• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Heathkit W4-AM amp

A few weeks ago the power transformer on my recently acquired W4-AM amp bit the dust. It looks like finding a used replacement may be very difficult and expensive if even possible. So I am wondering about trying to extract the core and consider rewinding it. I have never tried to un-pot (is that a word?) one of these old transformers. Is heating the only way to do this? What kind of temperatures are necessary? Could I let it soak in an oven for a couple of hours to soften the potting compound (tar?). Alternatively, it there a chemical that would dissolve the potting? If anyone has experience with this I would greatly appreciate your sharing your experience and wisdom on this. Thanks.

Bill
 
My guess is put it on a hot plate on low for an hour or more till it falls out. I'd also consider using a heavy piece of scrap metal to transfer the heat. Start with the transformer upside down. In the long run it would be easier to buy a new one. The W4s & W5s power transformers burn out is quite common. Many show with replacements on them... The originals don't seem to last.
 
Yes, heat is the way to soften the filling. But it's an extremely filthy dirty job to do and something I would avoid if at all possible. Your Heath W4 is a Williamson style circuit and the 5881 outputs are run with cathode bias. This means the power transformer is nothing special and surely you can find a suitable replacement. It probably won't be a physically potted style. But a double half shell vertical mount can work and look just as good. There are several manufactures that produce these like Stancor, Thordarson, Triad, Hammon, Edcor, and others. The W4 schematic is easily found on line with the transformer ratings on the diagram. And eBay is a good place to start looking. You should also determine why the transformer failed. Bad tubes, Output tubes overheating from leaky coupling capacitors, leaky filter capacitors etc.
 
I'd rather expect the transformer repairshop guys to throw things at you if you ask them to pull a burned transformer out of tar and rewind it ;) .

Anyway, crispycircuit knows of common failures of these transformers, and HollowStates notes cathode biasing of the power tubes. This means the mains transformer continuously runs at high power, and we might assume that the original component isn't up to task. So a bigger one might be the preferable choice.

Best regards!
 
Everyone - thanks for your suggestions. Nearly a miracle - my son found a really old Admiral TV free for the taking. It turns out the power transformer is nearly a perfect replacement electrically. Plate voltages are just a smidge high, as is the 6.3 for filaments. I have it installed and it works great. I think I may still pull the the original apart as I have never seen the insides of one of these old tar potted transformers. Way back when I was a kid, I added turns to an old tube transformer to power up some more modern solid state gear I was working on at the time. If I can find a toroidal transformer that would fit inside the can, that would be ideal.
 
Everyone - thanks for your suggestions. Nearly a miracle - my son found a really old Admiral TV free for the taking. It turns out the power transformer is nearly a perfect replacement electrically. Plate voltages are just a smidge high, as is the 6.3 for filaments. I have it installed and it works great. I think I may still pull the the original apart as I have never seen the insides of one of these old tar potted transformers. Way back when I was a kid, I added turns to an old tube transformer to power up some more modern solid state gear I was working on at the time. If I can find a toroidal transformer that would fit inside the can, that would be ideal.

A CL-80 thermistor would probably bring the voltages into line.
 
NTC's thermistors are around 10 ohms when cold, but as current flowed thru it, it becomes warm and resistance drops to below 1 ohm, i use thermistors in my builds...don't be shy about using these NTC's...
what is 1 ohm compared to the dc resistance of your traffo primary? very low....
 
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