C4S - Wrong voltage on one side [resolved]

[Yosh]

I just tried to integrate the C4S kit into my S.E.X. One side (A) is working fine with 75V on OA and both LEDs lit up. The B side is the problem. The voltage at OB reads 385V, none of the two LEDs on that side is illuminated. I double-checked all solder joints and tried again. Same reading.

Any suggestions much appreciated.

[Paul Birkeland]

I would do two things:

1.  Resolder the center leg of Q2.  This leg connects to the tab of the transistor, so it can sink a lot of heat. 

2.  Post some photos of your installation.

-PB

[Yosh]

Thanks, Paul. I did resolder the center leg. I'm using a 60/40 solder wire with flux rosin core. It looks like a good connection to me, the situation is unchanged however. Could the transistor be faulty?

I am attaching two pics - from the top and below.

Thanks.

[Yosh]

Here the pics

[grufti]

This response is more general and not directly to your question. The answer might fix your problem anyway. You need a good sidecutter. You have wire ends sticking out in many places that need clipping. Your problem description doesn't sound like those would be the cause here, but it's very easy to get a short from stray wires.

Your center leg of Q2 on the B side might still need more heat even now.

[Paul Birkeland]

I'd also say that one of the 249K resistors looks like it could be pulled out/is loose.  There may be issues in the underlying build that are causing the issues you're experiencing.  Your board looks great to me.

[Yosh]

Hi --

Thanks for staying on it and trying to help me figure it out.

Nothing can get pulled out. I have used the high flux solder wire you recommended. I am very sure there is enough heat as I see the solder is properly spreading and running along the leg to the other side of the circuit board. That's true for the resistor you mention, Paul, as is for the middle leg of Q2 which I have heated up long enough to see how the little bit of solder that made it to the upside of the circuit board is melting. I have tested for shorts or solder connections - but there are none.

I understand that a lot of issues arise due to improper soldering. I am quite sure, though, that the solder joints are solid.

What else could I try?

Thanks again.

[Paul Birkeland]

Temporarily put the 150K resistor back in on that side and remeasure.

[Yosh]

With or without the original 150kOhm resistor, I am getting 385V at OB.

But I found something strange: I wanted to check the soldering the middle leg of Q2 (which connects straight to OB on the circuit board). I tested with a pointed probe and get 385V on the middle leg. No surprise. I get that at the bottom soldering joint but also directly measured on the leg on the top side or touching the heat sink plate of the resistor (which I understand are both part of the transistor's collector). All good. However, I found the base and the emitter (pin 1 and 3 of the MJE5731) ALSO read 385V.

When I do that on the left side that's working, I am getting 75V on OA and then around 375V on the emitter and base. While I don't understand a CCS in-depth, I would expect that behavior but not that all pins reading the same voltage as is on the non-functioning B side.

I double-checked again for soldering bridges (with magnifying glass and scraping between contacts) - looks good to me. Makes me wonder about either the board or the transistor.

[Paul Birkeland]

With or without the original 150kOhm resistor, I am getting 385V at OB.

So to use the 150K resistor, OB needs to be disconnected from the circuit, then the 150K resistor can be temporarily soldered back into the circuit.

If that is what you did, then there's an issue with the original build, and some tighter photos of the wiring for that channel may reveal what's gone wrong.

[Yosh]

I hadn't disconnected OB - I had wondered but wasn't sure with the instructions in your prior response. I should have asked. Let me try

[Yosh]

So reconnected the 150kOhm resistor and disconnected OB. I am still getting 385V on OB and all three pins of that transistor for that matter.

I measured resistance between pins and solder joints. These contacts are fine.

When I measure resistance between the pins, I get 777Ohm between pin 1 and 3 (or out of range if I change polarity) - and no connection between pin 2 and 1 or 3 - so definitely no short. Also both sides’ transistors measure the same making me think the transistor on the B side is good.

The kit is working fine with the 150k resistors. So that build is good.

I’m sending high res photos of the C4S top and bottom. Or do you want another pic? 

[Paul Birkeland]

So reconnected the 150kOhm resistor and disconnected OB. I am still getting 385V on OB and all three pins of that transistor for that matter.

OK, I'm obviously not communicating this well enough.  When you disconnect the C4S on that side, you need to disconnect the OB wire, then put the 150K resistor back, then measure the voltage on the side of the 150K resistor with the 0.1uF 400V cap (which is also where the OB wire would've gone).  When you have that half of the C4S disconnected from the circuit, there's no point to measure the voltages on that half of the C4S... it's disconnected.

Instead you are looking to see ~370V on one side of the 150K resistor and about 70V on the other side.  If you have the same voltage on each end of the 150K resistor when it's put back and installed in the SEX amp, then you have an issue in the build that has absolutely nothing to do with the C4S board, and you'll need to do a little more work to resolve that first, then you can put the C4S back in.

The kit is working fine with the 150k resistors. So that build is good.

This would seem to contradict what I'm reading above.  Did you put both 150K resistors back in?  Now I'm completely confused.

[Yosh]

The OB cable was disconnected but I had misunderstood you wanted me to measure at OB on the C4S circuit board.

I think I understand you now. You want me to do a voltage check on the ü modified B side With the original resistor.

I did. I’m getting 390V at lug 20 and 345 V at 16.

That’s wrong. I remember checking these voltages as part of the SEX assembly. They checked out and I used the amplifier for a month before adding the C4S. So something must have happened when I removed the plate load resistor.

I redid the other voltage tests and the values are on target - but not 16.

[Paul Birkeland]

What's the voltage at pin 9 on the 12 pin socket on the bad side?