should have either 5V or 6.3V, depending on what tube is used for the rectifier.
RE: Testing a power transformer - georgetownjohn - 11-13-2024
Thankyou, now I have a plan to follow--John
RE: Testing a power transformer - georgetownjohn - 11-13-2024
Since I can't afford a variac at this time, should I not try to test it? Thanks, John
RE: Testing a power transformer - morzh - 11-13-2024
Well, this is where you might use that dim bulb.
And, as for "can't afford",
https://www.ebay.com/itm/404967839817?_skw=variac+500VA&itmmeta=01JCM133AWT5T3EWBWEHKWYSKV&hash=item5e49f6d849:g:MV4AAOSwrmRjGtd5&itmprp=enc%3AAQAJAAAA8HoV3kP08IDx%2BKZ9MfhVJKmD%2BBwD%2BTbHD58W%2FswwXPPwE2ZFjaAn46jiqK3IbiX1Ja20P9tVCeA76gYncQYEc1La21ClnKdJ1tIMm0s%2BkVqXBIByrLJMuBvhw%2BvGgXa2MNiCDDchgoFMA2akuAiw3gEKfA%2BLS223v%2FsCOb4qqL8vUu4PBXyBwLxq5NRbGhDasRAd%2BbekXJwkReWsNOixCMgeC6vpu66c%2FbeFsAgvCobhiGXgOGdcCShWHynniylEHqWMUgujSpCFI%2BwyhXSxccreNlNzI4CPzaKUt2d98l3BwpmbEzfROK45ictB%2F5XUwg%3D%3D%7Ctkp%3ABFBMwrWMgeVk
A $50 one? If you cannot afford that one, it's tough....a Variac is one of the very basic necessities.