Audio transformer sizzles & smokes.. - Printable Version
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+--- Thread: Audio transformer sizzles & smokes.. (/showthread.php?tid=26410)
Audio transformer sizzles & smokes.. - ODPILOT - 04-07-2026
My radio: Philco 53-1750
Audio transformer: 32-8242-10
Specs: Impedance - Primary: 4.1K-ohms Secondary: 3.14 Ohms
DC resistance: Primary 435 Ohms Secondary: .91 Ohm
This set has been playing nicely for years. All of a sudden, the volume became is pegged at maximum and turning down the volume control did not lower it at all. This is true for the radio and the phonograph volumes.
I unbolted and turned upside down the radio, tapped around and noticed that the green wire (see red arrows) connected to the volume control were loose as the solder connection had failed. I re-soldered the wire back on. While I was in there, I also checked all the electrolyic capacitors as well as their ESR values and they're all good. I also rechecked all the tubes in a tube tester - and they all had good readings with no shorts.
When I plugged the set back in and turned it on from the volume control, the audio output transformer immediately sizzled and smoked. Yikes !! I immediately shut it off and removed the plug.
So - my questions are:
1. This audio transformer has only 3 wires. There is a black wire at each end, with a reddish wire in the middle. On one end of the transformer, the black & red wires are right next to each other and the other black wire is further away on the other side (see blue arrows). To check the transformer impedance and resistance of the primary and secondary - which two wires am I hooking up to the multimeter to check primary and secondary resistance? Is it the the black & red next to each other for the primary and then the red and other black for the secondary?
2. I'm trying to figure out if this problem is just the audio output transformer in the process of burning out OR is it getting the wrong voltage somehow, thereby causing it to fry?
3. One side of the transformer is puffed out and one side looks normal.
4. Do I have to remove the audio transformer from the circuit to test the primary & secondary resistance?
4. Does anyone make a triad audio transformer that I can use to replace this old transformer?
Thanks.
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RE: Audio transformer sizzles & smokes.. - morzh - 04-07-2026
Schematic please. On-line Riders does not have it.
What is a triad audio transformer?
Do you mean the Triad brand, which exists and is old by Mouser, or something else?
To measure resistance (promary, the xfmr usually needs not to be removed from the circuit, but the resistance is not very important; inductance is and for this you do have to remove it.)
You need to measure inductance to see if there is a turn short, which is probably what it is.
I suggest that the transformer by now is bad.
Also, sometimes it does not go bad on its own, and a tube (or tubes if it is a pushpull) make it bad.
You need to check the output tubes also.
RE: Audio transformer sizzles & smokes.. - ODPILOT - 04-07-2026
Triad transformer is the wrong term (I've edited and removed the term). It does have only 3 wires coming out of it. See link below for 53-1750 Philco radio/phonograph:
https://philcoradio.com/phorum/attachment.php?aid=7312
RE: Audio transformer sizzles & smokes.. - morzh - 04-07-2026
Now, the three wires are there because one of the output leads is grounded and the 4th wire (to the speaker) goes via the chassis/GND.
Otherwise, again, I think your xfmr is gone.
Disconnect; if you can, measure inductance; if not, see if that bulging side smells and darkened. The resistance measurements may or may not be useful.
Check the 35L6 tube. It could've caused this.
Replace the transformer and the tube if needed; measure voltages. It might just work after that.
RE: Audio transformer sizzles & smokes.. - RodB - 04-07-2026
For this transformer I use a Bogen T725 transformer. Be sure the 35L6GT.output tube pin 8 measures 130 ohms to gnd. That's the cathode resistor and the voltage across it sets the tube bias. I just looked at the schematic and maybe the resistance chart calls for 180 ohms. Check the color code on the resistor for the correct value.
RE: Audio transformer sizzles & smokes.. - DaleHCook - 04-07-2026
morzh Wrote: Schematic please. On-line Riders does not have it.
??? We have it here in the 1953 Yearbook, pp. 101-104.
RE: Audio transformer sizzles & smokes.. - morzh - 04-07-2026
I usually go to the Nostalgia, and there they did not have it.
In any case, it is usually a good idea to give a link to the sch being discussed or to insert a PDF with it.
RE: Audio transformer sizzles & smokes.. - DaleHCook - 04-07-2026
I created the Philso Service Index so that people can find our resources.
RE: Audio transformer sizzles & smokes.. - ODPILOT - 04-15-2026
Thanks everyone for your help. I resoldered 2 cracked/loose connections to the back of the volume control. I was able to test both the DC resistance and the impedance of my audio output transformer - and it seemed like it was normal. The transformer, although puffed out on one side, didn't' smell burnt. All other caps and resistors tested normal. As far as the 35L6GT tube, it tested good in the tube tester, BUT I had an extra NOS tube and used that one instead. I powered the radio up (expecting to see fireworks !!) and PRESTO - plays like new again. There were probably a combination of factors going on here - but all is good. Thanks folks for your expertise.