Noisy sex

[Doc B.]

OK, now put the 100 ohm resistors from C1 to C3 and from C2 to C3 back in, and see if that helps.

[RW]

100 ohm resistors are in. It helped a little, but not a lot, it's still audible.

[Paul Birkeland]

Just out of curiosity, can you measure the AC voltage from ground (on the headphone jack is fine) to the red and white wires that go to the headphone jack?  You'll likely need the mV scale to get a useful reading when taking this measurement.

[RW]

My headphone jack isn't secure to th chassis right now because of disassembly.
If I put my neg probe on lug 13, I get around 350mv on the red and white.

[Paul Birkeland]

My headphone jack isn't secure to th chassis right now because of disassembly.
If I put my neg probe on lug 13, I get around 350mv on the red and white.

Hmm, are you sure it's not 0.3mV?  350mV would show as about 0.4V on a higher scale.

350mV is way, way higher than normal.  The immediate thing that comes to mind to cause this would be poorly connected power supply caps. 

[RW]

I'll measure again. I'll be home in 30 min. Does it matter if I use safety, or signal ground when I take this measurement?

[Paul Birkeland]

Signal ground is better.

300mV of signal on sensitive headphones would be audible from about 3 feet away, so do have a careful look at the units.

-PB

[RW]

I just measured the headphone jack again. It's definitely 350 mv AC on my meter. I get .3 mv DC. Unfortunately my other meter doesn't read that low, so I can't confirm it.

[RW]

This may sound like a silly question, but I'll ask anyway. Am I supposed to have continuity between all of the conductors on the output of the impedance switches, to the headphone jack? If I test the Jack and have one probe on black, I'll get continuity when I hit red or white. It doesn't matter what color I keep the probe on, the other two will beep when touched. Same with the impedance switches. If I keep one probe on the positive terminal of the impedance switch, I can hit the other 3 outputs and it will show continuity. But, headphones work fine. I'm looking at the schematic and I don't see where the two should cross (or four in this case)

[Paul Birkeland]

I just measured the headphone jack again. It's definitely 350 mv AC on my meter. I get .3 mv DC. Unfortunately my other meter doesn't read that low, so I can't confirm it.

Can you post a photo of your meter grabbing this reading?  It doesn't seem like a reliable measurement, as your sensitive headphones wouldn't handle that much voltage. 

[Paul Birkeland]

This may sound like a silly question, but I'll ask anyway. Am I supposed to have continuity between all of the conductors on the output of the impedance switches, to the headphone jack?

Yes, the output transformer secondaries will show continuity.

[RW]

I'll post a pic of the measurement when I get home from work. ~7pm Est

[RW]

Here are a few pics. Using terminal 13 for ground. There's an AC, and a DC measurement for each lead.

[Doc B.]

I think there is something in the way the measurement is being done that is throwing off the result, simply because 300mV of hum on the output would make the amp un-listenable. Your description is of hum that is there enough to bother you, but not something that completely overwhelms the music, so I will assume that the hum you are hearing is a couple orders of magnitude lower than 300mV.

What we often find when trying to do AC measurements that that the test leads themselves can pick up stray hum. What I would suggest is to make that AC measurement again, with the red and black test leads twisted together (this helps to reject interference) and with the black test lead clipped to the negative terminal of the headphone jack, right next to the red test lead clipped to one of the positive headphone jack terminals. 

[mcandmar]

There is no ground wire from the headphone socket to 23U so the secondary's are floating. See top paragraph of page 29 in the manual.