[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

[klondike98]

Welcome to the Phorum!


Just a thought... you might swap out that 20uF electrolytic from the previous repair.

[Radioroslyn]

Hi Mike and welcome,
http://www.nostalgiaair.org/pagesbymodel...013339.pdf
Well let's try grounding pin 3 of the 7C6. That will tell us if the hum is in the power supply/ audio stages. If it persisted pull the 41 tubes out. If the hum is still pretty loud you've got a problem in the p/s probably filtering. You can also try pulling out each tube except for the 84 and see if the hum drops. Reinstall each tube before removing the next. That will give you a clue as to which stage is problematic.

GLOM
Terry N3GTE

[Mike Schettler]

Thanks Bob and Terry. I'll try your suggestions.

Mike

[fifties]

Are you receiving any stations, along with the hum?

[Mike Schettler]

OK, Bob and Terry. I tried both of your suggestions.
Bob, I installed a new 20 uF/350V  electrolytic in the spot where the previous tinkerer put in one-half of the dual cap. Powered it up and stepped thru AC inputs from 60V to 110V. Hum started around 80V and seemed just as loud as yesterday. So that doesn't appear to be a solution.

Next, I grounded pin 3 (grid) of the 7C6 per your suggestion Terry. The hum was still as loud as before. Removed the shorting wire.

Next I removed both 41 tubes. Ahhh, quiet throughout the land, with my input voltage variation of 60 - 110 V. Volume and bass/treble controls had no effect on the quiet. So it looks like the power supply is not the problem, correct?

Now I'lll start with the front-end 7G7 tube, and one-at-a-time, remove a tube and check for hum. Going down the line thru the 5 tubes before the dual 41's. Have I got that right? Thanks again for your suggestions.

And fifties, I have yet to hear any stations at any time.

Mike

[Radioroslyn]

>Now I'lll start with the front-end 7G7 tube, and one-at-a-time, remove a tube and check for hum. Going down the line thru the 5 tubes before the dual 41's. Have I got that right? Thanks again for your suggestions.

Yup.
Sounds like it's in the amp. By grounding pin 3 you are shorting out all of the signal from the previous stages. Could have a bad 7C6 tube, heater to cathode short or one of the 41's. If you wanted to double check the p/s you could have a look at the dc outputs (B+ and - bias) with a scope to see if there is any ripple. Us old school guys can use a pair of hi z fones and a cap to listen for it.
Terry

[Mike Schettler]

Hi Terry
I removed one tube at a time, starting at the front end and moving thru the stages. Turn off, next tube, turn on, repeat. Variac held AC input voltage at 100V. Every tube removal had the same result. Plenty of hum. I included both 41 tubes also, one at a time, and got plenty-o-hum.
OK, time to break out my Tek 475 o'scope to check the B+ and B- supplies.
New 7C6 and 7A6 should be arriving in the next couple of days.

Thanks
Mike

[fifties]

If you are not getting any broadcast stations, I would do a quick n dirty check to see if the oscillator is functioning, using the second radio test. Place a working radio next to it, tuned to about the center of the dial, 1000 Kc, turned on and volume up. Then sweep the dial of this set. You should hear an oscillation in the working radio at the IF frequency difference, either 550-560 and/or 1450-1460 Kc, if the oscillator is functioning.

[Mike Schettler]

Problem solved! There were 2 miswires in the B+ supply. The connections to the field coil were "swapped" resulting in nearly the entire secondary load being connected to the input side of the FC, instead of the output side of the FC! Partly due to 2 identical capacitors used in the secondary PS filter (C61 and C58).

Sigh, it's always where you least expect to find the problem...

Mike

[KCMike]

 always feels good to finally find the problem.

[Mike Schettler]

To be honest, I had run out of ideas. Time to ask my good friend to get another set of eyes staring at the problem. Took us an hour or so, but he found the problem. Less than an hour to verify and implement the fix.

Lesson learned. If you are at the end of your rope, phone a friend.

Mike

[klondike98]

Well done!!

[morzh]

Lesson learned. If you are at the end of your rope, phone a friend....

Mike

....or go to bed and get to it the next day.