Slight hum from 500C.

the_nines

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It played flawlessly since the rebuild a couple of years ago. Last week I noticed a slight hum.

The hum comes from both channels, is not volume dependent (doesn't get louder when I turn up the volume), and is present on all seven selector positions (phono, aux, fm, etc...).

The unit was rebuilt a couple of years ago with a full kit from Jim McShane. Attached is the kit list of parts that went in, and a pic of what came out.

Only the four output tubes have been replaced from the original Fisher 7591 to EH7591. All the other tubes were not touched as they tested fine at the time of the rebuild.

Thoughts?

Edit: I did a search on AK (with Google) but couldn't find anything similar to what I have described above.
 

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Assuming it still has the original power cord, first flip the plug in the outlet and see if the hum changes in any way.
 
Before you start playing with the tubes eliminate other connections as the source. Isolate the 500c by unplugging all connections other than the power. Turn on the aux input and see if there is a change in the hum. I once had an antenna connection create a low level hum such as you describe.

If that is not it first try just wiggling the preamp (V14 and V15) tubes in their sockets. Usually a oxidized pin connection will only be in one channel but it is worth eliminating as a cause.
 
I have the same symptoms in my highly-modded 500-C. It's a very low-volume hum that I can hear only when using headphones. I will try the solutions offered above, but I am suspicious... I have built and installed the IBAM circuit. The B- and ground UNINSULATED wires from the external IBAM board are routed through the chassis holes between the 3 can caps. Inside the chassis these wires are wrapped around what looks like a metal, cylindrical tube through which the TAPE HEAD wires are routed. And next to this tube are all the other audio signal wires (phono high and low, tape in and out, etc.). I know those signal wires carry AC which generates electromagnetic fields however small in this case. Could my uninsulated IBAM wiring be the cause of the hum? Thorne
 
Thorne- any & all wiring on the input side, especially low-level, should be shielded, just like fisher did. You can substitute shielded cable in place of your open wiring, such as rg-58 or other small coax, just ground the shielding (outer braided layer) to chassis ground. Or, wind some fine, as in 22-24awg solid wire around the wires in question, ground one end. Try it, I hope it helps. Catman
 
Thanks, Mr Cat. Never thought I could wrap wire around an uninsulated wire to make it insulated. Thorne
 
Did you use bare or insulated wire? In either case, be sure to use insulated single-strand wire. You're just simulating a shielded wire by wrapping with the aforementioned wire. Probably easier to use pieces of coax if you've access to any. If not so late this evening, I'd make a mock-up & send ya a pic. Can do tomorrow p.m., if you want.
 
Nah, thanks, Cat. I'll just remove the old wire, buy some insulated wire and reinstall. I didn't do it already because I'm not convinced that's the cause of the hum. To do the job right I should remove all possible causes (attached devices and mods) to see if any of those items might be the cause. I posted because I thought someone might know for sure. Not that I'm doubting you, but I'm betting you would agree that with all those mods you know I installed, the hum may have something to do with them (or attached devices and/or their wiring and lack of grounding).
 
I wonder if the OP has had any success running down the hum? Any news?

Thorne, I'm a little puzzled by your theory for the hum in your unit. IBAM wiring is not signal wiring--it just carries the DC bias voltage and the included electrolytic caps, in fact, shunt any AC the wiring may pick up from the tubes immediately to ground to prevent cross-talk. I've never had an IBAM unit introduce any hum in a unit I have so configured (including the Fisher 400 I just re-worked) and I have used only insulated, solid strand wire for the connections, not shielded.

The shielding of the input wires (by routing them through the spiral wound shield tubes you mention) is to keep the input signal wires from picking up stray induced currents from AC sources such as filament wiring, transformers, etc., not the other way around.

Insulation of the wire would have no effect on a conductors' emitting or picking up of electromagnetic fields, it just keeps the DC or AC voltage carried by the wires from shorting out against the chassis or other wires. Plastic insulation as such is transparent to electromagnetic fields.

In both of these situations, one question is whether the hum is 60 Hz or 120 Hz. 60 Hz is a deeper tone that comes from a lack of shielding of input signal wires or a tube cathode-to-filament failure (with an AC powered heater) that often follows the volume control. 120 Hz, which is higher-pitched and sounds like more of a buzz, is usually a sign of a power supply issue like a bad cap or a ground issue with the caps early in the B+ string.
 
Dave - The confusion you experienced is a result of my amateur usage of electronic terminology. I never thought shielding and insulation are different, but you've shown me they can be.

The hum I'm hearing is the lowest B note on a guitar. That would be the B that is 2 octaves below A440 which is 123.47 Hz. I guess that makes the issue one of grounding. I'm going to try process of elimination to see if that process turns up anything. I'll report back.

By the way, I've got the SDS Labs board a previous owner installed to replace the stock caps in the B+ string (I think... if I know what you mean by "B+ string" which is possible I don't). Pictures attached.
 

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No worries, Thorne! I've always been fascinated by that board and maybe one day will try one--it's a neat solution to replacing the B+ can caps, it appears. Yep, the "B+" supply string is slang for the Resistance/Capacitance (R/C) filtering network in what the Brits call the 'high tension' power supply that supplies various DC voltages to the plates of the different stages in an amp or receiver. A faint 120 Hz buzz may possibly be an artifact of the installation of this board, or the grounding thereof. The subject of grounding (star grounds, ground bus, ground loops, etc.) and hum chasing is one that has produced a lot of consternation and thought, I have found. Good luck running it down!
 
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