[EdHolland]

I just remembered, I have some hi temp silicone rubber material which could do the trick. Or a piece of FR4 laminate. The other idea I had was to fold several layers of alumin(i)um foil and attach it with Kapton tape. That would be a safe and effective heat shield.

[Arran]

Ed;
One material that I have seen, but never tried for this was material for making exhaust gaskets, it's similar to asbestos but is not of course, some auto parts stores like NAPA sells it.
Regards
Arran

[radiorich]

hello Ed,
how about that special tape used for ducting it's like foil or how about thin piece of
aluminum roof flashing and use really good double back tape.

Sincerely Richard

[EdHolland]

Good ideas, thank you Arrange and Rich. I have the adhesive aluminum foil already and can try that immediately.

More later.

[EdHolland]

I completed the insulation on the cabinet interior (Al foil and hi temp silicone rubber), and reglued the speaker gasket.

FM front end alignment completed, chassis bottom cover back in place, tidied up the flying lead that is used to couple the line cord to the FM antenna I had everything back in the case.

Time for a test - all sounding great on FM, dial alignment is pretty good, ditto on AM. And then the trouble started.. loud crackles and crashes again on AM. Almost certainly SMD again So I shall be going in to clean the micas in the remaining AM cans. Not unexpected, I suppose but not something I really wanted to be getting into at this stage.

[Paul Philco322]

That's a bummer but get them now. Paul B

[EdHolland]

Yes - better it happened on the repair bench than returning to the owner.

The other question is - do I also dissect and clean the remaining FM IFs as well? That's would make a total of five cans to work on...

(I'll get the AM side sorted first, then decide)

[radiorich]

Hello Ed,
yes, I like letting projects run for few days to make sure they don't start acting up .
Sincerely Richard

[EdHolland]

I got into the first AM IF can today, and found its mica was cracked. Amazing that it was still working. This one makes up three capacitor sections 120 pF, 120 pF and 240 pF. The last two share a common plate.  Fortunately, I had suitable silver mica caps on the shelf. I cut some sections of insulating sheet to replace the original mica, so the contacts don't short out, and reassembled with the new capacitors concealed in the can. I find there are three or four stages of note taking beginning with the basic chassis wiring and orientation of the can, then recording to which tags the wires of each coil are attached, and lastly the arrangement of contacts to pads on the mica sheet, orientation of the mica etc., quite time consuming.

Figuring what should go where for cap substitution was tricky, and reference was made to the schematic, and the original mica's silver pads to get the configuration correct. Thankfully after getting it back together, the radio was still receiving AM, and IFT1 peaked up cleanly on the signal generator.

Of course, it still crackles and crashes because the problem is in IFT2

But at least I didn't end up in a worse situation than I started - there was some fear that I'd mix up connections or miss something, and end up with a dead radio.

I see that the last remaining AM IFT has three coils and three capacitor sections, so more delicate surgery lies ahead.  . One thing I figured out was a good way to hold the parts while resoldering wires and caps into the can.

[EdHolland]

IF2 will need replacement caps. The arcing was severe enough that the mica is pitted and holed where the HT flashed over. There is tell-tail charring on the phenolic parts, which was partly visible even before I separated the capacitor assembly.

I'll get pictures for later. Back to the bench - lunch break is over

[EdHolland]

Here's what the inside of AM IFT2 looked like, and the other pic shows the discs I made to take place of the mica and keep the contacts from short-circuiting.

   

   

I completed the recap, which called for 2x 120 pF and 180 pF and the radio is working again, but STILL crackles. I'll need to go through the remaining FM IF cans to solve this one. It will be a bit of a slog, and I'll probably need to order some more caps, as I don't have the smaller values found in the 10.7 MHz transformers. At least my OCD was right after all 

[Arran]

Ed;
One thing that I will say about these is that Brand Z was considerate enough to make these mica caps easily accessible by undoing a machine nut, unlike the Automatic style ones where you need to drill out a brass eyelet, and take most of the IF transformer apart to get at the caps. The latter was considered disposable, if something went wrong the intent was that they be replaced, which would not have been an issue up until the early 1970s maybe, but not 50 years on from that. Then there are the ones who think some components are eternal parts, like the filter caps in an RCA Radiola 17 or 18, or molded mica caps, I have encountered bad ones in both cases, most mica caps are fine, but some of the post war Aerovox mica caps are often bad, the light brown ones, and it seems to be an issue with age, not with voltage given the circuits they were in.
Regards
Arran

[EdHolland]

Good points, Arran. I haven't encountered this problem with any other radio (all earlier vintages), so am learning here. It is a benefit to have everything screwed together, rather than riveted.


All the cans are done, and all seems well through the RF chain, but I still have one persistent, and very intermittent "pop" which seems to plague AM. One thing I notice when this occurs is that the neon indicator in in the bench power strip flashes. It usually doesn't light (the thing is old) but there is a coincident flash when the radio lets forth a pop from the speaker. The pop noise is controlled by the volume pot, so it is either a strong interference from a fault in the AC circuitry, or something faulting in the HT line.

Whatever and wherever it is, it's very hard to locate. I disconnected the cap across the line (an old ceramic disk) - no change. Poking and prodding under the chassis doesn't seem to provoke the trouble repeatedly, although sometimes it seems there could be an effect. Not convinced. The line cord and all internal mains wiring has been replaced.

[EdHolland]

I think the problem is traced - to noise on the AC line. It sounded so local, that it seemed certain it was generated within the set. The thing is so sensitive, that any interference is hard to hear on another radio, but I have convinced myself this is actually the case. Coincidence of "pops" with flickering of the neon in my power strip was eventually demonstrated using another receiver - with and without the Zenith plugged in and running. A good thing too, as I was runnning out of ideas excepting possible heater to cathode shorts in one of the valves.

Last thing today was breaking down and replacing the wiring for the pilot lamp. This was insulated with some plastic or rubber that had become very sticky and unpleasant. Access to the connections on the chassis was difficult, as was opening the bulb-holder, but I wanted to replace the wires throughout.

I'm declaring this one done! (with some follow up pics when it is back in the cabinet.)

[Arran]

Ed;
It was very likely vinyl covered wire given the vintage of the set, there was some rubber/gutta percha wire used in sets in the immediate post war era whilst there were shortages of materials, but much of that was gone by around 1950. From late 1945 to 1947 or 48 there was a pent up demand for consumer goods of all sorts, particularly radios and cars, so some radio manufacturers were not only using war surplus parts for building sets but were using pre war surplus parts like dog bone resistors, and rubber/gutta percha wire for a while, along with field coil speakers, and even cloth covered power cords. This was what surprised me with that Bush DAC 90A I have, it was build in 1956 near as I can tell, but they used rubber/gutta percha wire, most of it is still good other then the wiring to the big ballast resistor, that was just not being used anymore in either American or Canadian stuff by then.
The stickiness is caused by the plasticizer leaching out of the vinyl, but usually doesn't harm the core of the wire, so what I and others have done is to wipe it down with mineral spirits, or naptha, on a rag or paper towel, and it comes right off along with the dirt that sticks to it. I think it really depends on whom supplied the wire as not all vinyl covered wire goes that way, I guess some just used too much plasticizer where others did not. Were these pilot lamp wires red by any chance?
Regards
Arran