[wiatrob]

I recently fixed a σ22 I used to have in my EHHA (Bad Q1). I rigged it for 12V for another project, but it Looks like it somehow got toasted again on the bench (I had no negative output at all). I replaced the mosfets on the negative rail, and now I am getting the full rectified voltage (~21V) at the output.

Positive rail is working perfectly (~11.7V), LED lights. I checked the FETS and they seem OK. Replaced Q2 and Q12. Still no love.

Any thought for the next approach? I will check all the BC546/556 on that side but I'd thought I'd post before I replaced another set of FETS...

TIA

 

[linuxworks]

did you replace the .47 ohm r's?

if you replace the mosfets, you may also have to replace the power R's.

 

[Anonanimal]

X2 on linuxworks' suggestion. I've heard a few stories of these resistors causing problems but giving no visual sign that they were bad.

 

[amb]

wiatrob, make sure the negative rail MOSFETs are both IRF9Z34N (P-channel). Hopefully you didn't inadvertently install N-channel devices in there.

 

[wiatrob]

Hmm, I'll take those suggestions on the resistors, they ohm out pretty close to .47R in situ (not the best test, i know).

From memory, the P channels have the shinier tab correct?

 

[amb]

Quote:

Originally Posted by wiatrob /img/forum/go_quote.gif From memory, the P channels have the shinier tab correct?

Not necessarily. That varies from batch to batch. The current stock I have look identical between the N and P channel devices. The only way to be sure is to look at the markings.

 

[wiatrob]

Quote:

Originally Posted by amb /img/forum/go_quote.gif Not necessarily. That varies from batch to batch. The current stock I have look identical between the N and P channel devices. The only way to be sure is to look at the markings.

DOH! Hope I wasn't a dummy, and replaced them by memory.

Will check markings and the other Qs this evening and report back.

 

[wiatrob]

DOH! Intermediate DIY complacency: they were indeed the wrong devices. Replaced.

Kids, inspect all your part numbers!

Now I get +3 Volts on the neg rail. i suspect something else is amiss. Perhaps those .47Rs?

EDIT: the .47s test fine. Will begin checking Q12 again, then 5,6,9,10

 

[Anonanimal]

Glad you got those swapped! I can't think of anything else off the top of my head but it looks like you're at least getting there. Keep us updated with what you find.

 

[wiatrob]

Q2, 5,6,9, 10, 12 test OK (tested 10 out of circuit). will check voltages again...

 

[wiatrob]

Not getting any voltage at R6/R7 - CR4 blown up? Q12:
C: 0V
B:-21.0
E:-21.x

EDIT: Hmm, this is nuts, it worked for a minute. rechecking all my solder joints.

 

[wiatrob]

The 'just started working' a clue.

Insidious bad solder joint C19 ->ground.

And a bad Trafo. She's up and running now, letting her burn. Thanks Ti and all!

 

[oneplustwo]

A bad trafo? How did you figure that out? What was the symptom?

 

[wiatrob]

Quote:

Originally Posted by oneplustwo /img/forum/go_quote.gif A bad trafo? How did you figure that out? What was the symptom?

Heat, and smoke, eventually.