Gallien krueger MB200 problem

[angelothewolf]

Hi there! got this GK MB200, is a ICEpower 200W class D amplifier (50ASX2) board. It has an AUX power (25vdc unreg) wich is regulated with a pair of 7815/7915 extrernal regulators, wich feeds the rest of the external circuits, Preamp, etc.
The unit was not 'turning on'. In fact it was turning on, but without the 7915 negative rail, i found R15 (10ohm) fuse resistor open at the input of the regulator.
Replace that resistor to see wht happens, and the -15V came back, the 'On/protect' (blue or red) led light up staying on for 5 sec. and then became blue and stars to smoothly blinking, then i realize U7 was getting very hot. I take the IC out and put a socket in place, the IC is not bad, but when i take it out of the circuit everything works fine. Im not understanding why the ic is getting hot, in fact i would like to understand what is that 'diode- optocoupler-jfet' arrangement arround U7....

[Jazz P Bass]

Are D8 & D9 zener diodes in good shape?
Or are they possibly shorted?

That whole diode/ opto mess looks like a feedback path from the speaker.

[angelothewolf]

Are D8 & D9 zener diodes in good shape?
Or are they possibly shorted?

That whole diode/ opto mess looks like a feedback path from the speaker.

D8 was shoted, i forgot to mention but i replace it. No changes, U7 still getting hot, i lift R58,R60,R69 and the fet Q2. Same thing...

[Enzo]

See if Q2 is shorted and putting DC on the IC. Sockets? OK, then pull U7 and see if DC is on any pins other than power supply. Pin 3 especially.


I could be wrong, but I see a sample from the speaker coming through the 3k R53 to the LED in the opto. The diodes are there to direct it s polarity. Pick positive or negative to study. If the speaker sample goes over 15v, then the LED is lit, turning on the opto.

Meanwhile, Q2 is just a signal shunt. Voltage at its gate turns it off, allowing signal to pass. Trimmer R65 looks to set a threshold for the Q2 gate. The opto grabs onto the voltage to the gate. When the opto turns on, it pulls the Q2 gate voltage down, which allows it to start shunting.

1M R58 leaving U7 seems to be a way for the IC to limit itself on peaks.

So all that appears to be a limiter.

[g1]

Not sure if that is a trim pot or a user control, but agree with Enzo, looks like a limiter circuit.
Aside from checking for DC on U7 ins and outs, the outputs of U7 go directly to the next page which we can't see, so there could also be issues there, or even connector problems, solder bridge, etc. (J18 pin 4 & 7).

[Jazz P Bass]

O/K. U7 gets hot.

How about a few voltage measurements on the pins?

[angelothewolf]

OK, i discover that the IC heats up when i connect pin 7 to the input of the pwr amp, there is not other page because the whole power amp is the ICEpower board wich is another brand, and i dont have the schems, i have that PDF uploaded in my first post, wich explains pins connections, protection circuitry and so...

I tested the power amp separately and it works fine! if i lift pin 7 of U7 and inject a sinewave on J18 pin 4 the amplifier works nice.

im measuring around 400mV on the inputs and outputs of the IC... any ideas?

[g1]

You said D8 was shorted and replaced, maybe you should just replace D9 in case it is leaky but measuring ok.

[Jazz P Bass]

You had a blown D8.
So possibly something went wrong on the Ch1 input side if the ICE.

This point was not connected on your J18 pin 4 test.

I'd be looking at Ch1 input of the power amp.

[angelothewolf]

You said D8 was shorted and replaced, maybe you should just replace D9 in case it is leaky but measuring ok.

I replace both!

[MarkusBass]

As JPB says, the input of the power amp is DC coupled to the preamp (U7). If D8 was shorted, there must have been high DC voltage on pin #1 and also on pin #7 (since U7-B is just an inverting amp). It means that the power amp got very high voltage on its input. Maybe its input has failed. D9 is also suspected. Both diodes limit the input signal amplitude for the power amp (to 4V).
You can try to disable the power amp (simply be disconnecting P100 plug) and powering the preamp from external power supply.

Mark

[angelothewolf]

You had a blown D8.
So possibly something went wrong on the Ch1 input side if the ICE.

This point was not connected on your J18 pin 4 test.

I'd be looking at Ch1 input of the power amp.

Well the thing is that pulling j18 out, and injecting signal right into the amplifier inputs it works ok, except for the fact that the two GND pins are acting like inputs, yes, the signal gets in, only using the chasis as reference (not the GND pins), and then pins 4,5,6,7 act as inputs. WTF??



Protection>



Amp block>

[MarkusBass]

Check the preamp separately. It seems that the power amp is very well protected against various incorrect conditions.

[angelothewolf]

As JPB says, the input of the power amp is DC coupled to the preamp (U7). If D8 was shorted, there must have been high DC voltage on pin #1 and also on pin #7 (since U7-B is just an inverting amp). It means that the power amp got very high voltage on its input. Maybe its input has failed. D9 is also suspected. Both diodes limit the input signal amplitude for the power amp (to 4V).
You can try to disable the power amp (simply be disconnecting P100 plug) and powering the preamp from external power supply.

Mark

OK, i think i have some sort of problem in the amp inputs. Disconnecting J18 solve the IC heating issue and feeding signal through the preamp i got a clean output on U7. I will try to troubleshoot the PWR amp inputs, its difficult because there is not schems for it available, and its all SMD, but i can see some discrete components before the signal enters to an ICE custom chip. Maybe reflow?

[angelothewolf]

Check the preamp separately. It seems that the power amp is very well protected against various incorrect conditions.

yep, is what i thought too!