:Opps I hit enter to soon, This is a US Radio & television Model 41, I have replaced all the caps and off value resistors , the schematic says this has neutralizing condensers which appear to bridge across the tuning cap, but aparently they have been removed sometime in the past, today I removed the protective cover from the power transformer and along side of it mounted on the chassis is a large black rectangled box underneath it appears there were 4 wires coming from it but someone cut them off?? could this have been the neutralizing caps the schematic shows? I am not familiar with these , what are they for ? and are they needed to make the radio operate? as of now all it does is hum when you turn it on, if they are needed what value should they be ? the box has no markings of any kind on it , I removed the top of it and it is filled with I think epoxy, which I think proves this is some kind of old fashioned cap, the rest of the radio checks out fine voltages and tubes , but just a hum , this did not have electrolytics in the power supply , but regular 1uf caps which I replaced with modern replacments, as well as the other paper caps.
Can someone explain what neutralizing caps purpose was, I am at a dead end on this as it stands... thanks for any advise you may have.

Those would not be neutralizing caps in the power xformer. Those caps are in the RF section. They cancel out the inter-electrode capacitance of the tubes to allow higher gain w/o breaking into oscillation. Inserts a small amount of negative feedback.

I'm afraid you have more pressing problems. This is an old set and may have an open coil. Presence of plate voltage on your tubes will indicate continuity on the primaries, the secondaries with a ohmmeter check. Those cut wires are another concern. Good Luck!

Richard

Richard, any ideas as to what this black box would be? it sits next to the power transformer had 4 wires coming out of the bottom and they have been cut off it appears there wee two black wires and one red and one white, I do have plate voltages 100v in the RF, 175 on the audio tubes and about 68 v on the detector, could this box have been a old multi-section cap for the secondary? if so where are the neutralizing caps ? there are no caps conected any where near the RF section???
Thanks
:Those would not be neutralizing caps in the power xformer. Those caps are in the RF section. They cancel out the inter-electrode capacitance of the tubes to allow higher gain w/o breaking into oscillation. Inserts a small amount of negative feedback.
:
:I'm afraid you have more pressing problems. This is an old set and may have an open coil. Presence of plate voltage on your tubes will indicate continuity on the primaries, the secondaries with a ohmmeter check. Those cut wires are another concern. Good Luck!
:
:Richard

The box with the cut wires could very well be the original power supply caps. Can't really say for sure.

Would not dwell too long on those neutralizing caps at the moment. The radio should be doing something even if they are misaligned. They have to be located in close proximity to the RF coils. Are the RF coils shielded inside aluminum cans? Expect to see neutralizing caps inside those shields. If the neutralizing caps were indeed missing then would expect to hear wild squealing noises.

Your voltages are reasonable.

Have you checked the volume control? They tend to open up. If open, the RF cathode voltages will float to a high positive voltage shutting off the RF section.

Richard

Richard, after doing more checks I have found the audio transformer is open on the secondary leading to the 45 outputs, is it possible there could be a modern replacment for a coil this old ? Or a way to some how by-pass this audio coil ?
Thanks
:The box with the cut wires could very well be the original power supply caps. Can't really say for sure.
:
:Would not dwell too long on those neutralizing caps at the moment. The radio should be doing something even if they are misaligned. They have to be located in close proximity to the RF coils. Are the RF coils shielded inside aluminum cans? Expect to see neutralizing caps inside those shields. If the neutralizing caps were indeed missing then would expect to hear wild squealing noises.
:
:Your voltages are reasonable.
:
:Have you checked the volume control? They tend to open up. If open, the RF cathode voltages will float to a high positive voltage shutting off the RF section.
:
:Richard

Glad you find a definite problem. That is a common failure with a radio of that vintage.

No way to bypass. Will need to get a replacement audio output xformer. Radiodaze.com has a good selection. Should have a primary impedance of approx. 4000 ohms with center-tapped primary for push-pull operation. Impedance is not critical as long as you are within 1000 ohms. On secondary side if can't determine speaker's impedance, they were typically 3 to 4 ohms from that period.

Radiodaze part no. HX125J is a universal replacement that should work fine assuming it will mount OK in the radio.

Richard

Thanks Richard, I was checking out the replacments at tubes and more but I will check out the one you mentioned at radio daze , once I get it I will let you know if I get it working.
:Glad you find a definite problem. That is a common failure with a radio of that vintage.
:
:No way to bypass. Will need to get a replacement audio output xformer. Radiodaze.com has a good selection. Should have a primary impedance of approx. 4000 ohms with center-tapped primary for push-pull operation. Impedance is not critical as long as you are within 1000 ohms. On secondary side if can't determine speaker's impedance, they were typically 3 to 4 ohms from that period.
:
:Radiodaze part no. HX125J is a universal replacement that should work fine assuming it will mount OK in the radio.
:
:Richard

:Glad you find a definite problem. That is a common failure with a radio of that vintage.
:
:No way to bypass. Will need to get a replacement audio output xformer. Radiodaze.com has a good selection. Should have a primary impedance of approx. 4000 ohms with center-tapped primary for push-pull operation. Impedance is not critical as long as you are within 1000 ohms. On secondary side if can't determine speaker's impedance, they were typically 3 to 4 ohms from that period.
:
:Radiodaze part no. HX125J is a universal replacement that should work fine assuming it will mount OK in the radio.
:
:Richard

Guys,

You can get a pretty good approximation of the speaker inpedance by measuring the DC resistance of the voice coil and multiplying that by 1.25.

Radiodoc

Probably part of your hum problem is the lack of balanced push-pull. Hum can actually be caused by push-pull, but on good radios, hum can actually be neutralized with push-pull. If one of the 45s is not conducting, hum may go away somewhat once it is connected with a new transformer.

Also, you can try removing various RF and AF tubes and see if this eliminates hum. One of the stages may be malfunctioning, causing both your hum and lack of music. Don't forget to connect an antenna to a set this old. You really shouldn't operate it without both 45s working, but you could do some signal tracing on it.

Thomas