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Troubleshooting hum

c.coyle

c.coyle

Fighting the Dunning-Kruger effect.
Capacitor coupled receiver. Nothing connected to it except two speakers. Hum on all inputs, both channels, regardless of volume [EDIT: Meaning the hum level is not volume dependent]. Flipping the plug makes no difference. Plugging into another outlet on a different circuit makes no difference. Hum remains audible after turning off power until caps discharge.

Same as above with no speakers, just headphones.

Most likely source is power supply or bad ground, not a ground loop. Agree?
 
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With the volume at zero, you should be able to eliminate any circuitry before that as the cause. Different receivers put the volume control in different locations, so there could still be significant circuitry after it. What receiver? How much hum? Most hum problems turn out not to be caps... unless they're the problem! I'd start by looking at all grounds, especially anything that relies on a screw for contact. AC couple the scope and look at ripple on all power supplies. It could be a volt or two p-p on unregulated supplies, but down in the mV or less for the regulated supplies that usually supply the low level circuits. Sometimes, depending on the low level circuits, I'll short the inputs of the power amp directly to ground. If the hum remains, it's in the power amp. If not, it's before. You don't want to make any mistakes doing that. When using the scope to troubleshoot hum problems it's important to choose the scope ground wisely. Receivers can have surprising differences in ground potential at different locations.
 
Thanks, Conrad. I'll do all of that tonight. It's a Sansui 350A. The hum is minor. When playing FM or a source through AUX at low volume, I can hear it if I listen for it. If I crank the volume up, I can't hear it. With the volume all the way down, [EDIT: or with with the tape monitor on] or without anything other than speakers hooked up, it's easily noticeable.
 
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c.coyle said:
Hum remains audible after turning off power until caps discharge.
Click to expand...

This observation seems especially remarkable to me. (It's remarkable if I assume that the hum is at power line frequency, anyway. Can you say if that is true or not?)

Give that nothing else is connected except speakers, perhaps some other electrical device in the vicinity is producing interference that the Sansui is picking up. Not quite sure where that points to, diagnosis-wise, but I'd be looking at ground connections, especially those (if any) that involve shielding around the power amp or other post-volume-control circuitry.

Cheers,

chazix
 
chazix said:
This observation seems especially remarkable to me. (It's remarkable if I assume that the hum is at power line frequency, anyway. Can you say if that is true or not?)
Click to expand...

I was wrong about that. Upon further careful listening via speakers and headphones, I have to say that the hum ends when the power is switched off.

chazix said:
Give that nothing else is connected except speakers, perhaps some other electrical device in the vicinity is producing interference that the Sansui is picking up. Not quite sure where that points to, diagnosis-wise, but I'd be looking at ground connections, especially those (if any) that involve shielding around the power amp or other post-volume-control circuitry.
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I plugged my phone signal generator in, dialed up a 120hz sine wave, and A-B'd the hum and the phone. It sounds like 120hz. I'll put a scope on it tonight and look for PS ripple. I am going to rebuild the PS anyway, but I'd prefer to figure this out rather than throw parts at the problem.
 
1970, so bad caps are a definite possibility. It also uses the dreaded 458 transistors in the low level and the power amp. If the gain of those is whacked or they've gotten leaky (high odds on both) weird things could happen, including hum. I would definitely change them.
 
390 mv ripple on the main filter cap, which goes straight to the output transistor collectors. Full PS rebuild coming up. Thanks as always for the help.

And all the 2SC458s will be switched to KSC1845FTA.
 
ConradH said:
390 mV on the main unregulated supply doesn't sound like much. I'm guessing that's not it.
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Well, I was going to redo the PS anyway. While I'm waiting for Digi-Key, I'll continue with the procedure you outlined in Post #2. So far I have carefully inspected all grounds, visually and with an ohm meter, replaced the two 2SC458s on the amp board, and looked for ripple on the 25v output of the PS board pass transistor, of which there is basically none.
 
Just for shits and giggles is there hum when you feed TAPE OUT into another receiver? IOW is the TAPE OUT feeding hum out? IOW is the line level signal corrupted by hum at that convenient tap point?
 
I grounded the amp inputs as per Conrad's suggestion, and the hum remains, so I'll focus on the amp board for now. All grounds look and test good.

There's a local regulator on the amp board. Since new caps will be physically smaller, I think I'll increase the values. I also want to pull the little pass tranny (2SC815) and check it. It drops 47v to 45v, and I see about 300mv of ripple on its emitter

Loopstick: Good idea on the tape out. I'm going to give that a try.
 

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The ripple on that pass transistor sounds too high. Between the base cap and the later filter cap I'd expect it to be smaller than what you're seeing. Maybe low gain on the part? It's certainly possible that the regulator could have the same problem on both channels.
 
ConradH said:
For some reason I thought you had changed the caps, but if not, definitely the place to start.
Click to expand...

Well, I took some time out from shoveling snow and did the following, see schematic attached to Post #12:

- C823, the cap at the base of the 2SC815 pass tranny got changed from 220uf to 470uf.

- C822, on the emitter of the pass tranny to ground, went from 3.3uf to 47uf. (If that cap is there for filtering ((bypass maybe?)), 3.3uf seems kind of puny)

- C821 went from 47uf to 100uf.

- The two 2SC458s on the amp board got changed to KSC1845FTA.

The ripple at the emitter of the 2SC815 pass transistor is now 180mv, down from 300mv. The hum is substantially reduced and you really have to listen for it. [EDIT: It is now low enough that I can hear what seems to be a distinct hum depending on how I have the unpolarized plug in the wall socket. Neutral to neutral eliminates this.]

BTW, the ripple on the 24 volt regulated output on the main PS board is 3mv, so that pretty much says to me that the local regulator circuit on the amp board is the source of the hum. Maybe I should replace the 2SC815 with a higher gain transistor, per Conrad's suggestion, such as a ZTX603. Maybe kick up C823 to something > 470uf. What do you all think?
 
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This is where you can go down a rabbit hole, depending on how Sansui did the grounding. Read the stuff I did on the 881- http://audiokarma.org/forums/index.php?threads/881-receiver-bias-transistors.694713/

I'd pick some ground reference for the scope, say the input ground of the power section, and then probe every other ground. If you see differences in the amount of hum on the ground lines, there's probably a grounding issue and further component changes will have little effect. The transistor is still worth a try- I think the hum level should be lower there, but it depends on the reference point. You should trace out the charging loops for the power supply and insure that no signal ground is tied within that loop.
 
I have a tendency to get obsessive and to over-complicate. I think I have two hums.

The first is the one we've been discussing all along, and I think I have isolated that to filtering at the local series pass regulator on the amp board. I'm happy with the hum reduction to virtually inaudible, and other than maybe changing the pass tranny, I think I'll stop there. I think that dropping 47 unregulated just 2 volts to 45 regulated is a good indication for a higher gain pass transistor. This hum sounds like 120hz

The second hum I assume is a ground loop. It sounds different. But, I can eliminate it by observing proper plug polarity, and that is just fine with me. Nevertheless, I will probe grounds with the scope, just out of curiosity.

I consider the problem solved for all practical purposes, and I thank you and everyone else who chimed in.

Chris
 
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