Thread Rating:
  • 0 Vote(s) - 0 Average
  • 1
  • 2
  • 3
  • 4
  • 5

Hum on FM
#1

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?Icon_confused:
http://www.nostalgiaair.org/PagesByModel...025312.pdf

Eric
The Villages, FL
Member: Philco Phorum, ARF, ARCI & Radiomuseum.org

#2

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?
#3

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.

Eric
The Villages, FL
Member: Philco Phorum, ARF, ARCI & Radiomuseum.org

#4

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

Eric
The Villages, FL
Member: Philco Phorum, ARF, ARCI & Radiomuseum.org

#5

Theses Zeniths are very good sounding, many small enough to carry around the house. got a photo?

Paul

Tubetalk1
#6

Here you go, Paul.


Attached Files Image(s)
   

Eric
The Villages, FL
Member: Philco Phorum, ARF, ARCI & Radiomuseum.org





Users browsing this thread: 1 Guest(s)
[-]
Recent Posts
Jackson 715 not working
Now that you posted the schematic, I don't know why that Sprague electrolytic cap is across the meter, as it is not indi...MrFixr55 — 05:51 PM
HiFi (Chifi) tube amp build - but my own design.
What may be lacking in the PP Tube amps may be the 2nd harmonics, which some, especially RCA back in the day called &quo...MrFixr55 — 05:32 PM
Jackson 715 not working
Usually in an emission tester, the tube under test is measured as if it were a diode. So, some testers connect all the g...RodB — 04:17 PM
Restoring Philco 37-604C
Yep. F5 is green, D5 is Red. Red is Bad. Green is Clean.morzh — 01:30 PM
Jackson 715 not working
I did start to do that but I stalled out because I could not figure out how the grid and plate get voltage. In this diag...daveone23 — 11:52 AM
Restoring Philco 37-604C
(Insert Homer Simpson "DOPF" Here.) When all fails, look at the can. Took the Ron Ramirez advice, red Caig D...MrFixr55 — 09:23 AM
Philco 91 Speaker Replacement
From your text I am not sure if you intend to use the existing speaker with a resistor instead of the field coil. It wo...morzh — 08:44 AM
Philco 91 Speaker Replacement
My field coil is bad. I am still hoping to find an original, but if I can't I will go with a fitting Philco speaker, 125...dconant — 08:34 AM
Philco 91 Speaker Replacement
As Rod said, it is OK to use a fitting speaker, and then look for an original one. If you buy a Hammond 125 output tr...morzh — 08:15 AM
Philco 91 Speaker Replacement
Yes, I often have to substitute, then keep an eye out for an original. In the meantime, the radio is working and being e...RodB — 08:02 AM

[-]
Who's Online
There are currently no members online.

>