Zenith C845 - FM works, no AM

[jr_tech]

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

[dieseljeep]

Originally Posted by jr_tech

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

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.

[Jeffhs]

Originally Posted by jr_tech

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

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.

[jr_tech]

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

[Jeffhs]

Originally Posted by jr_tech

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.

[jr_tech]

Originally Posted by Jeffhs

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

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