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Picked up an old Zenith AM FM Model H724. Radio plays well for awhile and then gets bad static like a thunderstorm is approaching. Recapped and changed out all tubes. Tried tapping with rubber handled screwdriver both when static was present and when it was absent and it didn't seem to matter, making me think it is not an intermittent connection problem. I've read about this problem somewhere. Best I can remember the problem was inside one of the I-f cans. Maybe someone knows or has a better memory?
Thanks in advance.
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It could be the IF cans but before playing with those try giving the volume control and the tube sockets a good cleaning, it could also be a cold solder joint so it worth checking your work there. I am suggesting all of these first because breaking into an IF can and removing and replacing the capacitors/condensers inside is finicky work that should only be undertaken as a last resort. If they do turn out to be the culprit try to measure the exact size of the originals before diving in as they are not all the same size. I have replaced the capacitors inside an IF can just recently and contrary to popular belief they are not all 100 picofarads (micromicrofarads) , and you do not have to used silver micas as replacements, precision ceramics capacitors work just fine and are maybe 1/3 the cost of silver micas and are smaller.
Regards
Arran
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Agree with all, Key thing is time before symptoms occur. A diseased mica typically starts misbehaving from 0-60 seconds from a cold start. Note well (N/B) the comment about values, I've seen anywhere from 10 to 220 mmf in cans. Also could be any silver mica outside of the cans in the front end. If the covers to the cans come off easy, replacement shouldn't be too hard. In this instance it is OK to cut out the old part at the component body, loop the old lead and use a heat sink (transistor or roach clip, you probably have one or the other) in order not to disturb the coil connection joint. Then cut the leads of the replacement short and make another loop, join, crimp, and solder quickly and carefully. Then do the same on the other side. Well that's how I do it.
Could also be a resistor that is going bad if the set plays for, say 5 or 10 minutes before crapping out. For a nickel apiece I would replace 'em all, except for wirewounds and candohms, and I usually do this as I am recapping. The old resistors almost always drift high, and will continue to climb, so the set may only work for another year or two.
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IF it turns out to be the IF cans, and that possibility is VERY high, here are the typical values used in most Zeniths of that time frame. (thanks, Dan Schoo)
http://www.sparkbench.com/zenithifxfmrs.
It takes a bit of surgical skill to replace the mica wafers but it's not terribly difficult after you do the first one.
