Hello,

I have recently found one of these radios at a flea market. Upon inspection I have found that the power transformer is completely shorted, chard and burnt out reaking of a possible internal fire. A wax of some sort melted and oozed out of the wire hole. The wires were melted together. Can someone have a look at the schematic for this radio and recommend what type of transformer I can use in its place. Its a really neat looking radio and I hate to scrap it for parts.

Thanks

Greg

Hello Greg,
I not sure what to tell about a replacement transformer. I di find this gentleman http://www.sparkbench.com/d724album.html and he has a Truetone D-724 that he restored and he mentions the bad transformer. He can help you I am sure.
rick

A transformer of the appropriate physical size with a 300 to 325 volt center tapped high voltage winding should work well. You might also consider a 350 volt transformer, though that might be overkill, depending on the radio.

The transformer should also have a 6.3 volt winding and a 5.0 volt winding. Take the current rating for each 6.3 volt tube and add it to the next to obtain the total current draw. Include any pilot lamps. You will want a 6.3 volt winding that gives a little more than what's required.

The rectifier probably draws about 2.0 amperes. You want a 2 or 3 ampere 5.0 volt winding for the rectifier filament.

Take the total amperage drawn by the 6.3 volt tubes and multiply it by 6.3. This is the wattage drawn by the 6.3 volt tubes (and pilot lamp(s)). Take the total current drawn by the rectifier filament and multiply it by 5.0. This is the wattage drawn by the rectifier filament. Take the wattage drawn by the rectifier filament and add it to the wattage drawn by the other tubes' filaments. Subtract this wattage from the total wattage drawn by the radio. The remaining wattage is that required by the high voltage winding, minus inefficiencies of the transformer and wiring. Take this wattage and divide it by 325 (let's assume that the original transformer had a 325 volt winding...this is between 300 and 350, so it's a good estimate). The resulting number is the amount of amperes your replacement transformer's high voltage winding must be able to supply. Say the remaining wattage you came up with after doing the above math was 50. That means that the radio's B circuit was drawing 50 watts from the high voltage winding (which, hopefully, it was designed to handle, though by the looks of your transformer, it might not have been). Take 50 and divide it by 325. This will give you .154. That means that your new high voltage winding must be capable of supplying 154 mA of current. The number you come up with probably won't be that high, but it may be.

Thomas

When I say "depending on the radio," I mean to say, "whether or not it is a consol." Using 350 volts and push-pull to drive a little 5 or 6 inch speaker is overkill. The speaker can't handle that much. On the other hand, with a nice big 10 inch speaker, you want good power and volume. Depending on the radio, it might have even been originally equipped with a 250 volt transformer. That is rather inadequate with type 42 tubes, though (you might get away with it if you use 6V6 tubes). 300 to 325 will give you nice rich volume.

T.

Hi

Don't think the radio had very high B+ voltage? The second electrolytic filter cap is only rated 225 volts. When replacing it use 450 volts.

http://www.nostalgiaair.org/PagesByModel/892/M0023892.pdf

Although best to have a transformer with higher ratings 6.3 volts winding can be 3 amps and 5 volt @ 2 amps. Whatever you buy it has to fit on the chassis. Higher rated transformers will be larger.

Norm

:When I say "depending on the radio," I mean to say, "whether or not it is a consol." Using 350 volts and push-pull to drive a little 5 or 6 inch speaker is overkill. The speaker can't handle that much. On the other hand, with a nice big 10 inch speaker, you want good power and volume. Depending on the radio, it might have even been originally equipped with a 250 volt transformer. That is rather inadequate with type 42 tubes, though (you might get away with it if you use 6V6 tubes). 300 to 325 will give you nice rich volume.
:
:T.

I guess the electrolytic's rated voltage is a good clue. There have been many circuits of this type which have a 300 or 325 volt h-v winding in the transformer, but there are also those with a lower voltage. Take, for instance, my Airline radio with its teledial. Loaded B+ is only 150 volts. They could have made the radio without a transformer, and obtained the same results. Still it performs fairly well at those voltages.

T.