It's a PCB, The tracks aren't great and there are a lot of wires soldered to the underside, a real mess.
I scoped the input (right before C94) and the hum is already there and even before the volume knob (visible on both channels in mono and just one channel but double the amplitude when in stereo).
I scoped the input (right before C94) and the hum is already there and even before the volume knob (visible on both channels in mono and just one channel but double the amplitude when in stereo).
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I don't have a TA button but from what I see on the schematic this would be equivalent to the PU button (activating the DIN input on the back), hum doesn't change nor in tone nor in amplitude.
There is also a FA button on the front, no idea what this is supposed to do but no matter what input I select this button has no influence whatsoever on the sound.
There is also a FA button on the front, no idea what this is supposed to do but no matter what input I select this button has no influence whatsoever on the sound.
Ok, if the hum stays if you press the TA/PU button, i.e. disconnect the radio part, the hum must be induced somewhere into the AF section. But I'm rather clueless now, as you've performed any possible tests without plausible outcome.
FA stands for ferrite antenna and indeed has nothing to do with the audio amplifier.
Best regards!
FA stands for ferrite antenna and indeed has nothing to do with the audio amplifier.
Best regards!
Unless there is something wrong with the switch itself that doesn't disconnect the fm/am part I also got to the conclusion the problem can only be in the AF section. To confirm that I disconnected the input of the volume pot from any other section and the hum was still there. If I turn the volume pot it doesn't really increase but there is some change in tone, don't know if this indicates something.
The FM reception is actually surprisingly good so even if there was some issue it would have been worth repairing.
The FM reception is actually surprisingly good so even if there was some issue it would have been worth repairing.
have you ever worked on a tube radio receiver?
if so, very well, I say nothing.
but if not, you can already start buying a whole panel of capacitor in pF and variable capacitor, always in pF
and take yoga classes.
I hated and I only kept one that I had the chance to buy in new condition years ago, a grundig rt50 with stereo decoder tubes.
And yes, I can say that it works really well.
if so, very well, I say nothing.
but if not, you can already start buying a whole panel of capacitor in pF and variable capacitor, always in pF
and take yoga classes.
I hated and I only kept one that I had the chance to buy in new condition years ago, a grundig rt50 with stereo decoder tubes.
And yes, I can say that it works really well.
I've worked on some tube gear but this is my first radio receiver.
I lifted all the coupling caps and tested them and they're all fine (even if my tester doesn't apply the same voltage to them I think I can rely on the result). I'm not looking to restore this one, just get it to work properly, even with an external source the sound is quite pleasing.
I lifted all the coupling caps and tested them and they're all fine (even if my tester doesn't apply the same voltage to them I think I can rely on the result). I'm not looking to restore this one, just get it to work properly, even with an external source the sound is quite pleasing.
ok good luck then.
if Kay Pirinha did not lift the wolf, it's unlikely I'll do it.
you will end up finding, sure, you must not lose patience and know fire a pose when it becomes too tense.
I bought a tube amp on a well known but completely modified two years ago, and I just found out why it worked badly
if Kay Pirinha did not lift the wolf, it's unlikely I'll do it.
you will end up finding, sure, you must not lose patience and know fire a pose when it becomes too tense.
I bought a tube amp on a well known but completely modified two years ago, and I just found out why it worked badly
Found the problem .
Actually it was part of the previous attempt to repair this before it ended up on a flee market. Two pads of the circuit board were accidentally swapped which ended up connecting the side of the 220ohm bias resistor that should have been at ground to the live trace of the filament supply injecting a nice 50hz sine wave in the system. This is also why I didn't see it with continuity test as that part is connected to ground through just the filament. I corrected that and now there is a very faint hum on both channels that will probably even get lower when I take the 150W dim bulb out of the equation and replace the 32µF axial cap that shows signs of fatigue.
Thank you for all your help, if I did't try what you suggested I might have ended up never finding the issue.
. Actually it was part of the previous attempt to repair this before it ended up on a flee market. Two pads of the circuit board were accidentally swapped which ended up connecting the side of the 220ohm bias resistor that should have been at ground to the live trace of the filament supply injecting a nice 50hz sine wave in the system. This is also why I didn't see it with continuity test as that part is connected to ground through just the filament. I corrected that and now there is a very faint hum on both channels that will probably even get lower when I take the 150W dim bulb out of the equation and replace the 32µF axial cap that shows signs of fatigue.
Thank you for all your help, if I did't try what you suggested I might have ended up never finding the issue.
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