Left channel is out [resolved]

[Skipperrik]

It all started when I noticed that the music sounded veiled for the first 45 minutes or so after turning the amp on and then suddenly it opened up and became 3-dimensional, like it's supposed to sound. Swapping tubes didn't cure it so I suspected there was a cold joint somewhere. I carefully inspected every joint and didn't find any that looked troublesome.

I decided that while I had it upside down I would trim some of the leads on the 220uF, 250V capacitors that I had left a little too long. That went smoothly and I didn't identify any other potential problems.

I reconnected the amp for a test run and there was no sound from the left channel. I determined that it wasn't the headphone cable or the tubes, and switching to different inputs had no effect. I put it back on the workbench and discovered that none of the LEDs were lit on one of the High Current C4S boards and only two were lit on the low current board. Additionally, only one of the LEDs was lit on the input tube. Thinking it was an issue with the capacitors I had worked on, I began checking voltages around them to see what was going on. Unfortunately, I shorted the lead of the 270Ω resistor that attaches to terminal 24L. I checked the resistance and it measures 264Ω but the capacitor that attaches to that terminal could have been damaged. Downstream, I noticed that one of the LEDs on the C4S board is no longer lit.

Here are the voltage readings:

Low current board with 2 LEDs lit
IA  125.2
OA  73
KregA  4.68
bRegA  125
IB  0
OB  110.1
KregB  1.16
bRegB  0

High current board with 3 LEDs lit
IA  197.1
OA  124.8
bA  0.4
IB  0.3
OB  78
bB  124.5

High current board with no LEDs lit
IA  211
OA  209
bA  0
IB  0.1
OB  1.385
bB  209

I suspect the problem is the 220uF capacitors but don't know how to check them. Any help diagnosing the problems would be greatly appreciated!

[Paul Birkeland]

I suspect to the wire to OB on the center board is broken or not well connected on one end.

[Skipperrik]

The wire from OB to 14U checked for continuity but I re-flowed the connections at each end and that seems to have taken care of the problem. All of the LEDs came to life.

One of the LEDs on one of the high current boards is still out. I re-flowed all of the wire connections coming into it and swapped tubes. Nothing had any effect. The voltages check the same as before.

[Paul Birkeland]

If a bunch of LEDs came to life, voltages will change and that would be good information to post.

[Skipperrik]

High current with one LED out
IA  191.7
OA  118.1
bA  0.4
IB  0.4
OB  76.8
bB  118.1

High current with all LEDs burning
IA  185.7
OA  150.1
bA  0.2
IB  0.3
OB  83.2
bB  150.1

Low current
IA  117.4
OA  73
KregA  5.04
bRegA  118
IB  150.1
OB  72.4
KregB  3.72
bRegB  150

[Paul Birkeland]

It's possible to remove the connections to the center board and rotate it 180 degrees.  If bRegA remains the same, that would be one direction to go in, and if the bad voltage moves to bRegB, that would be a different set of recommendations.

[Skipperrik]

Okay, the board has been turned 180 degrees and everything has been reconnected. The same LED on the high current board is not burning.

Here are the new values:

High current with one LED out
IA  196.2
OA  149.8
bA  0.5
IB  0.4
OB  82.7
bB  149.8

High current with all LEDs burning
IA  192.3
OA  177.9
bA  0.3
IB  0.3
OB  88.2
bB  0.3

Low current (center board
IA  177.3
OA  72.5
KregA  6.44
bRegA  177.3
IB  149.7
OB  72.7
KregB  8.66
bRegB  149.7

[Paul Birkeland]

Low current (center board)
IA  177.3
IB  149.7

Going from 117 to 177 wasn't expected. 

I would be looking for loose connections on the 7 pin socket and the wiring to the board and on the board itself on the high current C4S board with the LED that's not glowing.  You've verified that the center board is working properly and the 12A7 socket is wired properly, so you're left with the 6080 socket wiring and the 6AQ5 regulator tube and high current C4S wiring.  If you pull the 6AQ5 on the offending side, what OA and OB voltages do you get on the high current C4S board on that side?

[Skipperrik]

OA  195.6
OB  91.4

The LED next to the one that wasn't burning is now not lit.

[Paul Birkeland]

Ok, so this tells us that the high current C4S board on that side would like to operate properly.

What I would be looking at next really carefully would be the resistor attached to pin 1 on the 7 pin socket under that board, as well as the resistor between pins 5 and 6 on that same socket.  One of those that's loose or broken will make some issues like this.

[Skipperrik]

Pin 1  214
Pins 5 & 6  236

The problem is on the board above the B socket. I got the same resistance readings on the D socket.

I re-flowed the connections at the resistors as well as all of the pin connections at the B socket and the C socket. There was no change in the resistance values and the LED is still out.

Could it be a transistor issue?

[Paul Birkeland]

If the problem was on the board above the socket, I would expect more issues when you run the amp without the 6AQ5 installed that sit above it.

You can check by swapping the high current C4S boards from side to side.

[Skipperrik]

I'm getting sound out of the wounded side but at a diminished volume. Does that tell you anything?

[Paul Birkeland]

No, listening to an amp that doesn't pass its voltage check is never a good idea.

[Skipperrik]

Okay, it's a lot of work to swap the boards but I'll give it a shot.