C4S board Problems

[Mordicai]

Had my Sex 2.1, plus C4S board & Output Transformer Switches  for  over a year and just love it. A few weeks ago the Left(black) channel started clicking off and then back on. Yesterday it clicked off and stayed off. I traced into the C4S board where the (B) D-1 LED was barely lit, and the two 150k resistors had blacked areas on their leads and around their leads on the board. They still measured 150.7K. I resoldered the D-1 LED leads and both the 150K resistors leads. Fired it up and it worked fine. After a 30 sec it clicked off, back on, off and then back on. Its been playing for about a half hour with no problems. I assume the blacked leads on the transistors are from a poor connection, but I really don't know enough about all this to assume anything. I live in the country and my AC is 125. Any feedback would be appreciated......Max

[Mordicai]

Played for about an hour and and then went back to right channel only. Play with it again tomorrow.

[Doc B.]

Have you swaped tubes to see if the problem moves with a tube?

[Mordicai]

Yes Doc, Thats the first thing I did. Switched tubes and rca inputs. Even though the slightly fried 150k resistors still test ok, I wonder if they could change when they warm up. Perhaps I need to try replacing them.

[Mordicai]

Is it normal for the 150k transistors  on the C4S board, to have heat blackened leads ?  If so I'll stop worrying about it.

[Doc B.]

No, it is not normal. It would be a sign they have been overstressed with too much current.

[Paul Birkeland]

It would help to know what voltages accompany the problem.  The main voltage you want to measure as at the output of the board on each side (OA and OB), as well as the voltage across the 1000uF capacitor on each tube socket.

-PB

[Mordicai]

Paul, when measuring voltage across he 1000uf just clip on each side? No attachment to ground ?  Or  clip to ground and one side of the condenser?

[Paul Birkeland]

The striped side of the cap is grounded, so you can just measure from ground to the end connected to each tube socket.

-PB

[Mordicai]

Paul the voltage numbers are: OA 66.2 ; OB 56.1 
Voltage across 1000 cap is L 19.4;  R 18.9 

Cleaned the tube sockets for a third time and like before, the headphones played great for a few minute and then the Left track quit. Changing tubes again did not help.

[Paul Birkeland]

Are those the voltages after the left channel faded out? 

[Mordicai]

The amp was only firing on the right channel. I moved it to the bench, flipped it over, plugged it in,turned it on and took those measurements. There was no source plugged in and no headphone attached, so I can only assume it was still only working on one channel. If it would help I can test it tomorrow as its playing while hooked up to the system

[Paul Birkeland]

Yes, you'll want to confirm that the offending channel has dropped out when you take measurements.  A cheap (small) test speaker and some music playing through the SEX from your phone can make this process a lot easier.

-PB

[Mordicai]

Paul, Sorry about that. Wasn't thinking. When the left channel clicks off the voltage at OB is 0 volts. OA is still 66.2. I reflowed the connections at OB with no success. It played for  1/2 hour and sounded great, and then started clicking on and off and finally just off. Again the only obvious problems with that B side of the board is the black burned ends where the 150K attach to the board. Both still measure 150. The 499 ohm resistor  on the B side has slightly burned ends. The 499 on the A side is still bright and shiny. Both measure 499.
 

[Paul Birkeland]

Does the voltage across the 1000uF cap on that channel also drop to 0V when this happens?