Issue with A Side C4S Board A [resolved]

[giggsy]

Hi,

Have an issue with the voltage through 32U after attaching the board.

Let me preface by saying I accidentally broke the 215 1/8 watt resistor from A3 to the center socket pin when working on A4. I had a replacement resistor to hand but although the markings are the same, the size of the resistor is larger than the ones supplied (see pic) so it could simply be that i used the wrong resistor.

My issue is the voltage at 32U measures about 98V. By turning the potentiometer I can get it up to 100V.
On the D side 46U initially read 158V but was easily adjusted to 175V.

Here are some readings I took

        A Side    D Side
IA        508v      508v
OA      295v      296v

32U      ~98v   
46U      158v adjusted up to 175v

On A board
Kreg on A side = 6.88v
Kreg on B side  = 6.75v
voltage across 4.99k Resistor = 2.47v

 

[Paul Birkeland]

Turn the blue trim pot ten turns clockwise, then measure Kreg A.  Now turn the trim pot ten turns counter-clockwise and remeasure the voltage on Kreg A.  Let us know what those voltages are.

 

[giggsy]

Turning 10x clockwise, kreg went down to 4.95v.
Turning back 10x counter-clockwise took the kreg back to 6.88v

 

[Paul Birkeland]

Can you post a photo of the top of the board in question?

 

[giggsy]

Thanks for helping Paul.
I've attached some pics of the board.

 

[Paul Birkeland]

I would try reflowing the solder joints on the MJE5731A transistors.  You'll know when they are heated enough because the solder will wick through the board to the other side. 

 

[giggsy]

I reflowed the solder on all the transistors. Still getting the same reading.

 

[Paul Birkeland]

If the blue trim pot is turned to be 0 ohms, you'll get the voltage that's across the R3 resistor that's next to it. You have reported this as 2.47V, but you haven't gotten that at your Kreg output.

I suspect either the Kreg to Breg wire has an issue or you aren't turning the pot ten full turns.

 

[giggsy]

You are quite right to question my counting skills  but my screwdriver has marking on one side so pretty sure I did it right this time.
I redid just in case but got the same result.

The reading at BREG and KREG are the same so guessing the connection is ok.

Is there anything to be gained from switching the boards over?

 

[Paul Birkeland]

Sure, you could swap the boards side to side to see if the issue follows one of the boards.

What I would suggest though is to set your meter to read impedance, then attempt to adjust the blue trim pot so you see 0 ohms between Kreg and the "R" pad on the 431.  You can do this with the amplifier off.

 

[giggsy]

The ohm reading between the R side of 431 and KREG was 696. I was able to adjust it down to 0.3ohms but not all the way to zero.

 

[Paul Birkeland]

OK, 0.3 ohms is plenty close to 0, that 0.3 is just your meter leads.

If you power the amp up with the pot set that low, you should see 2.5V coming out of Kreg, can you confirm that?

 

[giggsy]

Reading at KREG is 2.476v

 

[Paul Birkeland]

OK, what's OB?

 

[giggsy]

OB is 96.5v

 

[Paul Birkeland]

OK, now turn the pot up so that same measurement shows 10K instead of 0.  Power up the amp and measure Kreg again and OB.

 

[giggsy]

It got up to 9.6Kohms.
OB reading is only 102v

 

[giggsy]

OB reading is only 102v
KREG is 7.27v

 

[Paul Birkeland]

I would reflow the MJE5731A and PN2907 joints on that board.  Could you also provide a DC resistance between ground and the center pin of that 9 pin socket?

 

[giggsy]

I am not getting any DC resistance between ground and the center pin.
I am getting 212ohm reading between A3 and center pin.

On the D side I get a 4.9ohm reading between the ground and center pin and 314ohm between D7 and the center pin.