[Vic Gibson]

Completely re-stuffed this 42-327 but still has a nasty feedback oscillation. Suspect first IF - off the mixer tube plate. Primary and secondary OK but there is a cap. across the primary. Riders doesnt say what the value is and the code is one I cant find any reference to. The arrows (hope you can see in the attached) point in odd directions so I dont know how to read it. Any ideas on what the value is or where I can find this code? (google was unsuccessful). Also, my first post - hope the picture link works. I used photo bucket.

[Image: http://i1275.photobucket.com/albums/y456...d249f4.jpg]

http://s1275.photobucket.com/user/vgibso...4.jpg.html

[BrendaAnnD]

Are there tube shields that should be in place, but aren't?

[Vic Gibson]

Nope. No tube shields on the chassis - apparently that model doesnt have them. Tubes are OK. Resistors in tolerance. Can see a nice 455 Khz IF signal on the mixer plate with my scope. Don't see it on the secondary - though that may be a limitation of seeing a voltage wave form on a grid...

[Mondial]

That cap appears to be the 3000 pF capacitor used to tune the primary of the transformer to resonance. If you can peak the slug on the primary then the cap is probably ok. If you still want to change it, you could try a 3000 pF silver mica.

Oscillation in the IF sounds more like a bad bypass cap in the screen or plate decoupling circuits.

[Vic Gibson]

Hey thanks for your thoughts. My "guess" was 3000pF but I do not know which dots are 1st and 2nd etc... Anyway, the slug has very little effect on the output - hardly any change over multiple turns of the slug. But, you could say there is sort of a peak - however broad. Thing is I've already changed all the caps - except the ones in the IF coils. Putting my scope probe on the plate of the 2nd 7B7 (pin 2) kills the oscillation and I actually get stations. ??!! Dont have a 3000pF cap. Wonder if a .0047 would work? I'm at my wits end with this thing. I get how radios work and the IF works, but clearly I have something more to learn! - just can't beat this oscillation thing.

[Mondial]

The 7B7's do not seem to be well bypassed. You could try connecting a .05 uF cap from the cold ends of the IF primaries ( red leads) to the cathodes and see if it makes any difference. The circuit seems to be relying on the electrolytic filter cap for a bypass, which may not be adequate with the new parts.

[Vic Gibson]

Mondial - Really good thought on the 7B7s. I did replace the .2 mF with the wrapped coil wire - using a new .2 with a somewhat smaller diameter - so it may not be working as well as it should. There is a .05 bypass on the screens (no. 28). Anyway, tomorrow I will reinstall the IF transformer - probably wont change the 3000pF. Then I will try your idea with the .05 and I'll post again. Thanks.

[John R]

Vic,
This same cap was used in many prewar Philco's, 41-290/41-295 etc., and has been the topic of discussions on this phorum before.
Typically, as in your set across the primary of the 1st. IF. It is debateable whether this is a molded mica or molded paper due to the relative high value of 3000pf. Some have been found to be paper not mica. Micamold caps do have a reputation for failure. Silver mica was generally not employed in consumer electronics till post WWII. I would disconnect one side & test for leakage & value. Personally, I would replace it with a new silver mica, which I did in my 41-295, since it is inside the IF can. That way you don't have to think about it any longer. John

[Vic Gibson]

JohnR and Mondial,
I reinstalled the IF x-former - tried a .047 bypass everywhere - no joy. Unfortunately I have no way to test capacitance. And I dont have a 3000pF around. But yeah, replacing it would eliminate it. I see the Tube-Store has them - I will order ...and wait. Hate that. Meanwhile I would also note B+ is running 104VDC (schematic says 83V) and about 2vac ripple - PS seems normal except it now has new 22 mf filters instead of 20 mf. And I have a modern meter with way more than 20,000 ohms/volt like they used back then. Dont know why B+ is high. Also got plenty of volume. Have run the IF alignment a dozen times and it seems to work alright - each stage peaks OK - except the slug on the 1st IF seems to have a very broad peak if you can call it that. When tuning now, anywhere near a station I get whistling - anywhere else on the band I get a 60hz buzz/feedback.

[Vic Gibson]

Did a site search on 3000pf and sure enough, it’s come up before in a similar circuit in a 41-296 - like JohnR suggested. Apparently W5AMI replaced it but still had a flat or flatter peak with the slug tuning. I’m still OK with replacing it and tubes-store.com has them, but it is in Russia! And thetubestore.com while in the US has only up to 1000pf – as does most everyone else. Mouser seems the only place to get them – and they want pretty high prices. Anybody have a source for 3000pf silver micas?

[John R]

Vic,
justradios.com out of Canada. $3.19 @.

[Vic Gibson]

Thanks John. Just ordered from them.

[codefox1]

Agree, seems to be as stated and veriried from schematic, and you will be fine. Just to pass along, for unknown values, sometimes the color dots just fall off or are so discolored or of a propriety nature that even a smart kid-old F**T team can't tell.

[Vic Gibson]

Hey All,
Turns out the “mystery” cap is indeed a 3000 pf. I acquired a cap meter and it measured 3003 pf. But I installed a new one anyway since I ordered one. Unfortunately it still has a bad oscillation – motorboating really.

So I thought maybe since the B+ was 20v higher than the schematic, it might be the problem. So I put 470 ohms in series with the field coil and dropped B+ to within a couple volts of the schematic reference. Did me absolutely no good.

So I thought maybe the 2 volts pp was too much ripple (really only 2%) so I added filtering to bring it down to less than 1 volt pp. Did me absolutely no good.

So I was poking filter caps everywhere in the circuit, but places it stopped the oscillation either killed the IF or the audio. Until I poked a vacant pin on the 2nd IF toob. Pin 5 had no connection, but just touching it with a screwdriver killed the oscillation. Looked in the toob manual and pin 5 on the 7B7 is labled IS – not NC. Looked up IS and found that it is an Internal Shield. So I just connected pin 5 to B- and presto – motorboating is gone. And I had already added a toob shield to that toob.

Did the alignment and it now works – on local stations. Even the shortwave works.

[John R]

Vic,
Nice troubleshooting...yes pin 5 of both 7B7's are shown tied to the cathodes of the respective tubes which are at B- per the schematic. Check your 1st. IF to verify it is also wired the same if you haven't done so...probably you have. Who would've thunk! John