S.E.X. 3.0 C4S - No Sound Left Channel After Spark Show [resolved]

[TurbOSquiD77]

Hello, it's been a while!

6DN7 Conversion - success
C4S Installation - success

Sounds great! Functioned as it should for quite some time. Then sparks and smoke came from underneath the chassis plate. The resistor from terminal 35 looks like it was touching, or very close to, terminal 12 of the PT-10. I'm unsure if this is what caused the issue, but the lead looked pretty long, so I snipped it.

Here are the symptoms after snipping the lead:
Left Channel has no sound.
Swapping left and right RCA's from the DAC source results in the same issue with the left channel.
Swapping tubes results in the same issue with the left channel.
 
Here are the measurements after snipping the lead:
Terminal Voltage (DC)
2      64.9V
3      17.18V
4      9.6mv
5      0mv
6      369V
7      394V
10      372V
11      17.27V
12      .1mV
13      394V
16      -1.064V
17      1.6mV
18      -.559V
19      0mv
20      -.255V
21      -.240V
24      -.3V
25      1.5mV
26      0mV
27      -.336V

H2/H5      3.06V/3.06V
H4/H7      -2.9V/-2.9V

OA  65.5
IA    364V
IB    .75V
OB  -1.1V

Thank you,

-T

[Paul Birkeland]

You have no high voltage at IB.

What voltages do you get at 21 and 20?

[TurbOSquiD77]

You have no high voltage at IB.

What voltages do you get at 21 and 20?

Hey PB how's it going?

I read approximately .44V at terminals 21/20.

I apologize, those measurements I originally posted must be from before the incident. I could have sworn I took them after.

[Paul Birkeland]

The 680 ohm resistor feeding those terminals is likely open or not properly soldered.

[TurbOSquiD77]

Thanks, Paul.

I've updated the measurements in my original post. Will try resoldering first, and see what happens.

[TurbOSquiD77]

Clean/resolder/clean = similar voltages.

Noticed that the upper 680 ohm resistor blue stripe is, well, not blue anymore. The leads are also looking a bit fried. Will replace.

Based on the measurements, what is the likelihood other parts were damaged by the incident?

[Paul Birkeland]

You can turn the SEX off and wait till the high DC voltage is bled off, then measure the resistance of the 680 ohm resistor.  You must have normal DC voltage on the voltage doubler side, and we know you have no voltage on the other.  This can only happen if the resistor is open (what I think is going on) or if there is a short after the resistor (then it will burn itself up until it's open).

Your meter will confirm the condition of the resistor.

[TurbOSquiD77]

Finally got around to fixing this. A new 680-ohm resistor did the trick. Thank you!