Quote:
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.