[Tubesforme]

   


I have a problem with a model 20/21. Specifically a model 21 with a model 20 chassis. Schematic is attached. There is a very low hum the starts immediately when the receiver is turned on and it gets louder as the tubes come up to operating temperature. It is present without the receiver being tuned to a station.It isn't tunable and doesn't vary with the volume. The interstage transformer had secondary 1/2 open and was replaced with a NOS Stancor A53-C. All tubes have been substituted several times. All grounds have been checked and re soldered. All parts in the circuit except for no 16, the 250uuf capacitor have been replaced, some of them twice. Presently all parts test good and are NOS. I have rebuilt the primary filter capacitor twice, with original value capacitors and both times different brands. The speaker isn't the original and its field coil measures 3.9K ohms. The output transformer was replaced with a universal P/P type, set for type 71 output tubes. All voltages are well withing spec except for the ones shown here on the detector and the audio amplifier. This radio does play, however in a normal room the hum makes it impossible to listen to it. I was told, by one person that perhaps one of the 2.5V tubes had shorted at some point and that the hum was being caused by a shorted power transformer winding. I have tried an external 2.5V transformer and no change. I have also disconnected the pilot lamp in case there was a crack in the insulator for the bulb. That didn't help.

#24 detector plate reads 100V and should be 35V
#24 detector screen reads 9V and should be 2V
#24 detector filament is good at 2.3V
#24 detector cathode is good at 8.7V
#24 grid voltage is -.1V and should be +1V

#27 audio grid reads -7V and should be +1V
#27 plate reads 117V, which is the source voltage so it is ok
#27 cathode reads 7.5V which is ok
#27 filament reads 2.3V which is ok

All B+ voltages are well within specs. I wasn't able to ground this chassis, otherwise all voltages reads were taken from Riders page 2-3. The voltage divider resistor measures correct resistance between all sections. Point F reads +12VDC and what should be point B, off the .5meg from the 27 audio tube reads -7VDC.

The grid voltages for the PP 71's is withing spec at -52V.

Any ideas anyone? I wish I had a scope, but I don't. Tubesforme

[Ron Ramirez]

The same subject is being discussed here:
http://www.philcoradio.com/phorum/showth...?tid=13340

so I am locking this one.

Please, people, if your thread is not receiving replies, don't start a new one on the same subject.