led not working [solved]

[mozarti]

I have had periodic problems with one channel of my Beepre.  Most recently, the leds on the B side of CS4 board are not lighting up. The A side works fine. I have also noticed that the center leg of the transistor between the two Leds is red hot.  Any thoughts as to what might be wrong with this channel?   Thanks in advance. 

[Paul Birkeland]

How are your voltages?

If part of a transistor is glowing red hot, that would be very unusual (something I've never witnessed), and should be addressed.

It could be as simple as a flaky solder joint, or something more sinister.

In either event, one LED not working would be enough motivation to want to go over the board, touching up all the solder joints and tightening the heatsink mounting hardware.

-PB

[mozarti]

Thank you, Paul.  The only abnormal reading is 5.47 v. for terminal 1.  The terminal 16 on the good side has the acceptable 5.06 v.  I will go over the solder joints.  By the way, LED lights sometimes come on and sometimes not.

[Paul Birkeland]

Yeah, that sure sounds like a flaky solder joint!

[mozarti]

Paul,

I re-soldered all the points on the B side of the board, and all the wire connections, but still the led on the b-side do not light up.  The A side still lights up but they do so several seconds after the diodes on the other channel light up. As to the measurements, terminal 1 measures 10.7 v, terminal 2 has jumped up to 155,  terminal 4 moved up to 10, and terminal 6 starts at 175 and moves down to 146.  It looks like I made things worse by resoldering things. Few questions:

1.  Can I be reasonably sure that the problem is on the board?  If so, I may just buy a new board and start all over again. 
2.  Before I try this, do you have any guesses which solder joint is likely to be a problem?

Once again, I very much appreciate your help.

[Paul Birkeland]

You can remove both big C4S boards and swap them side-to-side, that will isolate the issue down to the PCB, or perhaps something else in the preamp.

[mozarti]

Paul,

I decided to give it another shot and re-soldered the CS4 board again, but I got the same results (the A side lights up but not the B side with terminal 1 measuring 6.7 volts.). So I decided to check the resistance and I got some strange numbers: terminal 1 measures almost 1Kohm and terminal 4 measures 96 ohms with the 300b in.  Any guesses?

[mozarti]

The Problem found and solved: A loose wire on 5L.  BeePre is singing again!  Thank you.