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philco 37-650 low audio issue - Printable Version

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philco 37-650 low audio issue - Kenradio - 04-30-2017

I have a Philco 37-650 chassis that I'm restoring and it is working other than the volume going down shortly after it warms up. I checked and subbed the 6K5 & 6F6 tubes and that didn't fix the problem. Two odd things I noticed is one the plate of the 6K5 goes from 200 V down to 90 V and the volume goes down as well and the tone switch has very little effect on the tone---the tone/on-off switch is good and the tone cap block #54/54A is recapped and connected properly. The bias voltage on the 6F6 tubes is -16.5 V and the plate/screen voltages are very close according to the service literature. The voltages on the voltage divider are -17.2 v & -2.7V. The RF/IF section is working great and the shadow meter is working. I checked the output transformer on the H-26 speaker and the primary is measuring 660 ohms/970 ohms compared to 330/370 ohms that is shown on the schematic. The field is measuring 465 ohms--wondering if the output transformer might be the issue or if it is something else--I have fully recapped the chassis and checked/rechecked all connections and I can't find any other problems. I've restored other 37-650 chassis before but this one is the worst one I ever worked on.   Thanks,  Ken

RE: philco 37-650 low audio issue - Jim Koehler - 04-30-2017

It sounds like that O/P transformer might've been a war-time fix. 
Does it look like it was replaced?
...Jim

RE: philco 37-650 low audio issue - morzh - 04-30-2017

Ken

Do other plate voltages go down as well? How about the centertap on the output xfmr (plus of the 57 cap)?

RE: philco 37-650 low audio issue - Kenradio - 04-30-2017

Hi Jim, the output transformer is original and riveted to the speaker. Hi Morzh, the plate voltages to the output tubes and rest of the radio are ok and stable only the plate voltage to the 6K5 drops to around 90 volts and when it does the volume gradually drops. I originally replaced the plate load resistor to the 6K5 with a 100K resistor and it is ok so far. The 6K5 is good and I subbed it out with 2 others I have in my tube stock still the same issue. I removed the 6K5 and the voltage at the plate pin on the socket went to 265 volts so there are no shorts around the 6K5 socket. I have checked every connection from the 6K5 to the 6F6 output stage and find no wiring errors. I noticed I can remove one of the 6F6 tubes and have no audio but pull the other one and still have audio but it is lower--is this normal for this oddball output circuit?  Thanks,  Ken