I have no Bass amplification on the 37-690 I'm trying to finish up.
All caps and resistors have been replaced
Known good 6K7G and 6J5G in place.
All the wiring around the 2 Bass Amp tubes have been checked and cross checked against the schematic.
That leaves signal tracing to find where the audio is failing. This thing should act like a basic amplifier circuit I suppose.
I injected AF onto the wiper of the bass control (120). That worked OK.
I then injected the signal onto the grid cap of the 1st Bass amp 6K7G. A decent pop, but no audio. That's where I lose the possible signal injection path.
Any tips on where to apply the audio signal to troubleshoot the Bass Amp section (lower right corner of the schematic)? Everything else in the radio works now.
Oh, I'm way beyond the basics here, I'm 13 weeks into this thing. I imagine you are referring to the band switch. Yes all is correct with it, as bass amplification only works on the AM band.
One thing to keep in mind is where the speaker is located while you are testing the radio. I recently worked on a 38-690 where the bass control did not have much effect while the speaker is on the bench. When I put it back in the cabinet, the bass would rattle the windows. Just something to think about.
Tony
“People may not remember how fast you did a job, but they will remember how well you did it”
Have been able to verify that there is no amplification occurring at the 1st Bass amp stage (6K7G). The 2nd stage (6J5G) is fine, it is just not getting anything from the 1st stage.
The voltages are fine, the grid is -2.88v and the screen grid is -2.5v, the suppressor grid +60v and the Plate is 235v. Injecting a 200Hz AF signal at the Plate of the tube, the whole bass amplification comes alive, with bass volume control working and everything. If I inject the same 200Hz signal on the other side of the choke I loose the amplification, but still can (barley) hear the AF tone and it is no longer measurable at the Driver tube.
[Backstory: this beast was recapped decades ago, but the dude screwed up and restuffed every cap block (36) instead of doing them progressively, checking ops as he proceeded. With all the problems I found, I can guarantee this radio never worked after he touched it]
I'm staring at this Philco capacitor block and all of a sudden it occurred to me, "schematic and parts list show there should be two 0.03uf caps in it...and I don't see a lead poking out and connected to the center contacts. Could it be that simple?"
Sure enough, removed the block and only one Brown Drop in there! It was the #143 from the 1st Audio output that he never installed.
Reinstalled everything with the new cap. Man, this baby is LOUD and FULL sounding. Only issue remaining is that it is amplifying the whole whole freq range. At least it has the lower end now that got filtered out in the earlier stages. Now to figure out what is wrong with the filter choke #104.
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