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Old 29th October 2011, 10:09 PM   #1
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Default need help with ac/dc am/fm radio

i could use some advice with a zenith ac/dc, am/fm radio. i got it, tried it out, seemed to work okay, but with filter cap noise. replaced filter cap section, the paper/bakelite caps, and a couple of ceramic caps. the radio plays without the a/c hum, however, i have two issues: the fm sounds clear, and doesn't drift off-station, but the volume comes and goes.(the selector is am, afc, and fm). also, i am gettin a lot of noise on the am side - little "pffft"s, crackle, etc. will going back in and replacing all the carbon comp resistors and ceramic caps help this out? thanks! larry s.
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Old 29th October 2011, 10:26 PM   #2
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Good, you've changed out all the usual suspects. My mica caps and disc caps in my 1960's amps and organs are original, no trouble there. (But dynakit and hammond were premium brands). Carbon comp resistors 100kohms and over make a tiny hiss, not really the kind of noise you are getting. I suspect arcs across dirt. I changed some tubes in my 1961 preamp when I was young for popping, but when I changed paper caps last year to correct unbalanced channels it was popping badly and the arcs turned out to be in places on the PCB's where I hadn't worked - ie 1961 left over solder rosin. I found the spots finally by turning off all the lights at night and staring at the pcb until it popped, then without moving my eyes from the blue flame image, turning on a trouble light. A good scrub with deionized water where I didn't work, but the builder did, and a couple of dry days to dry off the water, and no more popping.
You've obviously survived the high voltage once, but if you haven't read the safety rules lately they are on aikenamps.com tech button technician safety button. Oh, and radios often were directly connected to the power line without an isolation transformer, so think about that: especially the AM section with the 55 V tube and the set of tubes that added up to 120 VAC on the filaments.
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Last edited by indianajo; 29th October 2011 at 10:33 PM.
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Old 29th October 2011, 11:24 PM   #3
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Hisses and pops can also be bad connections between the grids and the tube socket pins. Pull your tubes and clean the pins with an eraser and thin alcohol. After you measure your tube sockets as safe, you can poke around in them also with a shaver cleaner brush or a pick or something. One way to test this theory, shake the tubes by the top while on. IF they get quiet all the sudden, you found it. Sometimes tube sockets have to be replaced.
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Old 30th October 2011, 02:05 AM   #4
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thanks for the tips! i cleaned the pins with a scruffy pad before i tested the tubes...had to replace most of them, actually, and the replacements were cleaned that way as well. there really isn't a way to grab the tubes while the set is on (assembled, anyway), due to the interlock plug. i turned it on in a dark room, and looked for flashes...didn't see any. this is point-to-point, by the way, not pcb. larry
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Old 30th October 2011, 02:58 AM   #5
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Well, dirt shorts & corrosion opens affect point to point wiring, too, just not as easy to see. Most radio/TV repairmen had "cheater" plugs to work with the back off. I saw some at Salvation Army resale last year, but I have enough of them. I think Newark still sells them. Old TV's sitting out for the garbage are another source of cheater plugs, about the only thing they are good for anymore. I quit trying to fix my TV when the electrolytic capacitor count went over 100. Now they don't pick up the signal anymore, and they don't have enough lines either. Oh, the rectifier tube in old tube color TV's is supposed to be something desirable for tube amps.
Volume could be the control, that is pretty common. You can check volume with the 2 VAC scale on a 100kohm/volt VOM or VTVM, or scope. Common DVM's seem pretty useless at audio frequencies on the AC scales.
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Last edited by indianajo; 30th October 2011 at 03:12 AM.
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Old 31st October 2011, 06:39 AM   #6
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i am gettin a lot of noise on the am side - little "pffft"s, crackle, etc.

Well, that's why FM was made.
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Old 31st October 2011, 02:21 PM   #7
drj759 is offline drj759  United States
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What model?

Pops and crackles on AM only could be due to a dirty tuning capactor.

One suggestion on the changing volume level: I have a Zenith K731 that changes volume level randomly. Volume control needs to be replaced. Touching the volume knob will restore the volume level. DeOxit helped some.

Those Zeniths from the 50's/60's are good little radios. I use a Zenith G730 as my "daily driver". Listening to it at the moment.

Last edited by drj759; 31st October 2011 at 02:23 PM. Reason: spelling
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Old 31st October 2011, 07:47 PM   #8
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zenith model M-723
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