Mackie SA1232 no 15v supply.. 2 of them!

[The Dude]

Oh, sorry. I thought your -15V supply was working and only the +15 was not.

[Indyryder02]

Yes. Neither. The whole 15v supply.
same with the other unit! So of course, it’s gonna be interesting to see how that one failed!
thays the other one with the burning R50. It was super high when I first tested it. Several meg. Replaced it and another resistor that was burnt. (Forgot which one now) it was ok for a while during testing.. then R50 started to slowly burn again…
Still can’t imagine the odds they both have the same basic issue but with different causes it seems.. or maybe they’ll end up same cause with different failures resulting in the same concern of not 15v supply. I’ll report back after I get C28 replaced.. hopefully that will get this one up again.. then I can go back to the other tk see what’s roasting R50… I’ll take your previous advice for checking for external shorts etc .

Thank you very much for your expert help! I Greatly appreciate it!

[Indyryder02]

I replaced C28, it was indeed shorted.
now when I tried to power it up, this unit also is trying to burn R50.. so they’re at least both on the same page now

didnt find any dead shorts etc from rails to ground or between them.

suspect Q27 but it seems to test ok.

edit…
I just tried to check voltages on the two transistors again..
now I have the 15v supplies.. and R50 seems ok… no clue…
didn’t change anything…

ok.. so.. since this one is pretending it fixed itself.. back to the first one..
still burning up R50 slowly.. large voltage drop across it.. something is pulling it down hard it seems

[nevetslab]

I've serviced a number of these Mackie SA-1232's, SA-1532's along with the SWA1501's and SWA 1801. Similar power supply-coupled switching to couple in the high voltage rails when signal demands it, once the levels exceed the +/- 25VDC that's always active in the power amp circuits. I've attached a Service Note for servicing the SWA 1801, which has applicable info to the SA-1232. All of them use that Opto-isolator switching IC HCPL-3100 in controlling the HV power supply to add the high voltage range to the output stage under drive.







Mackie SWA 1801 Service Notes.doc

I was using my Bruel & Kjaer 1027 Sine-Random Generator with a B & K 1617 1/3 Oct Filter in the loop to provide 1/3 Oct Pink Noise. Interesting that when selecting LF filters, here using 50Hz 1/3 Oct filter, you get random amplitude LF Sine Wave, which is very handy to show how this Mackie SA-1232 amplifier works. The waveform in the middle is the combined output, with the segments above and below the +/- 25V rails show that portion of the HV system turned on and off to just pass the high voltage peaks. This was captured on my Tektronix 7633 storage Scope.

[Indyryder02]

I remember now the other burnt resistor I replaced besides R50 on the first unit, was R75..

[Indyryder02]

That’s a very nice lab setup!

and thanks for the service notes.
mid found them previously somewhere on here and read through, but didn’t find anything pertaining to my particular issue. Still looking to find what’s roasting R50. Everything in line seems to test ok so far..

[Indyryder02]

Looks like I have a short to ground between R75 and R27 on the LF board.. seems perhaps C32 is shorted?… not sure where else it could be getting the short to ground at from that junction besides through C32..

[Indyryder02]

Yep. C32 was shorted.
Any idea what would cause these to short?
i normally work on guitar amps, don’t remember ever finding a shorted capacitor. Common for this type of gear? Bad batch of caps?

anyway. Both amps are back together and working again.

thank again for the help as always! Best site for techs hands down thay I know of.

[The Dude]

Glad you got it. Out of curiosity, what brand caps are the shorted ones? Electrolytic caps don't short that often, but it does happen.

[Indyryder02]

JH CD110 series. Jianghai. 100uf / 25v (little blue bastards haha)
gonna replace the rest while I’m at it, so I hopefully don’t see these big heavy units again!

is there any threads listing known bad capacitors batches, brands etc?

should be if not..

with the guitar amps, I see the grey IC branded caps failing constantly.. all the Fender amps have used em for years and they always wind up leaking.