Zenith C845 - FM works, no AM

[Jeffhs]

Originally Posted by Reece

Garbled and poor reception on AM with a new loop, and the radio having worked OK on AM before that, sounds like an open AM antenna loop. Check the loop for continuity with your DVM.

Also: work the bandswitch back and forth a dozen times which will tend to clean it even without any cleaner. Clean the pins of all the RF and IF tubes, one at a time, with some rubbing alcohol if you have nothing better, or even a little WD-40, applied to all pins sparingly with a cotton swab, and then while the pins are still wet plug the tube in and out of the socket several times to clean.

I will bet on the open loop and the dirty contacts. Good luck!

Reece,

Thanks much for the advice.

I found the problem tonight; it was the loop, which was open. There is a very small terminal strip on the loop to which its fine wire leads connect; one lead was disconnected from its terminal, effectively causing an open circuit. I took the lead from the open side of the loop and connected it to the AM tuning capacitor, connected the other (low or ground) side to the feedthrough capacitor terminal on the chassis, turned on the radio to AM, and it worked.

Amazing, the problems one little, teeny-tiny (and all too easy to overlook) open connection can cause! The only other problem I have now is what seems to be a poor contact on the band switch, resulting in intermittent FM reception. I'll order a can of Deoxit tomorrow on Amazon.com (thanks to jr_tech for the link) and give that switch a cleaning, which will probably be the first really good cleaning it has had in over a half-century. I'll clean the radio-phono selector switch too, while I'm at it.


Thanks again, Reece, for the advice on checking the loop. I had been looking everywhere else (except under the chassis) for the problem; I had mistakenly assumed that since the loop looked OK, it would work--I didn't think to look at the terminals until tonight.

[Electronic M]

Originally Posted by Jeffhs

Amazing, the problems one little, teeny-tiny (and all too easy to overlook) open connection can cause!

If you think that effect is amazing then picture the open inside an IF can or an audio coupling capacitor....That could prevent all reception...

[toxcrusadr]

Glad you got it sorted out!

Parts Express has Deoxit although Amazon may beat their price. Most people can't find stuff locally and have to mail order.

As for caps, there are a bunch of good sources out there. Audiokarma will have some 'cap source' threads if you search.

If you're going to fix up more radios in the future it's good to have things lined up.

[Jeffhs]

Originally Posted by toxcrusadr

Glad you got it sorted out!

Parts Express has Deoxit although Amazon may beat their price. Most people can't find stuff locally and have to mail order.

As for caps, there are a bunch of good sources out there. Audiokarma will have some 'cap source' threads if you search.

If you're going to fix up more radios in the future it's good to have things lined up.

Thanks for the information on capacitor sources and an alternate source for Deoxit. I have several vintage radios here that could stand new capacitors, although right now they sound pretty good for their ages (several 1950s-'60s Zenith tube sets, two solid-state Zenith T/Os, and one 1980 Zenith 9-transistor AM-FM portable). Can never tell, however, when an electrolytic capacitor might short and damage the power transformer in sets so equipped (if the line fuse, if used, doesn't blow first) or if they might start humming (a dead giveaway that a filter capacitor is going bad), so it's probably best to replace the old capacitors in vintage radios and TVs as a matter of routine.

I am in that very situation you mention of having to get just about everything in the way of repair parts by mail order. I live in a very small town that doesn't have anything as far as electronics parts houses go; to make matters worse (!), the TV shop in the next town won't touch vintage radios or TVs, so I do have to order by mail or by UPS anything I need to repair and/or maintain my old radios. I don't have a workshop anymore, but I try my best to work on my sets within the space I have available in my apartment, which isn't much. There have been times when I have soldered connections on my old radios with the chassis sitting, upended, on my bed; a poor substitute for a workbench, but it works for me.

[Jeffhs]

Why would an open loop antenna in an AC-DC radio cause garbled reception (if the radio receives anything at all) on very strong stations? I would think an open loop would kill all reception, even if the radio were literally within spitting distance of a local station's transmitter towers. I was only receiving the local station in my area (1kW, two miles or so distant from my apartment) on the AM band of my C845 with one end of the loop open; however, the sound was weak and extremely garbled, as if 60-cycle AC hum were entering the audio stages due to a defective filter capacitor or an H-K short or leakage in an audio tube (although H-K shorts in AC-DC radios usually kill the set entirely, since the short will remove filament voltage from all tubes ahead of the shorted one). The tipoff that the problem was unlikely to be the filter capacitors or tubes was there was no hum whatsoever on FM.

[old_coot88]

Originally Posted by Jeffhs

Why would an open loop antenna in an AC-DC radio cause garbled reception (if the radio receives anything at all) on very strong stations? I would think an open loop would kill all reception, even if the radio were literally within spitting distance of a local station's transmitter towers. I was only receiving the local station in my area (1kW, two miles or so distant from my apartment) on the AM band of my C845 with one end of the loop open; however, the sound was weak and extremely garbled, as if 60-cycle AC hum were entering the audio stages due to a defective filter capacitor or an H-K short or leakage in an audio tube (although H-K shorts in AC-DC radios usually kill the set entirely, since the short will remove filament voltage from all tubes ahead of the shorted one). The tipoff that the problem was unlikely to be the filter capacitors or tubes was there was no hum whatsoever on FM.

In most designs using a loop antenna, the loop provides the DC return path to ground for the converter tube's grid. When that path is broken, the grid is free to 'float' and becomes exquisitely sensitive to ambient AC fields in the air.

When a station is tuned in, its signal is heavily modulated by the resultant hum making it sound garbled. And it's weak because the tube's bias polnt is wacked 'way out, cutting the gain*. But when you tune off-station, you don't hear the hum at all. You hear it only when a station is tuned in. Thus the effect is called 'tunable hum'.
And it affects only the AM in an AM/FM set.

*Loss of gain also occurs due to the resonant circuit (of the loop and tuning cap) being broken.

[davet753]

The issue with hearing some sound with the volume control all the way down can be cured by adding a dropping resistor to get your B+ back down where it should be. I have 2 of these sets, and had the same issue when switching to a silicon diode rectifier. The addition of the resistor brought things back in line again.

[DavGoodlin]

Jeff, I have two Zenith Interludes like yours with "noisy" bandswitches. I cleaned them a dozen times with everything from pencil erasers to methyl alcohol to de-Oxit, without much noticeable improvement. It never seemed to affect the AM. I used a chopstick to poke the switch contacts, narrowing the issue to two connections.
It was so bad in the one that in order to get clear FM (intermittent pop and hiss due to a high-impedance connection), I had to disable the AM and hard-wire across the switch, on the rear-most switch wafer IIRC.
I cannot imagine this was an issue when these sets were new but the oxidation and likely eroding-wear of the switch contacts seems to affect them after 50 years of gentle use.