[Texasrocker]

Im "scratchin" my head on this restoration of this simple small 1949 mod bakelight radio ac/dc Zenith AM chassis.!(hee hee!) Ive carefully replaced all the caps. The sets working with a "intermittent" shorting in-out from full vol to low vol prob in the front-end. This chassis is the simple 5-tuber, 50L6,& following minature tubes lineup of the era. The set is working, audio section is very stong until the vol "intermittently" shuts down drastically. I have re-checked all tube pins, socket connections, etc very carefully. Ive subbed in a known-good set of spare(s) tubes from another good 100% working (similar) chassis. The prob sounds like a bad-resistor trying to correct itself, but all orig resistors check within fairly near specs, or, very close.The orig antenna-coil checks good this time!!!(hee hee)
Ive traced the prob back to the 2-gang mechanical tuning condensers "mica" adjustments, and/or, the possibility of a IF coil intermittent short in one or both the 455kc cans simply by "tapping" on them. Since the orig 1 meg audio taper Vol/on-off control was bad, I already replaced that also. My voltage checks under chassis are all good point-to-point. When I tap on the tuning-condenser, I get full volume then it immediately drop off again to very low volume. Im overlooking something simple here, and hope I dont need to pull the IF trans if not necessary? Your thoughts please. The orig loop antenna is good also, and the set performs very well when the front-end is "intermittenly" working properly for those few minutes after complete tubes warmup. This set has the isolated ground B- , and 5 tube filament string. Any Ideas??

[rghines1]

This may be the dreaded silver mica disease inside the IF transformers. The schematic shows they are adjusted with the ferrite slugs which are the most prone to this problem.

If so you are likely hearing some static thunderstorms. Sometimes use a simple RF grounding probe that is easy to build. Take a old test probe and wire a 0.1uf 630V poly cap in series with the probe. Connect one of the probe to -B with the other end used to ground out tube pins such as plate and grid. With patience it is possible to isolate the problem to a particular stage with the divide and conquer approach.

Richard

[AI2V]

If tapping the tuning cap does something as you note, try tuning to a station at the very highest point on the dial (minimum capacitance in the tuning cap) and see if it happens there. If the problem goes away, you probably have a shorting plate in the tuning cap.

If you problem happens with a minor rotation of the tuning cap, you may have lousy connections in the tuning cap rotor wipers.

A third thing to consider: a temperature sensitive resistor will always look just fine when measured with an ohmmeter; it's only when it's hot and current flowing that the problem will be seen. Let the radio heat up fully and when the problem begins, start spraying each resistor, starting with the largest wattage, with some freeze-spray. You can also speed up time by heating the underchassis with a heat gun- but not too much.

[Texasrocker]

Thanks for your help Pete & Richard! These type sets arent my faves to service!!! Im hoping the silver-micas in the tuning-slug type IFs are OK? I did get the set working again without the "intermittent" prob so far. I noticed while I was carefully probing around the under-chassis point-to-point connections, I got full-vol by pushing fairly hard on one of the orig solder connections at the 2nd IF primary connections. Low & behold, I added abit of new solder to orig Zenith( cold solder joint) and the set came on full vol as it should. Ive bench tested it now for approx 4 hrs without any dropouts in performance. The set now receives stations all across the dial. Since this is a customers radio, I will let it play on the workbench several more hrs to see if the micas are also bad in the IF cans before I rtn it to customer.
Another prob I found on my workbench, is my vintage AC isolation trans is "upping" my actual AC line voltage! I didnt realize that until I checked. My actual incoming AC line voltage is running around 124 volts, & my AC Isolation trans outlet was reading around 134 volts AC! If I had realized that, I would have cranked the Iso Trans AC back down with my variac!Wonder whats up with that happening with my Iso Trans also?
I copied your posts to keep in my files for future reference on these type chassis repairs!! Thanks again!! If the set fails again, I know what to check next via your suggestions! Randal