When Transformers Go Awayyyyyyyyy - Why?

[W1UJR]

The homebrew power supply on the HRO crapped out this week.
Actually the set kept playing, I was in the middle of a QSO on 160 when I smelled the burning and saw the smoke.
In fact, the HRO show no problems and was still working fine when I powered down.

This is a nice homebrew supply made back in the buzzardly times, well built.
Once on the bench I checked the components and determined the xformer was to blame.
The previous owner had removed the type 80 rectifier and installed two silicone diodes.
When I replaced the power xformer I reinstalled the 80 tube and removed the silicone stuff.

Questions:

1) Where do these typically crap out, is it a simple matter to remove the covers and repair the windings?

2) Aside from shorts, what causes xformers to fail? No faults with the RX, the replacement xformer bolted right in the set is playing fine now. The unit has been in use on and off for the better part of 6 months.

3) How do I prevent this from happening in the future? The installed line fuse, 2 amps, did not pop. Is a tube better than the silicone diodes?

Tnx - Bruce

[W1DAN]

Hi Bruce!

Sorry your transformer died. Yuk!

I have seen many HRO-60 transformers die duu to their in ability to provide enough power to the radio (HV windings too weak for the load, which is primarily the 6V6 output tubes).

Other reasons for transformers shorting are moisture in the transformer and very large voltage spikes.

To save the transformers:
On a known good radio (with no internal shorts) I install sucessively smaller fuses in a radio until the fuse blows, the go up one size. In my HRO-60's I put a 1 amp fuse (note National supplies 2 amp fuses whish wil not save the transformer!).

I sometimes also put two 1/4 amp fuses on the HV secondary output. This would blow if you have a filter cap short.

MOV's also help for inrush current limiting as noted elsewhere on this board.

If a radio is super rare, I use a Variac to ramp the voltage up.

Hope this helps!!

I also hope to chat with you on 75 after the holidays.

73
Dan
W1DAN

[WA1GFZ]

Two things cause transformers to fail
1 heat
2 moisture
(3 in the mil world vibration)

1. Keep it cool so the insulation does not over heat and fail.
         The hotter they run the sooner they die.
2 Some transformers are potted to seal out moisture Moisture reduces the insulation break down voltages.

All transformers can be repaired. I've rewound a few. George W1AJW is a pro.
yesrs ago I worked for a company that wound transformers. I was lucky to watch the production people and learn the tricks. Older insulation types are prone to die due to moisture or heat. newer materials take heat better and wire insulation has improved.
The good thing is the thikness of these materials is smaller so it is easier to copy a design.  Always use the same size or bigger wire to reduce heat.

[W1UJR]

Dan - tnx for the suggestions, this failure appears to have happened inside the xformer, so perhaps the low amp fuse on the primary would have helped.
How are things going for you? Don't hear you on much, but then again I am only just starting to get active again myself.
Did you sell off all of your HRO goodies?


Frank - heat, maybe...moisture, more probable.
The unit is kept in the barn, which until recently was not heated.
Have to think that thermal cycling during the day/night must have caused condensation to form.
Guess it bodes well for heating stored xformers up before use, or really sealing them well.
I hear W2ZM on a few nights back talking about pouring some sort of liquid laquer into old xformers, guess I'll have to talk to Bob about that one.

73 Bruce W1UJR

[WA1GFZ]

Bruce there are a number of different transformer varnishes and epoxy materials.
Tar is the old stand by.Yes trmperature cycles rub wire insulation.  Real high performance transformers are potted in a vacuum to suck the material between wires and layers.
George can tell you where to get this stuff if you want to roll your own.

[WA1HZK]

Boy do I miss those color TV transformers I used to get at the dump in the 60's!