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

[Indyryder02]

solid state is my weakness, I usually manage to come through.. but could use a little help.. these are pretty strange to me.
I have two of these brought to me. No sound, no power light..
traced so far to no 15v supply..
which would account for no power led. Etc.
R50 was burned open on one of these.. not sure why yet. Moved on to the other one in hopes of figuring more out with that one.. before tackling what’s going on with the other one..

Q26 and Q27 both have 25v on the collectors hardly anything on emitters. Should be 15v?


where is the 15v supply actually being derived from? Regulated from these darlington’s?
i think of being staring at these schematics too long to make sense anymore!
and I’m still trying to figure out how these transistors really work in this part of a circuit. Or if I’m totally missing it and looking in the wrong area.

thank you, and forgive my lack of knowledge. Hence why I appreciate you all so much and support this awesome site.

[The Dude]

The collectors of Q26 and Q27 should have approximately 25V. They connect directly to the rectifier circuit through 10 ohm resistors. That supply is then regulated down via the shunt regulator circuits consisting of the transistor pairs and associated components. If you are missing the +15V supply, yet R50 burned open on the -15V supply, it is possible that there is an external short between the two supplies. I'd first do a resistance check from +&-15 supplies to ground, and a resistance check from +15V to -15V supply to make sure there are no external shorts. If there is an external short, attempting to repair the supply will be futile. Also, it's important when you describe supplies to note whether you are referring to the negative or positive supply.

[Indyryder02]

Ok, thanks. That helps a lot already.. that’s what I was thinking.. but I didn’t know for sure. To clarify the Q26 and Q27 and associated parts are forming the resistor shunt taking the 25v down to 15v? And that would be found on the emitters of those two?

Since I have two of these units with the same basic problem, I
moved on from the one with the burnt R50 as the other one didn’t have that..
but the other one also has no 15v regulated supply.

are the two transistors not functioning properly or not getting something they need to conduct? Still trying to figure out how these work really..
thanks!

[The Dude]

"To clarify the Q26 and Q27 and associated parts are forming the resistor shunt taking the 25v down to 15v? And that would be found on the emitters of those two?"

That's basically correct. What are the voltages on Q26?

[Indyryder02]

Q26 currently reads
B 2.56v
C 24.3v
E 1.46v

I also just noticed (and I think they were even before, I may have caused this with testing without realizing..

i have -28 and +24 I was pretty sure there were equal before +/-
not sure what could have happened there just yet..

measures the same back at D2 also

[The Dude]

Base and emitter voltages should (obviously) be higher. Temporarily unsolder one side of R70 and remeasure to eliminate a possible load issue.

[Indyryder02]

Same after lifting R70

[The Dude]

Check R56 and Q28.

[Indyryder02]

I knew the voltages were off there, but Didn’t know which direction to look, either to the transistors or surrounding components .. everything tested fine. Unless my meter is getting tricked in circuit etc.. wouldn’t be the first time!

[Indyryder02]

Ok. R56 is good at 1k
Q28 tested fine diode test..
reads
E 0.00v
B .7v
C 2.6v

[Indyryder02]

Suspect C28? I wondered before about it. Not much left!

[Indyryder02]

Seems like a dead short between the emitter of Q28
and the base of Q27, going through C28.. less something else I’m missing in the circuit is causing that reading.

trying to figure this out as I go of course.. and of course am going to want to know the process in figuring this out!

[Indyryder02]

Just checked the second unit, it charges up resistance between the two points as expected through a cap.
the one I’m currently working on is a dead short there…

oh how I’ve been hoping to avoid removing this board!

[The Dude]

Emitter of Q28 is ground. If Q27 base also shows shorted to ground, then yes- I'd suspect shorted C28. But, I don't see how the -15V supply could be working if that's the case.

[Indyryder02]

Neither 15v supplies are working, that’s been the problem. I apologize I didn’t clarify that in the beginning.
i first suspected C28 had a broke solder connection earlier on. Even wiggled it’s. Didn’t expect it to be shorted as I rarely find shorted caps..
wild for sure… and the other unit also doesn’t have 15v + or _
- yet isn’t shorted in the same place… it’s the one with the burning R50.. I would never have imagine getting two of the same model speakers, both with no 15v +/- and it being different causes… this is crazy for sure..
looks like I’ll have to remove this board sadly.. to remove C28 and test it out of circuit to further confirm and replace it etc.

hopefully it’ll be easier than it looks.