[Guest]

I have a Philco 81 that I repaired last year. I recapped it and replaced the dogbones with new resistors. I changed the 4 meg resistor on the 2nd detector grid to a 2 meg resistor. I put in all NOS tubes. I also did an alignment. I rebuilt the bakelite blocks so they are still in the chassis as original. Basically, I did a full overhaul under the chassis trying to keep it as original as possible. The radio played very nicely and all the voltages were good. But that was last year.

I didn't use the radio for several months, so when I tried to use it recently it was dead. The audio section is ok, when I touch the grid cap of the 2nd detector, I get hum. I can touch the 1st detector grid cap and nothing happens. But when I remove the grid cap from the 1st detector the set starts to make noise and pick up stations. When I replace the grid cap the set plays normally. I suspect that the oscillator is failing to start, but something about removing the grid cap gets it going. I swapped the 1st detector and 2nd detector tubes and it made no difference. I haven't pulled the chassis out, but I am ready to now.

Where would you suggest I begin to determine if my problem is in the oscillator or in the RF front end? Are there any other suggestions? If the regeneration is not set right, could it cause this problem?

I have restored several Philcos and they work nicely when you get them repaired right.
This is a nice, original condition set and I would like to have it play well. And yes, I like Philcos.

[exray]

A couple of things come to mind. There may be a high resistance break in the input rf transformer. Very difficult to pin down. A good DMM or VTVM might pick it up - a VOM might not.

Bad solder joint associated with that grid? I would reflow the solder on the tube's grid cap because thats easy to try. You might also try replacing that tube with a known good one...or even switch it with the other 77.

Do you get the same effect if you pull the second tube's cap and replace it? How about if you disconnect/reconnect the antenna while listening?

You can check for oscillator operation by using a second radio alongside the Philco. Tune it to some random frequency about the middle of the band. Say 1200. When you tune the Philco's dial to about 940 you should hear the Philco's local oscillator carrier quite strong in radio #2.

See how it passes that test and if its the oscillator at fault we'll pick away at the options.

-Bill

[Guest]

Thanks Exray. I did the test with the second radio (Philco 41-226). When the Model 81 was dead, there was no oscillator sound in the second radio. I removed the grid cap from the 1st detector and the Model 81 started to oscillate and could be heard in the second radio. So we now know that we have a problem with an oscillator that fails to start. I have tried swapping the two #77 tubes, and that didn't help.

Disconnecting and reconnecting the antenna doesn't help. Disconnecting the ground doesn't get the oscillator going. Switching to police band doesn't get the oscillator going.

Now the Model 81 is oscillating and I can't get it to quit. It will probably have to cool down for a while.

[exray]

Good on your findings. Sounds like we can zero in on the oscillator.

I would bring in the following subjects for questioning...

Part #5 - .0014 cap
Part #10 - 6k resistor

I also wouldn't rule a out a high-R open in the OSC coil p/n #12.

I'm somewhat confused about the relationship between trimmer p/n #11 and the adjustment numbers in the alignment instructions. If you have a better handle than I do on the physical layout of the set that trimmer might have some effect on the ability to oscillate.

[Steve Davis]

Dr. Volts, Considering the fact that the problem occured after a period of non use, it could be the oscillator coil has absorbed some moisture. It may have to be removed and baked in the oven for a few minutes.
Steve

[Guest]

I pulled the chassis. Nothing seems to be way off. The antenna coil primary measures 3.2 ohms to ground. That could be a little high. The secondary measures 6.6 ohms total. One section measures 2.3 ohms and the other 4.7 ohms. Resistor 10 is new and measures 6k. Capacitor 5 is new and is 1500 pf, 630wvdc +/- 20%. It is a film capacitor. Maybe I can find the old mica capacitor. The oscillator coil section that is in the cathode circuit of the 1st detector measures 3.9 ohms to ground, which is normal. All of the tuning capacitors and trimmers show open circuit at DC. The oscillator coil 12 measures ok. The voltages on the 1st detector tube are: 275 v on the plate, 99.4 v on the screen grid, 5.04 on the cathode, and 0v on the plate. This was when the tube was not oscillating. I don't think anything changed when the oscillation started.

The police band switch doesn't seem to be interfering with the ciruit.

The symptoms are the same, the oscillator won't start until I remove the 1st detector grid cap and it keeps oscillating when the cap is replaced onto the tube.

[Guest]

Hooray! I got it fixed. Exray pretty much pinpointed the problem. There was a cold solder joint on the connection between the 6 k resistor, 0.0015 capacitor, and the oscillator coil in the cathode circuit of the 1st detector. I started tapping and pushing on parts in the problem area and I noticed that when I moved the 6k resistor I could make the problem come and go. A little more pushing showed up the loose solder joint. When soldering to the old wiring it's hard to get the solder to stick to the old wiring.

The radio picks up quite well and plays loudly, even on weak stations. It has just a little audio distortion. I wonder if I have the regen control turned up too far? It might be that the distortion is coming from the adjacent station that it is picking up. My favorite station is weak and it's only 40kc away from a powerhouse station whose towers are almost visible from my house. My Philco 89B separates them nicely, but it has all the circuitry to do so, and two extra tubes.

[exray]

Hooray! I got it fixed. Exray pretty much pinpointed the problem. There was a cold solder joint =.

Lucky guess

-Bill