S.E.X. 3.0 kit with C4S no sound right channel, voltages off on C4S board

[Wei]

Hello!

I tried searching the forum for a similar issue but I could not find it, and I'm hoping for some feedback.

I put together a very nice S.E.X. kit with C4S some time ago, which worked perfectly. I had to put it into storage for a while, and recently dug it out again with a new opportunity to use it again. My joy turned into chagrin when I found the right channel was not producing sound (and have tried swapping cables, speakers, input, etc) and have isolated the problem to the circuit of the right channel.

My resistances were as expected according to the manual excepting:
T30: 200 ohm
T31: 200 ohm
T32: 200 ohm

H2: 200 ohm
H4: 200 ohm
H5: 200 ohm
H6: 5.7K ohm
H7: 200 ohm

With tubes in, voltages were expected, except:
T2: 400V
T4: 11mV on startup, which went to 140mV after a few minutes
T18: 11mV (or is that close enough to zero, to be considered normal?)

I also noted two LEDs out on the C4S board.
OA and IA are both 400V
IB is 385V
OB is 70V

Thinking this could be isolated to the C4S board, I re-soldered the pins of the transistors and resistors on the board. The resistors are still intact and reading 300K and 500 respectively.
I also re-soldered the pins where I saw the resistance and voltage deviations.
Tubes are both glowing and qualitatively I don't appreciate any visual difference.
But the situation is still the same.

Any help would be appreciated!

[Paul Birkeland]

Can you post some build photos?

The IA/OA voltages being the same just indicates that no current is being drawn in the first stage on that channel.  It doesn't actually mean anything about the operation of the C4S, and the only solder joint I would reflow would be the center leg on Q2.  If you reflowed the solder joints for the LEDs, that can destroy the LEDs.

Could you let me know what DC voltages you see on pin 9 for each tube socket?

[Wei]

Hi PB,

Thanks so much for the quick reply! I've been out of town and now just returned to take the measurements.... and miraculously both channels are working right now, both sets of C4S LEDs are lit. So the pin9 DC voltages are 2.25v for each. But that's not helpful for diagnosis now. Naturally these things fix themselves as soon as one asks for help, but I'll take it and now enjoying the sweet music!

I'm assuming the most likely cause is a tenuous solder joint somewhere.
I wanted to still post the pics I had from before I left, and in the hopes that this could help anyone else with the same problem, and if you happen to spy a bad joint that I should target.

Thanks,
Wei

[Paul Birkeland]

If you have a soldering iron with an adjustable temperature, turn it all the way up.  If you don't have an adjustable temperature soldering iron, then the iron you have just doesn't get hot enough.