Smashup A side LEDs don't light up [resolved]

[antoineestephan]

Hi
Just finished my smashup boards and installed them in my Smash preamp
The LEDs on the board on the A side won't light up, those on the C side light up OK.
So did the voltage check and here's what I got
Terminal.                A side.                               C side
IA                           210.                                    209
OA.                          0.                                       148
+reg A side.             2.48.                                   2.48
IB.                           0.                                     148.5
OB.                          -5.                                     61
K reg B side.             2.97 and fluctuating up.        4.40
B reg B side.             0.                                     148.6
+reg B side.              0.                                     148.6
B1       0
B2.      0
B3.      3.1 and fluctuating upwards
B4       0
B5.      148.5
B6.        4.32
I re examined all the components on the faulty board didn't find anything wrong
I re did the soldering by wetting all the joints to no avail.
So guys I'm in need of your help once more.
Good day to all

[Paul Birkeland]

OA.                          0.                                       148

That's where I would start.  The 0V issue can often be remedied by generously reheating the center leg of the MJE5731A that feeds that terminal.

-PB

[antoineestephan]

Thanks PB
Tried reheating the center leg of the MJE5731A, actually I reheated and wetted all legs of both MJE5731A, that didn't help. Still 0V off OA.
I took a reading directly from the legs of the MJE5731A that's close to IA and OA:
I got 205V from the leg at the A side
I got 0V from the leg in the middle
I got 193V from the leg at the B side
Is it possible that my MJE5731A is blown?
Nice day

[antoineestephan]

That's where I would start.  The 0V issue can often be remedied by generously reheating the center leg of the MJE5731A that feeds that terminal.

-PB

Please PB
I'm still waiting for a response, desperately. Hope to hear more suggestions soon.
Thanks for your time and patience
antoineestephan

[Paul Birkeland]

A blow MJE5731A will fail shorted, and you'll have high voltage on all the terminals.

Can you pull the 6SN7 out of the socket, then recheck OA?  If it stays at 0V, put the 6SN7 back in and pull out the 4P1L's and try again.

-PB

[antoineestephan]

A blow MJE5731A will fail shorted, and you'll have high voltage on all the terminals.

Can you pull the 6SN7 out of the socket, then recheck OA?  If it stays at 0V, put the 6SN7 back in and pull out the 4P1L's and try again.

-PB

Thanks
Pulled out the 6SN7 and checked OA on the A side still 0V
Returned it and pulled the 4P1L on the A side, returned it and pulled out the other one, still 0V.
Switched between the two 4P1Ls and still 0V.
Took them both out and still 0V on OA.
All that time the LEDs on the C side were always lighting up and the voltages checkin out OK.
antoineestephan

[Paul Birkeland]

Can you post some photos of your build?  I may be able to spot a jumper that's out of place.

-PB

[antoineestephan]

Can you post some photos of your build?  I may be able to spot a jumper that's out of place.

-PB

Trying to upload a couple of pics, hopefully

[Paul Birkeland]

Can you confirm that the 431 regulators are in the proper holes?

[antoineestephan]

Can you confirm that the 431 regulators are in the proper holes?

I checked them with a magnifying glass and they seem OK. Nothing seems off.

[Paul Birkeland]

What I would do next is to run the preamp with the tubes in for ~10 minutes.  After the 10 minutes, power the preamp down, then measure the DC voltage at IA.  When it is below 10V, touch the heatsink on the MJE5731A on the A side.  Let me know if it's hot to the touch, or not warm at all. 

[antoineestephan]

What I would do next is to run the preamp with the tubes in for ~10 minutes.  After the 10 minutes, power the preamp down, then measure the DC voltage at IA.  When it is below 10V, touch the heatsink on the MJE5731A on the A side.  Let me know if it's hot to the touch, or not warm at all.

Hi, tried that and the heat sink on the MJE5731A on the A side is not even warm.
But Before I unplugged the power, I tried to measure the voltage on IA and I accidentally made a contact between IA and the leg of the MJE5731A that's close to the B side on the board, and that made the LEDs glow and I got a reading of 163V While I would get 190V if I measure on the same leg without touching IA.
I tried re flowing the solder on the legs of the MJE5731A but to no avail.

[Paul Birkeland]

You can try replacing the MJE5731A, but it certainly sounds like you have a flaky solder joint on that board or flaky wire connecting to it.

-PB

[antoineestephan]

You can try replacing the MJE5731A, but it certainly sounds like you have a flaky solder joint on that board or flaky wire connecting to it.

-PB

Hi again
I've been very busy lately and unavailable for some time.
It seems I can't find the MJE5731A locally.
I took out both boards and switched their places, put the good one on the A side, it worked. And put the faulty one on the C side, still not working. At least now I know that the error is inside that board, and that the connections to it are not faulty, but still can't visually detect anything wrong.
Can you please direct me to some procedure that can help eliminate possible causes? So I can eventually get to the real cause.

[Paul Birkeland]

Can you please direct me to some procedure that can help eliminate possible causes? So I can eventually get to the real cause.

I would start by replacing both transistors on the offending side.  If you can't find them locally, they can be ordered from us or any online electronics retailer.

-PB