Right Channel Issue, Continued...

[Paul Birkeland]

The middle pin of each level of the pot should give you a different AC voltage reading as you turn the pot. 

-PB

[oguinn]

One of these days I'll read slowly and thoroughly enough get your meaning the first time around. I was measuring at the RCA output level instead of the pot lugs. Here are the updated readings.

Right 50%: 36mV
Left 50%: 34.5mV

Right 100%: 403.4mV
Left 100%: 402.7mV

[Paul Birkeland]

OK, that is good, you have a working pot. 

The next test is to check for channel balance at G1 on each 12 pin socket.  Again, you can do this without the amp on.  This is the AC signal voltage at pin 10 on each 12 pin tube socket.

If those balance, power up the amp and turn the pot all the way up.  With the tone playing check and compare AC voltages at:

Terminal 4 vs. 18

Terminal 3 vs. 17

It also came to my mind that a 1000uF/35V cap that wasn't well connected would give you some pretty terrible performance out of one channel.

-PB

[oguinn]

A10 - 403.6
B10 - 402.7

4 - 6.53
18 - 7.5
3 - 59.2
17 - 89.5

[oguinn]

I also went ahead and touched up that 1000uf/35v cap, which didn’t make a difference.

[Paul Birkeland]

Swap the tubes and repeat the AC tone voltage checks on those last 4 terminals I listed.

-PB

[oguinn]

4 - 7.53
18 - 7.15
3 - 87.6
17 - 107.4

[Paul Birkeland]

Now measure the AC across the binding posts.

[oguinn]

Well. Something’s gone haywire now. The right side is making no noise, and the left side is making a god awful buzzing sound. Voltages were 3.4mv on the right and 402.4 on the left. I assume something is now definitely broken and I should repeat the voltage and resistance checks?

[Paul Birkeland]

Yes, you may have a loose connection somewhere that you've disturbed enough to now be able to locate.

[oguinn]

Voltages and resistances all checked out. A couple of the zero terminals had millivolts, but I assume that's ok and that I am just now noticing them because my meter is more sensitive.

[Paul Birkeland]

In essence, you have a decent amount of signal at terminals 3 and 17.  This signal is stepped down through your output transformers to your speaker posts.  If it isn't getting through, I would focus on the output transformer wiring. 

[oguinn]

Should I just reflow the right channel output transformer joints and then try again? Are there any voltage or resistance checks to tell me whether there's something wrong with the output transformer itself?

[Paul Birkeland]

You did the voltage checks...

You can measure the AC voltage between terminals 5 and 10 on each OT-2 if you like with the signal generator on. 

It is incredibly unlikely that there is an issue with the output transformer (you would be the first person ever to have a defective OT-2).

[oguinn]

Got it. I'll give it a shot.