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Mixer PS is on the fritz!

5/11/2000 11:21 PM
Dave	Mixer PS is on the fritz! Hi, Recently I repaired the powersupply in my Alesis recording console. What had happened was a .01 cap That was between the two rectifier diodes had shorted out. I replaced it and thought everything was all good. I powered up the board and everything worked fine for about 3 minutes. Then all the power faded out. No sound. No lights, no nothing. I checked the PS and there was no output voltage and the regulators were VERY hot. I let it cool down and tried again w/o the rest of the board hooked up to the PS. This time it worked fine. I left it on for half an hour with no heating or voltage drops. But, as soon as I connected the Board to the PS, it heated up and the power dropped out again. What do yall think the problem could be??? I'm pretty desperate as the board is the center of my studio. -Dave

5/11/2000 11:21 PM

Dave

Mixer PS is on the fritz!

Hi,
Recently I repaired the powersupply in my Alesis
recording console. What had happened was a .01
cap
That was between the two rectifier diodes had
shorted out. I replaced it and thought everything was
all good. I powered up the board and everything
worked fine for about 3 minutes. Then all the power
faded out. No sound. No lights, no nothing. I
checked the PS and there was no output voltage and
the regulators were VERY hot. I let it cool down
and tried again w/o the rest of the board hooked up
to the PS. This time it worked fine. I left it on for half
an hour with no heating or voltage drops. But, as
soon as I connected the Board to the PS, it heated
up and the power dropped out again.
What do yall think the problem could be???
I'm pretty desperate as the board is the center of my
studio.

-Dave

5/12/2000 12:18 AM
Brad	Dave, Did you check the diodes with your meter? Something blew the cap. A lot of times when one component blows, it may have taken something near it out, too. You may want to see if you can find a schematic for it, and look in that general area. That's where I would start. Brad

5/12/2000 12:25 AM
Brad	Wait a minute....you said the cap in the PS blew, you replaced it, and it worked if you didn't plug the board in. That, I guess would suggest something in the board, unless it's not blowing just because it doesn't complete a circuit. I'll shut up now, and not confuse you anymore....I'd have to see a schematic. Brad

5/12/2000 1:23 AM
Don Symes	Any black smudges or stanky spots on either circuit board? (I do actually mean sniff around both sides of both boards - you know what burning components smell like) I'd be looking closely at decoupling caps (especially the electrolytics) and op amps for a fried one (or twelve). Remove the suspect part(s) and see if the power stays up before you put the replacement part in - you may not have found all the bad ones, and they can still take out replacements. Good luck.

5/12/2000 1:23 AM

Don Symes

Any black smudges or stanky spots on either circuit board?
(I do actually mean sniff around both sides of both boards - you know what burning components smell like)

I'd be looking closely at decoupling caps (especially the electrolytics) and op amps for a fried one (or twelve).

Remove the suspect part(s) and see if the power stays up before you put the replacement part in - you may not have found all the bad ones, and they can still take out replacements.

Good luck.

5/12/2000 4:20 AM
R.G.	Not seeing the schematic is a hindrance of course, but here are some things to think about. A 0.01uF cap between two rectifier diodes may well be a spike suppressor on the secondary of the transformer. Of course, it may have failed from old age or inherent vice, but it's also possible it gave its life trying to hold out a really big transient. If so, it's also possible that (a) the spike got one or more of the diodes and (b) the spike also creamed one or more of the parts on the mixer board. It's certain that something did, as what you describe is an on-board short. The 3-minutes-to-fadeout indicates that something died quietly in operation and is now shorting the power outputs to ground. If it were mine, I would (a) rebuild the power supply. Diodes, caps, and voltage regulators are cheap; it sounds like you depend on this baby. (b) use an ohmmeter to figure out whether the short is +V to ground, -V to ground, +V to -V, or all of the above. (c) Inspect all the power decoupling caps carefully for signs of leakage or damage. Maybe you'll get lucky. Likewise, if the chips are socketed, pull one chip at a time and see if that one is the one that frees the power lines. Shoot, just feel all the chips and see if one is hotter than the rest. (d) if worst comes to worst: locate the power distribution lines. Cut the lines so that about half the board is powered, half not. Hooke the power supply back up and see if it still drops out. If not, the half you disconnected is where the flaw is. If yes, the half you still have connected is where the flaw is. (Note that it's possible that you have multiple failures...). Whichever half shows the short, slice the power lines about midway in THAT section, repair the cuts that you did first and test again. By continuing to divide the board in about half each time, you will quickly narrow it in. For instance, if you have a 32 way board, five cuts will narrow it down to one channel. When you think you're close, test everything in a reasonable sized area for shorting, decoupling caps and IC's first.

5/12/2000 4:20 AM

R.G.

Not seeing the schematic is a hindrance of course, but here are some things to think about.

A 0.01uF cap between two rectifier diodes may well be a spike suppressor on the secondary of the transformer. Of course, it may have failed from old age or inherent vice, but it's also possible it gave its life trying to hold out a really big transient.

If so, it's also possible that (a) the spike got one or more of the diodes and (b) the spike also creamed one or more of the parts on the mixer board. It's certain that something did, as what you describe is an on-board short.

The 3-minutes-to-fadeout indicates that something died quietly in operation and is now shorting the power outputs to ground.

If it were mine, I would
(a) rebuild the power supply. Diodes, caps, and voltage regulators are cheap; it sounds like you depend on this baby.
(b) use an ohmmeter to figure out whether the short is +V to ground, -V to ground, +V to -V, or all of the above.
(c) Inspect all the power decoupling caps carefully for signs of leakage or damage. Maybe you'll get lucky. Likewise, if the chips are socketed, pull one chip at a time and see if that one is the one that frees the power lines. Shoot, just feel all the chips and see if one is hotter than the rest.

(d) if worst comes to worst: locate the power distribution lines. Cut the lines so that about half the board is powered, half not. Hooke the power supply back up and see if it still drops out. If not, the half you disconnected is where the flaw is. If yes, the half you still have connected is where the flaw is. (Note that it's possible that you have multiple failures...).
Whichever half shows the short, slice the power lines about midway in THAT section, repair the cuts
that you did first and test again. By continuing to divide the board in about half each time, you will quickly narrow it in. For instance, if you have a 32 way board, five cuts will narrow it down to one channel. When you think you're close, test *everything* in a reasonable sized area for shorting, decoupling caps and IC's first.

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