C4S Upgrade Issue: Voltage on OB terminal

[pbvoorhees]

I just completed the C4S upgrade on my SEX 3.0 kit.
Prior to the upgrade, I listened to the SEX regularly for the last 8 months. All worked fine. No issues.
The current problems are:
1. Only 2 LED's light up. The two LED's that do NOT light up are on the B side. And,
2. The voltages are off on the OB terminal. The reading is -0.58

LED's on the A side both light up and the OA terminal voltage reads 86v.

I am looking for troubleshooting advice on how to resolve the issues listed above.

Thank you.
Pieter

 

[Paul Birkeland]

What is the IB voltage? Can you post some photos of the build?

 

[pbvoorhees]

Thank you for the quick reply!

The IB voltage read 69v on the initial reading. It read lower on the second reading: 53v

In addition, I tested each terminal. The readings were:
OA: 85.2v
1A: 371.6v
B-A/B: 0.2
1B: 69v (1st time) and then 53v (2nd time)
OB: -0.57

Here are some photos:

 

[Paul Birkeland]

The first big issue I see here is soldering. There's either lead free solder being used or your iron is way too cool. I'd reheat all of the solder joints on the bottom of the board other than the LEDs and really let that solder flow out.

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The red wire going to IB is either broken or not well enough soldered to properly function. Do double check that you see 350-400V at the terminal where it originates.

 

[pbvoorhees]

To answer your questions above, I am using Kester 60/40 solder. My Hakko soldering station is set to 700 degrees.

Following your instructions: I resoldered all the B side (except for the LED's) from the bottom. I left the A side 'as is' since the voltage readings were in line.

Also, I decided to just replace the red wire going to 1B. it did not appear broken, but who knows . . .

When I took the red wire off of terminal 20, there was a small piece of the lead from the removed 150K resistor that remained behind from when I clipped it for the C4S install. I removed this 1/4 inch bit.

After re-installing the red wire, I did a voltage test and my numbers were roughly the same as the previous post. And the terminal 20 voltage only read 134 volts.

Still no good.

I decided that I needed to resolder the red wire and the 450v capacitor at terminal 20. This is when I got bizarre readings. 1.19 volts at 1B and -403 volts at 0B (I don't even understand how a negative 403 volt reading is possible). I was ready to give up and come back another day.

As a last resort, I wanted to take a reading at terminal 6 and check the voltage there. I powered up the S.E.X again and as I was taking the reading at terminal 6, I heard small crackling and I may or may not have seen the tiniest spark at terminal 20. Concerning . . .

Terminal 6 tested 347 volts.

The crackling/spark at terminal 20 stopped as quickly as it began, so tested it. Terminal 20 tested at 346 volts.

I also noticed that all four LED's now lit up.

My readings on the board are as follows:
0A: 88v
1A: 347v
B-A: 0
1B: 346v
0B: 73.8v

I don't fully understand all happened but the C4S is now installed, the voltage numbers are in the correct range, and I am happily back to listening my S.E.X. amp.

PB: Thank you for your guidance/suggestions on the C4S install. It was very helpful.

 

[Paul Birkeland]

The negative voltage reading usually indicates a problem with a ground wire not being well soldered and providing high resistance. I'd go through the stock build and reflow all the solder joints around the 12 pin sockets just to be extra certain this doesn't come back.