Behringer 960M or 980M schematic

[Jazz P Bass]

Thanks for spelling that out Enzo.
Just knowing the Prote (at least what you have described) will surely help if the High side PS will not turn on.
(Never got to that point. Secondary arrived blown).
Heh Heh
And I was trying to trace this bad boy out.

[Jazz P Bass]

Alright, here we go.
Got the parts installed for the secondary power supply.
15 volts is good. The fan operates.
I also replaced U4 (comparator), U5 (controller) & U6 (555).
Checked & rechecked all replacement solder joints & connections.
As I feared from the beginning, the High voltage Pwr Supply will not kick in.
Sw 2 is closed.
Protect is on.
The base of Q7(which is also the 555 trigger input & U5 controller pin #2 input) is at 0.105 Vdc.
Therefore 555 output pin is High (3.82Vdc).
This is keeping the relay off. Q4 is high (0.669Vdc) turning off the input to Q3 (0.037Vdc).
Feel free to chime in, I am stuck.

[Enzo]

OK, S2 is closed as it should be.

PROTE is stuck low. Potential PROTE grounders:

Q6 and U8a, both in parallel acting through R39. In other words removing R39 takes them out of circuit.

U4a,b (pins 2,1) wired directly to PROTE, theyf function described above.

U4d,c acting through D12.

Q8, collector wired direct.


Q8 base ultimately controlled by "SC." More in a moment

U4d,c also involved with SC but also with Q5.


SC - up in the mains DC section there is a series inductor L1 in series with the primaries of T1, the main transformer. The push-pull juntion of the main switching xstrs to one end of the promaries and L1 at the other and on to the junction between the main filters. You probably have that.

There is a winding on L1 (which they call T106-2). One end of this winding is to power ground, (which we all know is not really...) the other end is "SC." SO as far as I can tell, SC is a current sense winding for the main switchers. I wouold hazzard a guess that it not only serves a protective function, but also may be involved with power factor correction or some such. DOn;t know.

SC feeds to inputs of U4 through R38, and it also through R62 is rectified by D13, charging C66. From C66, 5.1 zener Z3 feeds base of Q8 through R35.

[Jazz P Bass]

SC is doing nothing. But then again, how can it if the supply is not running.
C66 has 0volts on it.
I did monitor the Primary of the transformer T1.
It has Vdc on it & it appears to have Vac briefly (My Fluke meter shows Vac but it cannot display it.)
So the PS is trying to run but the Protect is shutting it down.
I have checked all the transistors & diodes mentioned & none appear shorted.
I did call Full Compass & they want $178.00 for the PS.
This is o/k but I still have not verified whether the power amp works.

[Enzo]

I listed all those transistors and diodes, not so much expecting one to be bad, but to describe the various paths that led to shutdown. For example Q8 may not be bad, but if we can determine that Q8 is the one shutting us down, that points us towards the problem. In that example, Q8 may be working as designed and something is turniing her on.

And we'd feel real bad replacing a $178 SMPS only to find it is being shut down externally.

In the case of SC charging C66, I think it is desirable not to have much voltage there, at least less than 5.1v because we DON'T want Q8 turning on. It is only after the current through SC increases to the point C66 charges over that zener that Q8 will shut us down.

And something else to consider, if you get zero volts across C66 for example, then try a resistance reading while the circuit is live. We already know there is no voltage there, so the meter should be safe. And then we find out if there is simply no voltage present or if the transistor is turned on and shunting voltages. You can tel the transistor is not shorted with power off, but you can;t tell if it is conducting that way.

When in doubt, I have a supply of those cheap Harbor Freight meters on hand. $8 normal, on sale for $2, and often as not, free with coupon.

I don't recall, did we determine you could disconnect the power amp from the SMPS? If the SMPS fies up without the PA, ...

I don;t know about this system in particular, but past behringer models have both the heavy wire main V+ and V- from SMPS to PA, but also a small wire 6-pin connector for low voltage and controls. If you only disconnect the V+ connector, the low voltage controls lines will detect that and shut down the SMPS, so on those boards at least, you had to disconnect the main V cable AND the low V cable to let the SMPS run free.

On the SMPS, opto U8 is turned on by whatever comes in at pin 2 of CN1 and CN5. Marked DC-OFF. I cannot find where this comes from, but the presence of two identical connections makes me think one per power amp channel. SO perhaps a thermal sensoron the PAs? Maybe you can find CN1 and CN5 and see where the wires lead? A stuck on sensor would thus shit down the SMPS

[Jazz P Bass]

CN1 & CN5 are not populated.
High side will not turn on whether the power amp is plugged in or not.
I would like to thank you for the assist.
Clearly, without the schematic, this is hopeless.
I am returning the mixer as a mixer.
Customer can purchase a power amp.

[Enzo]

Well, we tried anyway.

[Jazz P Bass]

Bagged & Tagged

I really do think we gave it the best.
I am like a badger when it comes to repairing.
I will not give up.
If I have to chalk up time to learning, that is o/k.
I did learn an aweful lot with your generous help.
Comes a time when you have to bag it.
Hey, the mixer works (after I resoldered the jacks & controls).
Customer has a badmember that is going to sell him a QSC amp.
Till next time.
Thanks Enzo.