Channel imbalance from only one input [resolved]

[mercury128]

Hi

I just completed my Crackatwoa build. For one of the inputs (closest to the power transformer, RI3 and LI3), there is a severe channel imbalance. If the balance knob is kept in the "middle" position, which is sufficient for the other two inputs, there is almost no sound in the left channel. By turning the balance knob almost all the way counter clockwise I can get equal volume between the two channels, but the volume knob has to be set noticeably higher with this input.

Switching to a different input without changing the balance or volume knobs from the setting for RI3 and LI3 leads to both a much higher volume and channel imbalance, with a higher volume in the left rather than the right channel.

I have tried to troubleshoot by reflowing the RCA ground tabs and center pins at RI3 and LI3, and have also re-flowed the relevant positions on the input selector switch (9 and 12 brown for the left channel, from LI3 and 6 and 3 orange from RI3). I also tried to reflow all of the joints on the balance and volume pots with no luck in fixing the problem.

Also, is it normal operation for the input selection when rotating clockwise to cycle through the inputs from outside (right) to inside (left)? I ask because this ends up with the knob turning in opposite directions of the position of the inputs, so for example when I have input 3, which is on the "left" or "middle" of the chassis being selected, the knob is turned all the way to the most clockwise or "right" or "outside" position.

Any advice appreciated

Thanks
Alex

[Paul Birkeland]

Can you post some pictures of the wiring around your RCA jacks and selector switch?  I'd imagine that there's a soldering or hardware issue on the offending jack.  Since it only happens on one input, this narrows things down to the RCA jacks and wiring between them and the selector switch.

[mercury128]

Thanks Paul. Some pictures at various angles for the selector switch and RCA jacks attached.

[Paul Birkeland]

The center solder joint on the RCA jack in question doesn't look like it has been heated enough to flow out.  I'd be tempted to say that the solid brown wire on the selector switch looks iffy, but technically you probably wouldn't notice any issues with the amp even if that wasn't connected. 

Another thing you can test is to put one meter lead on the outside metal of the left jack, then measure the resistance between that and the center pin.  With the selector switch not on that input, it should be over the limit on your meter.  With that input selected, it will be around 50K or so.

[mercury128]

Thanks Paul. I tried to reflow and reinsert the center wire into the left rca Jack a few times with no luck. I even tried to remove some of the old solder with solder wick and add new solder.

When the input selector is set for these jacks, the resistance between the center pin and the outside metal is 0.3-0.4 ohms, or essentially zero. In contrast, for the right RCA Jack it is measuring 56 kOhms.

[Paul Birkeland]

Remove the wire from the center pin of the jack, then recheck the resistance across the jack.  If it goes OL, then you probably have melted together the brown & brown/white wires where they first twist together on one end, and prying them apart will restore proper operation. 

[mercury128]

Thanks Paul! You called it. No issues at the RCA end, but there was a melted connection between the brown and brown striped wires at the input selector end. Prying them apart has fixed the issue.

Thanks again.