[Mike Schettler]

It's humming, but not in a good way.

I’m restoring a Philco 40-190 obtained about a month ago. The cabinet is in fairly good condition, but that’s not today’s concern.

During restoration of the chassis, I installed replacement parts in the chassis for:  
all rubber-coated wires, all the waxies, all resistors greater than 100K ohms, as well as the 2 low-value resistors in the power transformer center-tap (negative bias) circuit, all coils checked for continuity, and IF cans internal R’s and C’s replaced.  
The tubes were checked using my PACO 920 tube tester. The 84 rectifier was a dead short, so I got a new 84 tube (not used) from FindATube.  The tube tester indicated the 7A6 tube (2nd Detector / AVC) diodes were questionable.  Also, the 7C6 (1st Audio Amp) showed a healthy triode, but its diodes were also questionable. I should mention that a knowledgeable friend and tube guy said that vacuum tube diodes often don’t perform well on some tube testers. All other tubes tested good (7G7, 7J7, 7B7, both 41’s). Power transformer windings all show appropriate resistance values.
WIth no tubes installed, I slowly powered up the radio using a Variac and my Kill-O-Watt to monitor the input V & I.  Measured the unloaded HV secondary voltage from the power transformer, and got a reading of 510Vac, measured across the anode pins on the empty 84 tube socket. Filament supply was right at 6 Vac at all tube sockets. The #44 pilot lamp was on and bright.  Transformer remained cool. I also added a fuse in the primary AC supply line, with a 1 Amp fuse.

First issue: I discovered a previous owner replaced the two power supply filter capacitors, using a dual 20 uF electrolytic capacitor. Unfortunately, the dual filter cap has a single negative lead, which is not compatible with the existing power supply circuits. (Schematic shows 2 different destinations for the 2 filter cap negative leads.)  I left one-half of the dual filter cap in the circuit and replaced the other with a 20 uF / 350V electrolytic cap, each now has the proper negative lead connection.

Yesterday I installed all the tubes and performed my initial power-on test. I ramped up the input voltage to my Variac and Kill-O-Watt to monitor the applied voltage and resulting currents. The results were not what I had hoped for:

Input Voltage,  Input Current, Observations:
50,        0.13,   Dim pilot lamp & no audio
60,        0.14,   Low level hum from speaker
70,        0.17,   Hum is louder and does not vary with volume
80,        0.23,   Hum same as at 70V
90,        0.28,   Hum same as at 70V
100,        0.32,   Hum same as at 70V
110,        0.32,   Hum same as at 70V. End of test.

Tubes appeared to all be lit, although the 7A6 is a little hard to distinguish. There did seem to be a small change in the hum while changing the tone control.

I did a search on the Phorum for causes of loud hum in a similiar Philco radio. A posting recommending checking 3 bypass capacitors, C45, C37B and C37C. I had already replaced these 3 capacitors with new ones.

Replacement 7A6 and 7C6 tubes are on order.

Any comments / suggestions on a next step to consider are much appreciated.

Thanks
Mike Schettler WA6MER