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Power Transformer Temperature Question - Raleigh - 06-18-2007

I'm working on a 41-280. The power transformer has a little bit of what looks like wax at the edge of the hole the wires go in. I'm pretty sure at least one of the electrolytic caps was replaced at some time in the past, maybe all three. My question is about whether the wax indicates a past overheating problem that was corrected by replacing a 'lytic or whether the problem is something I need to worry about now. I replaced all the 'lytics last week, by the way.

After the radio has been on a while, maybe an hour, the transformer is warmer to the touch than I would have expected, not hot enough to burn, and not nearly as hot as the 84 and 41 tubes, but surprisingly warm.

How hot should these transformers get when they are operating normally? The metal cage around the transformer was removed and lost by someone before I got it.

- Terry Burns - 06-19-2007

Hi John,

Some transformers can get pretty warm. The line voltage is higher today than 65 years ago. I have a few that I can only keep my hands on for a count of 5. I see you replaced the electrolytic caps but have you recapped the whole radio? Positive voltage on any of the control grids of the tube will make the radio draw too much current. Usually they are a couple of volts negative. Check your 41output tubes for negative voltage. Pin 4(control grid) with respect to pin 5 (cathode) which in your radio is grounded. You should have around -15 volts or so. The wax probably was from the transformer getting hot. Check your windings for resistance and voltage also. The high voltage windings from center tap should be within 5 to 20 ohms each other.

Hope this helps
Terry

- Raleigh - 06-19-2007

Terry,

Thanks for your tips. I'll take some measurements later in the week, if not tonight.

I have replaced all the tubular paper caps. I'm in the process of ordering replacements for the molded paper caps in the picofarad range. (Assuming I know how to tell the difference between the mica caps and molded paper ones--I'm not too sure.)

I replaced 6 or 7 resistors that were more than 10% out of spec. Two of the ones I replaced were the 1.0 Meg resistors in the bias circuit of the 41 tubes (#53 and #54.) One of those was reading 2.5 Megaohm and the other about 1.25. I think that would raise the grid bias voltage, wouldn't it?

I am also slowly replacing all of the rubber insulated wire. I've done the easy ones now, and am working on the nasty ones in the tight quarters near the band selector switch. I haven't tried to get into the interstage transformer cans yet.

Sometime in the past, someone replaced the 16 microfarad cap (#27A) with a 20 microfarad. I kept the 20 mf value when I replaced the old 'lytics because I read somewhere that going up a little in value was better.

- Raleigh - 06-19-2007

Terry,

Here are the results of the measurements you suggested, and maybe a couple more. The negative grid bias of the 41 tubes seems a little less than you suggested.

Grid bias of 41 tubes: -10.2 and -10.4 V DC

HV secondary winding resistance to center tap: 133.8 and 144.3 ohms (Schematic says 150 and 160.)

Primary current: 1.5 mA AC at 120.3 V AC (about 3 mA AC during warm-up)

Rectifier cathode to center tap: 256 V DC (Schematic says 265)

Secondary voltage across rectifier plates: 503 V DC. Each plate to center tap is almost exactly half the total.

Plate to chassis ground voltage of 41 tubes: 180.3 V DC (the schematic says 175 V DC at that point. I assume that is measured to chassis.)

It doesn't look like a existing overcurrent problem now, unless you think the grid bias isn't negative enough. The wax must have come from an old problem. I guess the temperature of the transformer is normal. What do you think?

- Terry Burns - 06-20-2007

Hi John,
Things look pretty good to me. Sounds like you have got it pretty well under control. If you are still concerned about your transformer heating up you can make a voltage bucking circuit using a 12 volt filament transformer. Do a search and you will find some info. If you have a variac try lower the line voltage to 110v and check your transformer. I ran a check on one of my transformers with a temperature probe and after 35 min it reached 130 degrees and stayed there for 90 min. That was with a line voltage of 125V. Not one I would run all day.

Terry