Channel imbalance [resolved]

[NightFlight]

After not listening for about a month or so... (yikes) I came back fresh and immediately noticed a channel imbalance which sweeps right to left in the fine volume attenuator as the attenuation is decreased. Obvious once you are aware of it.  Anyone else notice?

 

[Doc B.]

You can measure the resistance of each step on each channel. Try doing that and report back any differences you measure between left and right. The most likely cause would be swapping resistors on one side.

[NightFlight]

Hey thanks Doc. Finally got around to checking it out.  As it turns out my one 1372F (13.7K) is reading 8.86K.

[NightFlight]

Checking the my coarse attenuator with reference to ground:

• Points 12/6 - 139 ohms
• Points 11/5 - 390 ohms
• Poitns 10/4 - ~1.1k/1.07k     ohms
• points   9/3 - ~3.15k/2.89k   ohms
• points   8/2 - ~8.9k/6.87k     ohms
• points   7/1 - ~25.12k/8.86k ohms

The labelling all seems right.

My 16.2 is reading around 8K on the one side (across points 1,2). The 5.76K reads 4.7K (across points 2,3). The 2.05K reads about 100ohms low across points 3,4. Everything on the other channel seems fine.

Very weird. I'm flummoxed as to what might have done this. I'm fairly certain I metered them during the build.

[Paul Birkeland]

Try turning the fine attenuator all the way up, then recheck.

With the fine attenuator all the way up (and nothing plugged into the RCA jacks), you should be able to measure the resistors a bit better.

For what it's worth, we do not carry a 4.7K RN55D resistor in our inventory, so I suspect something else is messing with the readings. 

[NightFlight]

Well setting the fine attenuator to 0db while measuring was the issue. Everything checks out. But that leaves me scratching my head a bit.

There's no change through the positions on the coarse attenuator.

By ear with some mono test tracks I'm able to confidently say that positions -7.5db,-6.0db,-4.5db all sound slightly right shifted. -3.0db, -1.5db and 0db sound centred. I had been thinking that -3.0db to 0db sounded left shifted, but it goes to show I'd become adjusted to the imbalance over time, and that it was only time away that let me notice it. Now I can't un-notice it.

Steps to isolation would be to resolder the fine attenuator, failing that replace the resistors?

[NightFlight]

Well, that's embarrassing...

Apparently I missed 6L and it wasn't soldered thus no proper contact.  All fixed.

... and thanks for the help.