Pioneer SX 3800 dead channel

[jefferylhenr]

I have an SX3800 that was a total mess and dead as a door nail. Got everything addressed and working but some odd behavior with the channel balance. The left channel gain was lower than the right. Then it got worse and went dead except with loudness engaged. No sound on normal from left channel then good sound (except a slight gain difference) engaged. I have measured the loudness switch and it’s good and solid.

Now I have no sound left with loudness engaged on the left or not, and the right channel is way over normal gain.

Is this just a bad balance pot? I am going to measure it today. What else should I be looking at?

Btw way, both amps are fine and drive normally with a separate preamp.

 

[zebulon1]

Great TS on the separation technique, finding the preamp inop.
That saves some troubleshooting.
I was worried at first for the amps.
The Balance pot is the first component in the circuit chain. Cleaning it would be the first line of attack.
Second would be checking for a line level signal before it and after it.
Have you read other posts on the use of a DMM in checking for a signal? A signal tracer works better if you have one.
Does the channel come back when switching from Tone to Defeat? On this receiver. Its "On" Or "Off"

 

[zebulon1]

Looking at the scat closer a really good cleaning and lube on all the switches and pots is probably in order.

 

[jefferylhenr]

Looking at the scat closer a really good cleaning and lube on all the switches and pots is probably in order.

Everything has been cleaned multiple times. I just pulled the balance control, bent the tabs and opened it up. Couldn’t see anything wrong. So here is the situation now, balance 99% left and muting on and it’s playing almost normal on both channels? So me thinks the balance control is a prime suspect?

 

[zebulon1]

Mute off?
Weird.

 

[jefferylhenr]

Without mute on … way over gain Al left channel zilch. It started with the loudness. As you can see, switches, resistors and some low value film caps all in an inter dependent circuit.

 

[painttoad]

zeb suggested a signal tracer, i'll 2nd it.

 

[zebulon1]

I don’t know. Something is weird.
I might pull a pair of coupling caps and trace the signals Before it gets to the tone control. Narrow it down. A pot can be checked with an ohm meter.
Any Gain over normal levels spells trouble. Im not sure what to think.
Maybe a few more posts so I can get my head around it.

 

[markthefixer]

pull the balance pot out, see how circuit operates.

get clip test leads and ohmmeter. take lots of on bench balance control readings...

 

[jefferylhenr]

pull the balance pot out, see how circuit operates.

get clip test leads and ohmmeter. take lots of on bench balance control readings...

I did pull the balance pot and even opened it and examined the traces and measured at all points of the rotation. As for a signal tracer, there is a signal it’s just not as it should be. Other than the pots and switches in this circuit, it’s resistors and low value film caps. If the balance pot is okay, as it appears visually and with the ohm meter and the switches have all been confirmed to be working after numerous cleanings, flushings and treatments with d5 and f5… what’s left, caps and resistors. Maybe it’s the volume pot? As with mute engaged, and balance highly favoring the left channel we get a somewhat balanced output at 9:00. When I increase the gain, I need to move the balance pot further towards the right channel or I get too much gain on the left which somewhat indicates that the balance is working.

Was hoping someone understood the circuit well enough to narrow down the search.

I found a salvaged balance pot, but wondering now that it could be volume?

I should have written the balance pot measurements down, but didn’t. I can tell you however they were similar on both sides with rotation out of circuit. It’s kind of a major project getting these switches and pots out as you have to pull the entire face off to get to them.

 

[jefferylhenr]