Zenith C845 - FM works, no AM
 

I replaced the 12AT7 converter tube in my Zenith C845 AM-FM radio. Now I get FM (all local stations in Cleveland and many distant ones), but almost nothing on AM. I say "almost" nothing because I can hear a local station (however, the sound from that station is very weak and garbled), but nothing else. I also replaced the AM loop antenna with one I salvaged from a Zenith MJ-1035; no luck. I am sure the loop is connected properly (high side to the tuning capacitor, low side to the feedthrough capacitor terminal on the chassis). I considered replacing the 6BJ6 RF amplifier tube with a 12BA6, but now I'm not sure I want to do that after reading that this is not a good substitution--the reception on both bands could be worse than it is now if I tried it (as someone in this forum, or maybe it was on ARF, mentioned recently). Because the radio receives the local AM station (about two miles away) at all, I am sure the AM section must be working, if only just barely. Where could the problem be hiding? If it is anything worse than a bad tube, I am not going to bother with it, as AM radio in this area isn't worth listening to anymore--except possibly for AM 740 in Toronto.

Something else puzzles me about this radio. The volume control works normally, but I can still hear some sound from the speaker when the control is set at minimum. I read somewhere, again I think it may have been here or on ARF, that this can be caused by excessive B-plus voltage due to the selenium rectifier having been replaced with a 1N4007 silicon diode, but no dropping resistor. The selenium stack in this radio is probably as old (53 years) as the radio itself, so I'm thinking it may have been replaced some time before I got the set.

Thanks for any and all replies.

73 (best regards in ham radio talk),

Were you getting any AM before or was the radio basically dead and the FM came to life with the new tube but not AM?

Got a schematic for this beast? I'm just wondering how separate the AM and FM sections are. Are they both using the 12AT7?

Has the set been completely recapped or are the caps original? If original there may be one or more bad or drifted interstage caps somewhere in the AM circuitry.

How about a dirty band selector switch?

Thank you much for the prompt reply.


The AM section was working before I replaced the 12AT7. I had to replace the tube because a pin broke off the original.

On the schematic for the C845, chassis 8C02 (which is practically identical to that for the C835), the 12AT7 tube is identified simply as a converter, which leads me to believe it works for both AM and FM. I can't be certain about that, however, since the tubes in the C845 are wired in series, and the radio will not operate without all tubes in their sockets.

This radio has not been recapped, to the best of my knowledge (I got it used about five years or so ago), so there could well be one or more bad capacitors in the AM stages.

Dirty contacts on the band switch could be causing the problem. I don't have any kind of contact cleaner here, so I'll have to put off cleaning the switch until I can get a fresh can of cleaner at Radio Shack (the nearest RS branch store is perhaps ten miles from where I live; I don't drive, so it may be some time before I can get there).

I did mention in my post that I am getting very weak and garbled reception of a local AM station in the next town south of here, so I am thinking the AM stages must be working, at least somewhat. What puzzles me, however, is why, if the radio gets the local station (distorted though it is), I am not hearing any other stations anywhere on the dial. :scratch2:

It's time to stop groping and do some serious troubleshooting.

First make sure the local oscillator is running, and covers the proper frequency range. Put a signal into the IF and see if it has reasonable gain. After that, you will have enough info to track down the problem.

Generally, you won't find that the 12AT7 functions on AM. You probably have something like a 6BE6 or 12BE6 for the front end on AM.

I found a schematic here:
http://techpreservation.dyndns.org/s...ics/Zenith.htm
Looks like the 12AT7 serves as LO for both AM & FM... Try another one perhaps. How did the pin get broken off... did the radio fall or something? Was the antenna also damaged somehow? Perhaps a more complete description of the events surrounding the failure would be helpful here.
jr

Bob, I appreciate your tips and advice, but I do not have any test equipment except a digital multimeter (gave up my workshop years ago when I moved here :no:), so I cannot do any kind of signal tracing or advanced troubleshooting beyond soldering bad connections, replacing tubes, etc. I would not know how to test the local oscillator in the radio, either, without test gear.

The converter tube in the C845 is a 12AT7, which functions as both oscillator and mixer stage. The RF amplifier, which functions on both AM and FM, is a 6BJ6.

If you look at the schematic, you'll see that the AM oscillator circuit is rather involved.
Per previous entries, stating a sub for a 6BJ6 vs a 12BA6 sub. They're both remote cutoff penthodes and can be tried, to see if there's an improvement.
It's strange, as I have several of these same receivers and they're the most reliable and best performing sets of their kind.
The only problem, I ever had with these sets is bad tubes, lytics and band selector switches that needed cleaning. Also dial cord restringing. :thmbsp:

I was trying to reinsert the original 12AT7 into its socket with the radio chassis still in its cabinet (I couldn't see the socket very well), resulting in several pins bending. One pin broke off flush with the base of the tube when I tried to straighten it. As for the antenna, I lost a connection on the loop (the wire broke off and was probably buried deep in the winding, where I could not find it), so I replaced the entire loop with one I had on hand (in a parts donor set). Still no reception on AM except one local station, as I mentioned in my original post.

I replaced the broken 12AT7 with a new-old-stock tube. This restored the FM reception, but still no luck on AM. I am beginning to think that there must be a problem with defective capacitors or resistors somewhere under the chassis; in a radio this old (1960), I would not be one bit surprised if it had one or several leaky or even open capacitors, since to the best of my knowledge this radio has never been recapped (I got it about five years ago from an antique radio collector in Arizona, a former VK member).

If the 12AT7 works as the local oscillator on both bands, I honestly don't think the replacement tube is causing the problem since it works on FM, and well (this radio gets FM stations from 50-60+ miles distant, more in summer and fall, using just the built-in line cord antenna here in my location one mile from Lake Erie). I don't know the exact condition of the replacement tube, as I purchased it on eBay from a seller in New England who claimed and all but guaranteed the tube was good--he must have tested it using a mutual-conductance tube tester before listing it.

Did the AM work normally just before the antenna loop was broken? Was the replacement the same as the original? What was the initial problem that you were trying to fix?

jr

Yes, the AM did work well before the problem with the loop. The replacement is from a Zenith MJ-1035 and looks identical to the C-845's original loop. I don't see how that could have any bearing on whether or not the AM band works; the only thing I can think might happen with a non-original loop is the alignment might shift a bit, but the radio should work regardless.

If I remember correctly, I was having a problem with no reception on either band when I removed the original 12AT7. I would still be using that tube if a pin hadn't broken off flush with the glass at the base. The replacement is a NOS 12AT7 I purchased on eBay (BIN). The seller said, in the item description, that the tube was good (presumably he tested it first) and that he guaranteed the tube would be good when I received it. I have no way of knowing the cathode emission of either section, but the section that serves as the FM local oscillator must be working since, as I mentioned, the FM reception is great--I can hear stations 60+ miles distant just using the built-in antenna.

There might still be a problem with the band switch. I'll have to wait a while before I can tackle that problem, however, since the nearest Radio Shack to here is about ten miles distant (I live in a very small town with nothing in the way of electronics parts stores, or much else for that matter) and I do not drive. Once I do get a can of contact cleaner, though, I will try it on the band switch in my C-845; perhaps that will be all it needs to get the AM band singing again.

I don't know the condition of the electrolytic filter capacitor in the radio, although it must still be doing a half-decent job since I hear no hum in the audio, even on weak stations.

Wow that's an unusual circuit. But the diagram gives voltages so you can at least measure those and see how close they are.

Odd as it may seem, Amazon sells Deoxit D-5, which IMHO, is better than the stuff RS used to sell:
http://www.amazon.com/Hosa-Cable-Deo...ords=deoxit+d5
Not Affiliated,
jr

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!

This is a long shot, but easy to eliminate as a possibility (assuming the bandswitch is wafer type):
Watch the switch while rotating it, to be sure the rotors are secure to the shaft and fully turning with the shaft.

I've seen this a number of times in wafer type TV tuners where a rotor was loose on the shaft or not turning at all.

Zenith C-845: AM sings again! :yes:
 

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.