[EdHolland]

I managed a bit more detective work on the radio today. Sometimes, it's helpful to have too many radios, and this was one of those times. I used a neat little communications receiver I own as an IF sniffer, just using a piece of insulated wire as a capacitive probe, with no risk of zapping it's delicate (solid state) front end.

The signal generator, set to 455kHz was loose coupled up to the Zenith, and the radio also tuned to 455 kHz. With this setup, I could listen in to each IF stage by setting the probe near the output of each IF transformer. The Zenith was allowed to warm up, at which point the fault started to make itself heard while receiving AM. I turned down the Zenith, turned on the generator, and listened on the probe radio. First and second IF stages seemed to be producing a clean signal, but the third stage was noisy. Switching the Zenith to a broadcast station I could confirm that the same noise heard at the third IF was also present on the Zenith's audio.

I'll think about whether there's any other possible source for the noise (check resistors etc.,) before tearing into the IF can, but - if you'll pardon the pun - I'm getting warmer

Ed

[radiorich]

Hello Ed,
yes , I have had noise disc capacitors and resistors plus even Mica capacitors .
I was restoring a RCA and after recapping the set there was a noise like that but it ended up to be a mica capacitor in the audio circuit set has worked flawlessly ever since.

Sincerely Richard

[EdHolland]

Looking forward to some more work on this today. First off, to isolate the source of noise once and for all. I have a cunning plan to (temporarily) isolate the last IF stage and take the voltage off the suspect transformer. I could then inject modulated 455kHz through the transformerpirimary which should be detected without noise in this configuration, unless the source is another component.

Ed

[EdHolland]

Well, the plan worked.

I unsoldered the wires to the primary of the third 455kHz IF transformer. A small toroid was made up as a 1:1 rf isolating transformer so that I could feed 455kHz from my generator, and apply HT to the IF primary without damaging the generator. This way guaranteed the test focused on the specific IFT. HT for the 3rd IF tube was also disconnected.

With just the generator connected, the modulated 455kHz signal came through clear and undistorted. Connecting 95V from the IF tube HT rail immediately brought on the familiar crashes and crackles which plague AM reception.

With that test concluded, it seems we "have our man", and I'm now about to descend into the dark underworld of IF can repairs...

[EdHolland]

Well that was an adventure, and a successful one.

I removed the transformer, and took off the screening can:

   

After desoldering the wires (noting all positions) removing the nut at the base released the coils, a top disc of phenolic, the mica disc, and its contacts:

   

I found on line a description of using a pencil eraser to clean the mica. This one had clear tracking between the transformer primary and secondary tuning caps.

   

After cleaning, and managing not to break the mica, it looked like this:

   

Wires reattached, and ready to go back on to the chassis:

   

I crossed my fingers and applied power. AM came up nicely, and stayed that way. No sign of the horrendous crashing noises, even after extended warm-up.  Not so FM! Reception has started to go downhill, and with much the same fluttery interference. Part of this was the band-switch - I found a hidden section lurking inside a screening can near the tuner. Cleaning this improved things a little, but FM was still distorted, flickery and a mess. Using my "other radio" trick to probe along the IF chain, I injected a clean carrier and listened on 10.7 MHz. IF1, 2 and 3 were all clean, but at the discriminator, I got the same noise pattern from the probe radio and the Zenith's speaker. Looking at the schematic, there are caps inside the discriminator can. I'll do the same test and treatment on this part. 

Fun day!