[Joe]

Hello All: Happy New Year!

Have a Philco 20 that was playing ok, but randomly if I touched a grid cap with a screwdriver. Went to play it the other day and all I can [/b]hear now is a VERY faint hum, no matter what I do with the volume. Took it out of the cabinet and checked the tubes with a TV-7D/U tube tester. They are all strong. Tapped each connection under the chassis with a thin blade screwdriver. Also tapped each grid cap with a screwdriver hoping to hear some scratching through the speaker, but nothing. Jiggled the tubes, etc. Still nothing. Any suggestions as to where to start? THANKS>>>Joe

[Elijah]

If it were me and I knew I'd replaced all filter/bypass caps and checked resistors for opens I'd start at the transformer and check voltages at the rectifier tube and across terminal 7 and 8 to see if voltage was getting to the output tube's filaments. Check the rectifier plate to center tap voltages, too. Just one place to start.

[TA Forbes]

If radio has not been re-capped, do so. Recommend you do not turn it on again until this has been accomplished.

Since the 20 is a TRF receiver there are fewer things to check. One thing it could be is your output transformer (#29) . Check it for continuity from plate to plate of your 71 tubes. Do this with the speaker plugged in to the chassis.

Next check your SP-PPG interstage transformer secondary (#22). You should have continuity from grid to grid of your 71 tubes.

Another common issue the model 20s have is the antenna/volume control circuit. The volume control (#1) has two sections, one should measure 250 ohms, the other should be 5000 ohms.

[Joe]

Well, got a few things to try. Thanks guys!

[Joe]

Hello All

Did some extensive checks & found/did the lollowing....

Replaced all resistors out of tolorence. Ordered caps so I can recap radio. While waiting for caps, checked all voltages. Found the following...

All power xformer voltages between the taps are good.
24A 1st & 2nd rf....
2.1v pins 1&5 filament plate 325vdc pin2 114vdc grid pin 3
0vdc grid cap 8.6vdc cathode pin 4

24a detector....
2.1v pins 1&5 filament plate 0vdc 6.6vdc grid pin 3
0vdc grid cap, 1.1vdc cathode pin 4

27....
2.1v pins 1&5 filament plate 150vdc pin 2 -0.6vdc grid pin 3
0vdc cathode pin 4

71s....
4.8v pins 1&4 filament plate 325vdc pin 2 -8vdc grid pin 3

80...
700vac between pins 2&3 5v pins 1&4 filament

Just a novice, but isn't the plate voltage on the detector tube 24a supposed to be around 35vdc. Can someone narrow down the problem for me? THANKS!!! Joe

[Ron Ramirez]

Here's what I would look for first:

Shorted 250 pF mica cap (16).
Shorted plate bypass cap (14), 0.25 uF.
Open resistors (17) or (18 ), 500K and 100K, respectively.

[Joe]

Thank you Ron...I'll check. Actually, have caps on order. Do you think some of the voltages that are there are a little high? Thanks again...Joe

[Ron Ramirez]

Bear in mind that unless you are using a 1000 ohm/volt voltmeter, as was used back in ye olden days, your voltage readings will be higher than that specified in the service data because today's modern multimeters are much more sensitive (reading in the megohms/volt) and therefore, do not load down the circuit under test as the old meters did that were used when the Model 20 was new.

That having been said, however, yes, the plate voltages of your RF amplifier tubes and audio output tubes are higher than I would expect.

[Joe]

Thanks for the info concerning the types of meters. Is there any way I can lower the voltages Ron? Thanks again for all your help!

[Ron Ramirez]

Fix your zero plate voltage at the detector problem first, then remeasure. Follow the tips I offered in one of my previous posts.

[Joe]

I understand what you mean Ron, that is to correct the main issues first, then check all voltages again. When the components I ordered come in, at least now I know what to do. I'll let you know how it turns out. and if there is anything else. Thanks for your time Ron, and everyone else on this forum. Joe

[Ron Ramirez]

Good deal. And good luck!

[Joe]

Hello All

Well, found the 1400 ohm portion (between lugs 1&2 )of the long power resistor to be open, as well as resistor 17. Replaced 17 and now have plate voltage on the detector. Reading through other questions/answers, I read that putting in a 10 watt 1500 ohm resistor in place of the open portion should work. Installed it and the radio now plays!! BUT, I still need to recap, and align, there is some howling etc. My immediate concern though is the 10 watt 1500 ohm resistor. I have the radio on now about 1/2 hr, and that resistor is hot! Can't keep my fingers on it. Question with that....

Should it run that hot? Can I get a heak sink for it? If I put it in permanantly, can I snug it against the chassis so the chassis acts like a heat sink? Voltage between lugs 1&2 is 55vdc. Lug 1 to ground 47VDC lug 2 to ground -7.5vdc. THANKS!!!

[codefox1]

You are doing great so far. Always good to snug power resistors onto the chassis.

Don't play the set anymore until you finish recap and while you are at it any carbon resistors out of spec (cheap and easy enough to just replace all of 'em at the same time.) Makes a huge difference when you get to the final alignment stage, and radio will probably play safely and reliably for decades maybe needing a tube or two somewhere down the line.

[Joe]

Thanks, it just made some sort of sense to use the chassis as a heat sink. BUT, do you think that 10 watts is enough to replace the 1400 ohm wire wound resistor? . It gets too hot to touch.