Sherwood S-8000IV - hum in phono/tape inputs

Interesting. No sub chassis here, but I'll go make sure the "bracket trees" things are soldered to have good connection with ground
 
yeah I've had that problem in things too. The rivets get crusty. If its a copper plated chassis, you can just solder the tabs to the chassis and sort that.
 
No dice. I even poked around with an insulated poker to see if I could stumble across a faulty connection, but no luck. Any other possible places this could be coming from?
 
whats the grounding back to the input jacks look like? The ones on my 7000 were basically just pressed through an aluminum sheet, and I had some random hum issues until I took a punch to tighten that connection up so it actually made contact with circuit ground.

Kinda guessing at it honestly, short of hooking a scope up to ID where the noise comes from I don't rightly know what to say.
 
Here's a pic of the back of the inputs. I don't see anything there that would have caused it in these specific two inputs. The big grounding wire that connects the tape inputs to the input jack is also secure (I reflowed the joints). I think it will take a scope though to find the culprit. However, it should be fine in the meantime....I'll have to learn to ignore it, haha. As always, I really appreciate your help, gadget! I'm sure I'll revisit the problem sometime in the future, and if I ever find the cause, I'll post it here for posterity.
 

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yeah thats the same setup mine uses. I used a spring loaded center punch to re-stake each of them to the panel. It seemed to sort it, though my problem was a hum, not a noise like you describe.

Honestly that sounds like noisy tubes or bad resistors but if replacing them made no change, it can't be that.
 
Right...the noise is just kinda mysterious. It REALLY does sound like a tube, but I swapped out each 6EU7 and it was still there, so it cant be one of those. And all the resistors in the phono are either brand new or dead on spec. Are the 6EU7's the only tubes (besides 7868) being used for the phono preamp? Any other small tubes in the signal chain?
 
Not sure the particulars on this one, my 7000 and 5500 use 12AX7's. On those, there are two that are used only for the phono. Then there are 2 more that are for the line stage. All inputs make use of those, so basically in phono mode you're running 4 tubes plus the phase inverters (2 more 12ax7's on mine) and the 7868 output tubes. The 7000 has a tuner too, none of those tubes do anything in phono mode. If yours runs all 6EU7, I'd figure 4 6EU7 plus whatever is in front of the 7868 tubes.
 
I think the 8000 is the same way. 2 6EU7 for the phono, one for phase, another for tone control. There are another pair that sit in front of the OPT's. I know they are in the signal chain, but I'm not sure what they do specifically. I can rule those 6 out. There is a 12AT7 and 12AU7 near the back next to the phono jacks though...are these just handing the FM?
 
The ones in front of the outputs should be the phase inverters, one per channel.

I *think* the 12au7 and 12at7 are in the FM multiplexer, but possible one is multiplexer and the other is the mixer/oscillator in the tuner.
 
Ah ha - I didn't know that. The schematic doesn't list it that way, but that makes more sense.

That's what I figured with the 12at7 and 12au7. I wonder if the buzz could be coming from the phono gain knob, which is going bad? I'm not sure how pots fail, so maybe not...
 
Possible. Could always jump the thing out of circuit to eliminate the chance that its your issue.
 
That's true. My gut says to leave it alone for now, but if/when I want to tackle this again, that's might be where I start
 
I've been following this thread, have an S8000 that I plan to tackle one of these days.

I had a similar problem with my Fisher X 101 B. The problem ended up being a bad resistor. When I un soldered one end, the resistor fell apart. It had a radial crack all the way around the resistor body, just 1/16 inch or so from the lead. The resistor was soldered firmly in place, initially looked fine.
 
I've been following this thread, have an S8000 that I plan to tackle one of these days.

I had a similar problem with my Fisher X 101 B. The problem ended up being a bad resistor. When I un soldered one end, the resistor fell apart. It had a radial crack all the way around the resistor body, just 1/16 inch or so from the lead. The resistor was soldered firmly in place, initially looked fine.

Interesting. Did it test alright? Never thought to look for a cracked resistor. I tested em all, and I assume if they test OK then they are alright...
 
Interesting. Did it test alright? Never thought to look for a cracked resistor. I tested em all, and I assume if they test OK then they are alright...
I didn't test ok. When I un soldered one end it fell apart. I spent a lot of time chasing that noise!
 
Well, that could very well be the issue with mine. In searching for bad components, I did replace the resistors and caps piggy backed on them (in the phono pre) as they tested WAYYYY out...more than 10x. Curious enough, the hum is now much quieter. Sometimes I can't really hear it at all, so it's progress. Next time I'm in there I'll look for cracked resistors. Thanks for the tip!
 
sensible. I've never had a noisy resistor that wasn't also off tolerance. Certainly not impossible that in-spec resistors could be slightly noisy but the gross offenders were always bad in other ways too. Carbon comp resistors are just not the quietest things. In a high gain spot like a phono stage, even a slight noise will be amplified a considerable amount.
 
Any chance this hum could be coming from a cap, like the filter cap that supplies the phono/tape pre amp? I'm not sure if the buzz classifies as motor boating, but I've read that that can be caused by a bad caps....
 
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can you get a recording of what exactly you're hearing? If its just a hum, that could be a lot of things. Motorboating is a very different sound though.

Have you tried other tubes? Does it do this with shorting plugs used on the phono input?
 
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