SR circuit install (resolved)

[Jamier]

I attempted the SR install this morning. 1A, 0A and KregA were all in line.1B was good but 0B and KregB were way off. KregB was 1.06 V and 0B was at 0.0 V. Any advice is appreciated. Oh, also the LEDs on the  back half of the C4S board do not light up.

Jamie

[Jamier]

Here are pictures of my board, front and back. Are there any basic tests that I can perform on the transistors  or the regulators with a multimeter that will give you useful information? The A side seemed
to be functioning properly. The B side was where the problem seemed to be. IB seems right at 395V, but with 0V at 0B, no current is reaching board output, right? And,that's why the LEDs on the back half of the
C4s board were not lighting up, I think.I checked all the resistor values when I built the board, so that leads me to believe that the problem must be in one of the transistors or the regulator (or the Dayton Cap, but probably not likely). The only other possibility is that I had the board connected to the rest of the amp circuit in error, but I checked it  for accuracy several times before the Voltage test. Your help is much
appreciated.

Jamie

[Paul Birkeland]

The two main culprits of the issue you're describing are flaky solder joints (I see one or two on the A side that I would reheat), and swapped transistors.  The PN2907 and TL431A look very similar, so you'll want to triple check that they are installed correctly.

Beyond that, there is also the potential of a short dragging the voltage at OB down to 0V, which would result from miswired jumpers (yours look fine), a miswire at the socket, or a shorted half of a 12AT7 (You can swap 12AT7's to see if anything changes).

Things that are very unlikely:

1.  A failed 0.1uF cap - this will cause the 220 Ohm resistor in series with this cap to burn up and explode.
2.  A failed transistor - this is incredibly unlikely.  Sometimes folks will replace transistors and bring things back to life, but the very act of resoldering the first transistor would give the same results. 
3.  A failed regulator - If the TL431 shorted out, it wouldn't cause a 0V condition at OB, you would get 0V at Kreg and low voltage at OB.

[Jamier]

Thankyou for the reply. I'll reheat the joints and swap the input tubes.I thought I was real careful about the transistors but now you have me wondering. Would transposing the regulator
and the PN 2907A result in damage to either (or both)? I'm away for a few days but I can get back to it on Friday.

Jamie

[Paul Birkeland]

You can measure DC resistance between each pair of legs on each part to look for shorts to see if they are damaged.  I would suspect that you could swap these parts back and they should still operate properly.

-PB

[Jamier]

I reheated the solder joints at the regulator and transistors and reinstalled the board. That did it.The voltages came in where they should be. Breakin it in now. Thanks for the help PB.

Jamie

[Paul Birkeland]

Awesome! Let us know how you like it.

-PB

[Jamier]

I have about 6 hrs on the SR upgrade. This is amazing! Doc's description of the circuit's effect, increases bandwith and dynamics, is too modest. I get the bandwidth part; the amp does now have greater range. The really impressive part is the control of the image.  Voices and instruments are now more discretely placed and the SII seems to have a deathgrip on those woofers. How is that possible? Well, at any rate, for those with SIIs or those considering one, this is a must have!