[Emooze]

Recently finished my sigma 11 (24V) the other night and last night I finally got to plug it in. Flipped the switch and the LED turns on for a few seconds and then turns off. I check the board to see what was wrong and find I put CR2 in backwards.

Today I flipped CR2 and plugged it back in and I'm having the same issue. No voltage is being supplied to the output pads, in fact, no voltage is present anywhere past the emitter on Q1. The FETs get the voltage but nothing is coming out of either of them. So far I think I damaged Q1 but would there be anything else that could be damaged?

Also, the voltage coming out of the transformer is at 30 VAC. C5 holds 40 VDC when fully charged. Would that affect anything?

 

[amb]

Use your DMM's diode check mode to test all the BJTs. I had posted previously about how to do this, so search for it. On the σ11, Q8's B-C junction is intentionally shorted (see schematic) so it will give a "false" failure, but its B-E junction should still test good.

If you're sure Q1 is dead, then it would definitely cause the PSU to output the wrong voltage, or no voltage at all.

 

[Emooze]

Q1 is definitely dead, giving me no reading whatsoever whereas all the other transistors are working as you described.

The diodes, both CR and Zener are giving me different readings both forward and backward which can I assume is normal?

 

[amb]

Quote:

Originally Posted by Emooze /img/forum/go_quote.gif Q1 is definitely dead, giving me no reading whatsoever whereas all the other transistors are working as you described. The diodes, both CR and Zener are giving me different readings both forward and backward which can I assume is normal?

You can't test the CRD using the diode check method because it's not really a "diode". The zener, when forward biased will test like a regular diode, but when reverse biased should read approximately the rated zener voltage, but most DMMs don't put out enough voltage to give a sensible reading.

 

[Emooze]

hmm I think I'll order diodes with the transistor then. Thanks for the help amb! Will update when I confirm what blew

 

[Emooze]

For future reference, Q1 on my sigma was the only thing that blew. Replaced that and now it works perfectly.