Hum on FM - Printable Version
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Hum on FM - Eric Adams - 09-15-2013
Well, here I go again. It looks like the most problems I have are some kind of hum and they're all different.
This one is a Zenith 79H20. After a recap, all tubes tested as well as resistors. AM sounds great. After an alignment FM pulls in many stations. Between stations there is a steady hum. To confuse me even more, when I touch the volume control shaft (using an isolation transformer) the hum gets a bit louder. I've checked all ground connections. Could I have missed a resistor or a mica cap?
This really is a great sounding radio. Any ideas?
:http://www.nostalgiaair.org/PagesByModel/312/M0025312.pdf
RE: Hum on FM - Mondial - 09-15-2013
Looking at the schematic, it appears that the volume control is not directly grounded to the chassis, but rather through a .01 uF cap. This is probably why you get a hum when you touch the shaft.
C16, connected to the slider of the volume control looks like it was a shielded cap with a ground connection. If you replaced it with a normal cap, it may be picking up hum. Is C16 near any line voltage AC wiring? There has to be some reason why the original cap had a grounded shield.
How are you doing with the sweep generator and FM alignments?
RE: Hum on FM - Eric Adams - 09-15-2013
Thanks, Mondial, I'll take a look at C16 and let you know. As far as the alignments go, first I want to thank you for pointing out this HP generator. It really seems to be getting the job done. Second, I'm starting to get the hang of it but my biggest problem is still knowing where to make the hook ups for gen and scope. I still haven't got a radio with visual alignment instructions but I think I'm starting to figure it out.
RE: Hum on FM - Eric Adams - 09-16-2013
It looks like that did it, Mondial. I wrapped a coil of wire around C16 and grounded one end. I'm not saying that there is no hum, but it is now to the acceptable level for an AC/DC set.
Thanks
RE: Hum on FM - Paul Philco322 - 09-17-2013
Theses Zeniths are very good sounding, many small enough to carry around the house. got a photo?
Paul
RE: Hum on FM - Eric Adams - 09-17-2013
Here you go, Paul.