AW23 tuner hum

[Kent King]

Ok folks, I need some ideas. I have an AW23 tuner that has significant hum. And it is the tuner: running this tuner on a different amp/speaker, there is hum. Running a known good tuner on this amp/speaker sounds great - the hum is in the tuner...so...

I swapped out the 6C6 audio tubes (all 3), no difference. Also swapped the 2nd det - no luck. Turning up the bass control really increases the hum, but there is a feedback loop in that circuit, so that doesn't really surprise me. With the volume at 0, there is a loud hum. Turning it up, the sound is fine, just the underlying hum. 

Any ideas where to go next? I have enough tubes to swap out everything on the tuner - but maybe I'm missing something else?

Kent

[Alex R. Whitaker]

Could be a broken shield wire, or some wire that is a ground return.  Make sure your filament wiring hasn't been moved over, and is now close to a plate or grid lead.  If you can't find something like that, then you'll probably have to replace the tubes one at a time until the hum goes away.  

[David C. Poland]

do wonder about a filament to cathode leak or short on some tube.

Is the hum there with the 1st audio removed? or either/both 2nd audios?

is it 60 cycle? or more of an un grounded audio cable kind of hum?

[Kent King]

Thanks to both of you - we're largely thinking the same things. Regarding the hum, it sounds 60 cycle. And Dave's idea of pulling the 1st audio tube - I went and tried that quickly, hum is still there. I won't have a lot of time the next few days to work on it, but it must be something after the 1st audio tube. I will ponder swapping out the interstage transformer - but will look at all other possibilities first.

Thanks all!

[Kent King]

I did go back and look at a couple things - one is interesting: if I pull the grid leads on the 2nd audio tubes (PP 6C6), the hum drops <significantly>. Not entirely gone, but about 90% reduced. Also checked all voltages, they are spot on. I am really beginning to suspect the interstage transformer. 

[David C. Poland]

So hum remains after 1 st audio tube is pulled. Looking at the diagram, next is the bass control - are both channels of your bass control switch in good condition? making solid contact.

I found a fault in another Scott with push-pull drivers. With one tube pulled, audio continued. But replacing it and pulling the other, lost most of audio. Was the tube. But seems it could have been half open primary or secondary of interstage transformer, and I suppose depending on where the break was, I can imagine hum like when an audio cable is not plugged in.

A break in the primary would be obvious for lack of plate voltage on the driver tube. But a break in the secondary winding would be subtle to notice in the grid voltage circuit I think.

[Kent King]

The bass control switch looks good, seems to have continuity. Both the 2nd audio tubes have the proper grid bias (about -7.5v). And the plate on the 1st AF is good...in fact, all voltages are well within tolerance. I am really thinking it may be the interstage transformer. I have a good spare on hand, but it may be a couple days before I have time for that level of surgery.

Thanks!

[Kent King]

OK - time for an update, as my problem remains and I have run out of things to try:

I have replaced the interstage transformer, all the caps and resistors in the bass control circuit and the 1M resistor connected to the volume control. I redressed every wire in the audio stages.  While experimenting today, I removed the grid caps from the PP 2nd AF tubes. The hum vanished but I still had audio! Decent volume in fact and the bass control impacted the sound appropriately, RF gain made it louder too. The only way the signal can be getting in is capacitive coupling from the grid caps hanging in space to the grid caps on the tubes - impressive, but the sound was so strong it makes me wonder if there is another "leak" somewhere, with audio getting to the 6C6s. If I pull the 1st AF grid cap - it does go silent. Any further thoughts? (BTW, it is 60c hum, I confirmed that)

Kent

[Norman S Braithwaite]

This sounds like a possible case of a bad ground connection somewhere resulting in hum and allowing the audio signal to travel through a ground wire.  Try jumping lugs and terminals that are supposed to be at ground to the chassis using a short jumper.

Norman

[Thomas Day]

One off the wall I can think of, check the minus supply for ripple or signal with a scope. 
i know you said that another chassis with the same supply works fine, but there may be something in this chassis that is not the same.