The PHILCO Phorum

I think I'm missing something here. I'm feeding a 455Hz signal in to troubleshoot. I get a good strong signal to the voulme control, and also on the top of C424 (.006mfd). But I get nothing on the top side of R419 right above it. Resistor ohms out just fine. Anyone lead me in the right direction? I hope this schematic is readable. Thanks.

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C423 shorted?

(09-03-2018, 04:07 PM)morzh Wrote: [ -> ]R423 shorted?

Did you mean C423?

Yes, I did.

No, C423 is O.K. I just checked it even though it's new.

I'm missing something too. If you apply a rf signal to the volume control you shouldn't hear it. The rf signal must be rectified to covert it to an audio signal. If you want the use rf @ 455kc connect it to the control grid of the 7H7 (p6 I think). Make sure your mode switch is good and clean.

What I did not appreciate was it was 455kHz. C423 simply shorts it.

(09-03-2018, 07:37 PM)Radioroslyn Wrote: [ -> ]I'm missing something too. If you apply a rf signal to the volume control you shouldn't hear it. The rf signal must be rectified to covert it to an audio signal. If you want the use rf @ 455kc connect it to the control grid of the 7H7 (p6 I think). Make sure your mode switch is good and clean.

I'm using an Eico 324 RF generator with internal modulation AF out, not a strict RF signal. I just finished recapping the chassis and am in the process of alignment. Guess I should have explained that. Sorry. I'm working backwards from the amp section to find out why I'm not getting audio.

(09-03-2018, 07:59 PM)morzh Wrote: [ -> ]What I did not appreciate was it was 455kHz. C423 simply shorts it.

It's AF modulated RF. I used to use a "Mosquito" for tracing. Remember those?

I restored a 48-482 back in 2014. The toughest part was aligning the FM converter, but it can be done using a regular modulated sig gen if you carefully follow the instructions in the Philco service paperwork, it tells this alternate method. You may find my thread on this project of use. http://www.philcoradio.com/phorum/showth...?tid=10134

Read through your thread. Might be some useful information there if I ever get through the BC part.

Patrick

Where exactly is it that you feed the IF signal to?
I am still not sure.

(09-04-2018, 07:46 AM)morzh Wrote: [ -> ]Patrick

Where exactly is it that you feed the IF signal to?
I am still not sure.

I took a step back, and used the Philco troubleshooting guide. I have a strong IF signal at the grid of the 1st IF amp. I have only a very weak signal at the plate of the converter. That would indicate a problem at the 1st IF transformer, Z400. So it appears the problem most likely is there, and I was just going off on a tangent somewhere.

Found the problem! Z404, a choke/cap combination, was wired to the white lead on Z400 instead of the green lead. The two connection points are right next to each other. That caused a very weak signal from the plate of the converter. Connected it properly, aligned the IF, and picks up stations. I really think the stock speaker is too small for this radio.