Channel imbalance on fine attenuator [solved]

[Loquah]

If I experience a channel imbalance when using the -3dB position on the fine attenuator, is it safe to assume that I need to resolder the 2 joints associated with that position?

In all other ways the amp is up and running and sounding great.

[Paul Birkeland]

If it suddenly gets louder in one channel than at -1.5dB, then yes, the resistor is probably not connected on both ends.

If you switch from -1.5dB to -3dB and both channels get quieter, but one much more so, then you probably have some resistors swapped. 

[Loquah]

I've checked that the resistors on alternate sides of the attenuator switch are the same (i.e. the centre contacts connect to identical resistors in each position) and I've reflowed the -3dB solder joints. I still find that there is a slight difference in channels where the right side is slightly louder than the left. Could this be a faulty resistor?

[Paul Birkeland]

You can measure the fine resistors in-circuit, just turn that control to -0dB and all the fine resistors are out of the circuit.

If you can determine which of the situations applies from my last post, we could narrow it down.

-PB

[Loquah]

It actually sounds like the move from -1.5dB to -3dB cuts volume from the left channel, but cuts no volume from the right channel (i.e. the -3dB sounds like -1.5dB in the right channel)

[Paul Birkeland]

Can you pop a photo up of that switch?

[Loquah]

Here's the offending component 

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Ffarm4.staticflickr.com%2F3817%2F11297599115_5f4bdce74e.jpg&hash=ff77234d2655e0985a8108449a52a77e2c3d3cf3)
WP_20131210_09_24_46_Pro by Lachfen, on Flickr

[Paul Birkeland]

Can I see it set on -3dB?

[Loquah]

Sure!

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Ffarm4.staticflickr.com%2F3755%2F11298884523_c2a4f70481.jpg&hash=d16b5a2ff4bb7c0cfe1bb80edc5aea4669c447eb)
WP_20131210_10_55_03_Pro by Lachfen, on Flickr

Unfortunately you can't see all the resistor labels, but I've confirmed that the same spec resistors are on either side of the contacts here (and the surrounding -4.5dB and -1.5dB are also matched on either side).

I haven't taken to the resistors with a test meter as yet to confirm correct resistance

[Doc B.]

I'd suggest measuring the resistance across the input for the right channel and then the left channel at those switch positions that are sounding different and see if the resistance measures differently from channel to channel, e.g., if you have a short between a couple of switch terminals.

[Loquah]

Thanks Doc - I'll try that out as soon as I can drag myself away from my headphones and the Mainline being connected to the power!

[Loquah]

I have a 2262F resistor reading 11.8k instead of 22.6k. I'll email for a new one.

[mcandmar]

Curious the measured value is almost exactly half.  Just double check the pot wasn't on that position when you measured it, i.e. make sure it was out of the circuit.

[Doc B.]

Yes you need to consider what other resistance might be influencing that measurement, that is to say some resistors might not read the printed value when installed on the attenuator beacuse they are part of a more complex resistance created by the interaction of the coarse and fine controls. A potentially more useful approach is to measure the resistance at each resistor on the left channel and then do the same on the right and see if the resistors connected at the terminals where the wiper is touching when you get the mismatched reading is different on each channel. It could be that one of the resistors is somehow shorted out of the circuit by an errant lead that is touching something it shouldn't, or maybe a switch wiper is making a funny contact at that setting. You can measure across each resistor or measure from each terminal on the switch to ground. Just be consistent from side to side. You could also measure the input resistance change as you rotate the switch by simply measuring from the center pin of the rca jack to ground on each channel as you go through the steps. It sounds like the issue is on the fine control so leave the coarse one set all the way up and measure as you step through the fine control.

We're all out of the office today, but tomorrow we could go through and measure the values at various steps to give you a point of reference.

[Paul Birkeland]

I have a 2262F resistor reading 11.8k instead of 22.6k. I'll email for a new one.

If the switch is on that position, there will be a 25K in parallel with the 22.6K.  Be sure the switch is off that position. 

If the switch is on that position, you can check both resistors, they should both read the same value (which won't be 22.6K).