Strange fault in Zenith radio!

[Hemingray]

This one is entirely new to me, this just appeared awhile ago while running my Zenith 7G04. It played perfectly fine for about an hour or so (feeding it signal from my XM Xpress receiver). Out of nowhere, the radio begins to spew forth with loud bursts of static. Disconnecting my Xpress from it and hooking it to the outside antenna, EVERY STATION it pulls in is jam-loaded with these static bursts. Any clue what's suddenly started this up? tapping the radio doesn't cause it to go away, ruling out a possibly faulty tube connection. The only tubes that were ever changed in this radio is the 19T8 and 35B5. All of it's others checked out fine when I last worked on it.

Forgot to add, this is the second time it's done this. The first time was with the radio tuned to a local station.

[mr_fixer]

the first question is ... Does the volume control change the volume of the noise?

[radiotvnut]

You could have defective IF transformers. IF transformers from the immediate post WWII era until the end of tube radio production in the late '60's often suffer from "silver migration". What happens is that the capacitors that are imbedded in the base of the IF transformer leak and cause current to flow from the primary side to the secondary side of the transformer. This action will cause the static burst that you describe. There are ways that the old embedded caps can be eliminated and modern caps can be substituted. The process looks to be tedious; but, is often the only way, since these older IF transformers are hard to find now. I have not personally replaced the caps in an IF transformer; but, I've got a '52 Philco that I'm about to try my hand at repairing the IF's.

[Reece]

Your radio may very well have "silver migration disease". Google that phrase and you'll find lots of articles about it. Sometimes with this "disease" if the radio is turned on after a long time being off, it will play for a few seconds OK and then start the static. Before starting to tear into the IF's, I think I would signal trace and be sure what stage the noise is coming from.

Reece

[Hemingray]

mr_fixer: It does go along with volume changes, however it follows across the entire FM dial. I haven't tested on AM yet.

[akent36]

http://www.ppinyot.com/if_transformers.htm

[mr_fixer]

Originally Posted by Hemingray

mr_fixer: It does go along with volume changes, however it follows across the entire FM dial. I haven't tested on AM yet.

Well that means it is before the audio stages, It very well could be silver migration in caps of the IF cans.