Sx1000tw slightly lower left output ?

anderslober

Yikes...not again!
I have a very nice sx1000tw here with slightly lower output on the left. When I turn volume up it gets louder but also starts crackling.
Funny thing is when I turn on the unit....output is equal for a couple of seconds, then the left goes down a bit.
Unit is cleaned/deoxited and fully recapped. All tr's on the control amp/head amp and main amp have been changed to new(exactly as my other fully working unit). What could this be?
Could the big 0.7ohm resistors be the cause?
 
I have been measuring the strength of signals going IN and OUT of the control amp board. When the signal leaves the VOL pot to enter the board, it is equal. When the signal leaves the board at pin8......it isn't equal. Lower on LEFT. I have checked the signals going into and out of the BASS/TREBLE and BAL pots too. BASS pot seems fairly equal.......TREBLE pot does not......of course BAL is not equal.
I have checked the HIGH/LOW filter switches too....they are clean and OK. The signal is lower on the LEFT going IN and OUT of the switches. So, the HIGH/LOW switches are not at fault.

So....what could cause this? With increasing volume , the LEFT sound gets very scratchy.......RIGHT sound is nice and clear.

The tr's on the control amp board are new ksc1845's. Could a resistor be failing?

Any ideas would be great.....thanks.
 
Am I looking in the right place? Could something on the amp board 'drag' down the LEFT channel sound?
 
see if any transistors in that area are getting hot.

check the signal levels at the junction of:
C613 -
R635
C617+

and

C614 -
R636
C618+

Before they are fed to the tone controls. THAT is the point of the output level of the control amp that is UNAFFECTED by the tone controls. IF those levels are the same, you are looking at the tone controls and filters, if they are different, then the active circuitry is to blame. THEN you start taking transistor voltage readings.

There ARE some 0.2uf caps that are APPARENTLY not on the control amp board, but rather connected between pins 9, S7 and the balance control, are they new? They may be polarized etc... and need replacement.


The high / low filters CAN affect the overall levels, because they form a resistance network with the tone control pots.

You need to verify the OHMS of those switches, preferably disconnected.

Take it easy on worrying about my posting timing, I'm posting a LOT slower lately. Outside world stuff keeping me busy and distracted (and hurting my hands)..
 
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Mark.....first of all...get well soon.......don't stress about these minor amp problems.
If you have time......I have measured the strength of the signals going in and out of the tone control board.
There are 10 pins on each side. I compared each pin to the corresponding pin on the other side.
The only pins with less strength were pin8 and pin10 on the left. These seem to be directly connected to the treble pot..........it has been deoxited multiple times. So its clean.
I also checked the two 0.2uf caps.....both sides are equal in sound....so they are ok........
I will check the junction that you mentioned earlier. Caps and tr's are new.
 
You may want to ohm out the treble control to see if the two sides match.
It may be clean but shot.
 
WWOOWW.....you read my mind...scary. I was again using my trusty signal tracer...tracing all the LEFT signals in and out of everywhere. As I read the scat'..pin5 on the tone board delivers signal to the treble pot......this signal is 'divided' at the treble pot.....some goes back to pin8 on the tone board.....the other part of the signal goes to pin10 GND. BOTH pin 8 and 10 have distorted sound.......this distorted sound MUST come 'out' of the treble pot.....so, yes
my treble pot could be the culprit.
On my unit the two(scat value) 0.2uf mylar caps, are in fact 0,047 mylar caps. They sit directly on the low filter switch......sound going in and out on the left side was distorted...this sound comes from treble pot.......so, anybody out there with a dual 100K treble pot?:))))
 
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