What happened is, my Dad got this radio from an auction somewhere. He said it worked for about 15 seconds, then faded out. He forgot to unplug the radio when he went to bed, and the transformer got so hot that the paint burned off the cover and the potting compound all ran out. I haven't checked the field coil but I'm worried it might be burned as well, depending on where the short was in the B+ circuit.
The problem is, I don't know the voltages for the different windings on the original transformer and have no way to find out.
Sorry for all the posts on this radio, but it's hard knowing where to start when you know nothing about it. I think it will be an easy fix once I have a transformer.
They biased the filament positive on the AVC UY227 tube. This was probably done to keep hum from being emitted from the filament. Don't see this as a problem with more modern 27 tubes. Use the same winding for all 2.5 volt filament tubes if you don't have the second one. You may eliminate R16, 17 and 18 if only one winding is used.
All your tubes use 2.5 volt filaments except 80 which is 5 volts.
Norm
:Ok, I have the schematics now, and I know where to get a transformer, but I need to know what the deal is with the AVC having it's own filament circuit. Can I substitute a universal transformer?
:
:The problem is, I don't know the voltages for the different windings on the original transformer and have no way to find out.
:
:Sorry for all the posts on this radio, but it's hard knowing where to start when you know nothing about it. I think it will be an easy fix once I have a transformer.
I have to say I have a lot to learn with this type of stuff. I have years of experience and education with solid-state stuff, but I feel dumb now :-)
Looking at the schematic, It looks like there are 4 windings- a 5v winding for the rectifier heater, a center-tapped high voltage winding, a 2.5v winding for the AVC heater, and another 2.5v winding for the other heaters that also has a center tap connected to ground.
This is different than the Philco radio I'm working on in that the B+ line does not seem to be connected on the other side of the field coil. It looks like the B+ line it is the same line that one side of the rectifier heater is connected to. The center tap of the high voltage winding is connected to ground through the field coil, and is also the line that is used to positive bias the AVC heater via the network of R16, 17, 18. C30 appears to be the B+ filter cap. Am I going the right direction here? So at the least I need a transformer that is horizontal mount, with 5v, 2.5v, and approximately 300vCT (150-0-150?) windings? Not sure what to do about the center tap for the main heater winding if the replacement transformer doesn't have it.
I can't quite visualize how the rectifier circuit works. Do the two outer leads of the high voltage winding connect to the two plates of the rectifier tube, which induces the full wave DC onto the heater/cathode? I know a bit about how tubes work, but the philco with its half-wave rectifier was easier to understand. Mainly what confuses me is all the center taps, especially the one going to the field coil. In solid-state electronics, the center tap is usually connected to ground when a full wave rectifier is used. I think I need to go to tube school, I was born in 1980 :-)
The high voltage winding should be more like 600 volts CT, 300-0-300. Exact voltage won't be critical but 300 CT is too low.
If your filament winding doesn't have a center tap one can be made. Connect a 10 ohm resistor from each filament side to chassis. A lot of radios do it this way.
C30 is the first filter cap. C15 is another one. In this radio the field is in negative side of the supply. This makes negative bias voltage for output tube and AVC circuit.
I worked with solid state for 30 years, at Singer Business Machines, Atari and as QA Manager, National Semiconductor.
Norm
::Hi Aaron
::
:: They biased the filament positive on the AVC UY227 tube. This was probably done to keep hum from being emitted from the filament. Don't see this as a problem with more modern 27 tubes. Use the same winding for all 2.5 volt filament tubes if you don't have the second one. You may eliminate R16, 17 and 18 if only one winding is used.
::
:: All your tubes use 2.5 volt filaments except 80 which is 5 volts.
::
::Norm
:
:I have to say I have a lot to learn with this type of stuff. I have years of experience and education with solid-state stuff, but I feel dumb now :-)
:
:Looking at the schematic, It looks like there are 4 windings- a 5v winding for the rectifier heater, a center-tapped high voltage winding, a 2.5v winding for the AVC heater, and another 2.5v winding for the other heaters that also has a center tap connected to ground.
:
:This is different than the Philco radio I'm working on in that the B+ line does not seem to be connected on the other side of the field coil. It looks like the B+ line it is the same line that one side of the rectifier heater is connected to. The center tap of the high voltage winding is connected to ground through the field coil, and is also the line that is used to positive bias the AVC heater via the network of R16, 17, 18. C30 appears to be the B+ filter cap. Am I going the right direction here? So at the least I need a transformer that is horizontal mount, with 5v, 2.5v, and approximately 300vCT (150-0-150?) windings? Not sure what to do about the center tap for the main heater winding if the replacement transformer doesn't have it.
:
:I can't quite visualize how the rectifier circuit works. Do the two outer leads of the high voltage winding connect to the two plates of the rectifier tube, which induces the full wave DC onto the heater/cathode? I know a bit about how tubes work, but the philco with its half-wave rectifier was easier to understand. Mainly what confuses me is all the center taps, especially the one going to the field coil. In solid-state electronics, the center tap is usually connected to ground when a full wave rectifier is used. I think I need to go to tube school, I was born in 1980 :-)
Try PTOP. Here is their transformer list.
http://www.oldradioparts.com/2a23efl.txt
Email Gary if you aren't sure which one can be used. Other places like AES, www.tubesandmore.com also have power transformers but they will be modern and not exact replacements.
Norm
:Looking for a transformer for an RCA R-8 tombstone radio. I can barely read the PDF scan schematic, but I think the stock number says 7344. I can check a couple sources I know of, but I could use advice if anyone has some.
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:What happened is, my Dad got this radio from an auction somewhere. He said it worked for about 15 seconds, then faded out. He forgot to unplug the radio when he went to bed, and the transformer got so hot that the paint burned off the cover and the potting compound all ran out. I haven't checked the field coil but I'm worried it might be burned as well, depending on where the short was in the B+ circuit.
A bit pricey though, not sure if the radio is worth it since the finish appears to not be original, and it also needs at least one tube (G1 cap missing) and a knob. Should I keep watching for a used transformer instead?