C4S Voltage Problem

[Adrian]

I installed the C4S upgrade to my S.E.X. and get 0 to -0.1 VDC at OB and pin B5 instead of 75 VDC.
All of the diodes light well and the valve heaters work.
I checked all other voltages and they are all good with the exception of B6 reading 0.1 VDC vice 2.5 VDC.
I removed the board and connections and thoroughly cleaned the board with Isopropyl alcohol.
I carefully reflowed all solder joints and replaced the twisted pair at IB/OB.
All board components are in the correct position and the transistors are correctly aligned with the 2907A transistor bodies not touching the pads.
I reinstalled the unit and reconnected with the same result:  all voltages good with the exception of OB/B5 at about 0 VDC and B6 at about 0 VDC.
I'm not getting anything out of the left channel (OB) - I should be getting the 75 VDC at the plate.

I'm at a loss here.  Could I have a damaged transitor?  Are there other checks (resistance or voltage) I can take to narrow in on the problem?

 

[Adrian]

I've gone through the threads on "C4S Voltaage" and resoldered the center leg to the MJE transistor going to OB. I checked the continuity between the center leg of the MJE transistors and A5/B5 on both channels and I have good continuity to A5 but an open circuit on B5.  I think I have isolated my problem to the connection at the left channel MJE transistor.  I'm going to remove the C4S from the circuit so I can better reconnect the MJE.
Is there anyway to check the transistor to see if it is still good (not damaged from heat, etc.)?

 

[Adrian]

I found that the MJE transistor center leg (collector) for the left channel did not have continuity between it's pad and the OB pad on the board.
I put in a jumper between the 2 pads (picture below).
All resistances and voltages are "spot on" and it sounds great!

 

[Paul Birkeland]

Yeah, 0V at the output is most often that pesky center leg.

While I don't recommend the jumper, I do recommend not using much solder, but heating it until you see it suck into the board a little bit, which means it is wicking along the center leg through the plated hole in the PC board.

 

[Adrian]

PB:  I really tried to not get too much solder.  We'll see how this jumper works. Maybe it will last forever and work great.

 

[Doc B]

who you callin a little bitch?

 

[Adrian]

Paul/Doc: Actually I was tempted to call the PC Board a little bitch.
There was no connection (continuity) between the two pads and I did not want to order another board and strip this one (or maybe just buy a new C4s), so I did it the DIYer way and got the job done.

 

[Paul Birkeland]

Oops, autocorrect strikes again