gallien-krueger MB150s Protection circuit issue

[angelothewolf]

Hi there, im with this GK MB150s combo solid state bass amp and is in protect mode, i tried everything and all seems ok, but the protection circuit is muting the output. i start to suspect of the protection circuit and i discovered after join base and collector of q1 (mpsa56) that the amp start working (ok), but the ON light still off.
ill appreciate any idea. thanks

[Enzo]

Do you mean join collector and emitter?

The protect circuit is based on a common 555 timer IC, and it seems that yours is not timing out. COuld be a bad 555, or the timing cap may be faulty. Q8 Q9 could be faaulty, even S1 could be open. It is not a very complex circuit, just check averything around that 555.

[angelothewolf]

Originally Posted by Enzo

Do you mean join collector and emitter?

The protect circuit is based on a common 555 timer IC, and it seems that yours is not timing out. COuld be a bad 555, or the timing cap may be faulty. Q8 Q9 could be faaulty, even S1 could be open. It is not a very complex circuit, just check averything around that 555.

oh, thanks for answer, i think that the 555 is ok `couse when i turn on the amp, the red light is on for 5sec and then off (that means its working ok and the timing cap too, isn`t it?) i changed Q1, and checked all the resistrors and diodes and... nothing. S1 is closed too.
i want to try to replace Q8 & Q9 but the MPSA06 not exist where i am. do you know any more common replacement for that Tr?

Thanks a lot, i want my blue light back!!!!

[Enzo]

No, thurning the amp off bings a light on that stays until the power supplies discharge. I would not take that to mean the 555 was OK. it may be, but that wouldn't be why.

If Q8,9 are not shorted emitter to collector, they are not your problem. For that matter you could remove them and the amp would come back to life if they were what was wrong.

A very leaky C15 would act the same as a shorted Q8,9. A leaky C12 would never charge up and let the timer finish its cycle. The value of either of those caps is not critical. A larger C12 would make the delay longer, a smaller one shorter, so if you have no 33uf cap, try a 22 or even a 10.

[frickecello]

Hi guys,

I have a GK 400RB Mk IV head that does something weird...

The users manual states that after turning the amp on there is a 4 second delay (to allow the power supply to charge I guess) similar to what Enzo mentioned in this thread... The thing is that the amp seems to do that and after the 4 second delay it turns the mute circuit off (thus turning the power amp on which is normal) but only for an instant (less than a second) then something triggers the 555 again and the power amp turns off again, then again 4 seconds and the mute circuit is defeated just briefly and the same happens again every 4 seconds...

I have checked everything around the 555, the RC waveform seems fine, S1 is always closed (The circuit is identical to the one attached by the OP, some values do change I guess) and the FAULT signal turns on and off intermitently every 4 secs. which made me think that the power amp is OK since the FAULT signal would be always ON and is not, also the heatsink remains cold and the fan doesn't turn on...

Could this be just a bad 555 IC?

Thanks in advance.

[singa]

Hi, If I may chime in.Yes it's quite natural to suspect 555 timer circuit but if
no fault is found what then? So look carefully 555 gets trigger signal called
"current" and where does it go? It go to R52 and Q10 MPSA56.Now Q10 gets
trigger signal from Q11 2SC3478 and Q11 gets trigger signal from the
outputs.Aha.... .My guess is shorted output or at least one of them but also check D16/17, D13,14,15 and lastly "fuse" R56 1K ohm.

Good Luck.

[frickecello]

Originally Posted by singa

Hi, If I may chime in.Yes it's quite natural to suspect 555 timer circuit but if
no fault is found what then? So look carefully 555 gets trigger signal called
"current" and where does it go? It go to R52 and Q10 MPSA56.Now Q10 gets
trigger signal from Q11 2SC3478 and Q11 gets trigger signal from the
outputs.Aha.... .My guess is shorted output or at least one of them but also check D16/17, D13,14,15 and lastly "fuse" R56 1K ohm.

Good Luck.

Hi Singa,

I'm referring to the GK 400RB Mk IV, the protection circuit is identical to the one posted by the OP... To my understanding, the signal CURRENT goes from R52 to the protection circuit isn't it?

I don't think the output transistors are bad because the amp would stay "protected" and as I mentioned the signal FAULT (or CURRENT in the schematic attached by the OP) would stay in the same logic level which doesn't occur in the amp.

I don't understand why the amp keeps switching every 4 seconds from warm-up to ON briefly, it doesn't make sense to me.

[singa]

Originally Posted by frickecello

Hi Singa,

I'm referring to the GK 400RB Mk IV, the protection circuit is identical to the one posted by the OP... To my understanding, the signal CURRENT goes from R52 to the protection circuit isn't it?

I don't think the output transistors are bad because the amp would stay "protected" and as I mentioned the signal FAULT (or CURRENT in the schematic attached by the OP) would stay in the same logic level which doesn't occur in the amp.

I don't understand why the amp keeps switching every 4 seconds from warm-up to ON briefly, it doesn't make sense to me.

Hi frickecello,
It is very obvious the protection circuit 555 ic receives a signal
from the output transistors as I have traced out so there must be a dc offset
to trigger the shutdown if not then either the speakers will go to heaven or
the outputs will sacrifice themselves.Of course there is a possibility that
any one component in the chain can "trigger" the 555 in pseudo fashion.

OK you claim the outputs are functional can I assume you have checked them with a diode test/low ohm buzzer?If they are really good then you should check the other components in the chain,it's a matter of elimination.

You have to understand this is not a catastrophic failure so the power rails are not slammed to one of the opposite polarity.If you have checked the 555 ic circuit and all parts are functional,common sense will tell you it must be somewhere else.So the chain is the way you trace it by following it either forwards or backwards.Sooner or later you will come across a defective component.The amp stays in protected mode because a fault is present
and until you rectify it it will stay that way as designed.It's like a nag screen
in software and this is hardware style.

If you can show the schematic of your amp so much the better and then
I can reference the part numbers and you check and verify against them.
Ps if you suspect the power supply do by all means check the circuit but please be reminded to discharge the filter caps with a 5W- 10W resistor of sufficient resistance to ground before
laying hands inside.You are forwarned.Do you see any burned or charred parts and smell?
Singa

[frickecello]

Originally Posted by singa

OK you claim the outputs are functional can I assume you have checked them with a diode test/low ohm buzzer?If they are really good then you should check the other components in the chain,it's a matter of elimination.

I replaced all output transistors a week ago, and after turning the amp on I did not noticed anything strange, no smoke, no funny smell, no sparks, etc.. the heatsink remained cold, the speaker was not fried... Just the same thing happened... it just keeps switching intermitently from: staying in warm-up (red LED) for about 4 seconds to power-on (blue LED) for less than a second...

Originally Posted by singa

You have to understand this is not a catastrophic failure so the power rails are not slammed to one of the opposite polarity.If you have checked the 555 ic circuit and all parts are functional,common sense will tell you it must be somewhere else.So the chain is the way you trace it by following it either forwards or backwards.Sooner or later you will come across a defective component.The amp stays in protected mode because a fault is present
and until you rectify it it will stay that way as designed.It's like a nag screen
in software and this is hardware style.

OK so, probably what I haven't understood yet is how exactly the protection circuit works, correct me if I'm wrong: Since the output transistors are always tied to the power rails and this is not interrupted by the protection circuit then nothing would protect the output transistors from frying, if this is the case then why is the FAULT signal not held at the same logic level if the power amp is bad? It switches from OFF to ON in the same 4 sec.-brief instant fashion I described above.

Originally Posted by singa

If you can show the schematic of your amp so much the better and then
I can reference the part numbers and you check and verify against them.
Ps if you suspect the power supply do by all means check the circuit but please be reminded to discharge the filter caps with a 5W- 10W resistor of sufficient resistance to ground before
laying hands inside.You are forwarned.Do you see any burned or charred parts and smell?
Singa

Please find the schematics attached... I've checked with a scope both +/-60V and +/-15 power supply rails no problems there... There are not burned nor charred parts inside the amp...

[singa]

Hi frikecello,
Relax it does not help if you are anxious so be calm and
think rationally.You have a scope ,good but at this point it is not necessary.
The schematic has some "important" voltages at the input circuit and
elsewhere,that should guide you to the fault if it is there.

The only difference from the other model is your's have
a fan circuit but I don't think this is the suspect.What I said for pervious
schematic still holds for your own.Fault finding is a logical process which means understanding how things work.

OK so, probably what I haven't understood yet is how exactly the protection circuit works, correct me if I'm wrong: Since the output transistors are always tied to the power rails and this is not interrupted by the protection circuit then nothing would protect the output transistors from frying, if this is the case then why is the FAULT signal not held at the same logic level if the power amp is bad? It switches from OFF to ON in the same 4 sec.-brief instant fashion I described above.

The fault detection circuit consists of Q2 qhich is tied to the outputs as
well as the bias circuit.If a fault develops it biases Q11 and goes to Q12 and then to pin2 of 555 ic which is the "trigger".

I am not an expert but after reviewing the basics of 555 timer theory
I have a stroke of enlightenment. It would appear that this amp is a
tough nut to crack but here is what I think, as we tend to think that this
problem is difficult so just look at the 555 timer circuit yourself ,there is a switch S1 (105C) according to theory if S1 is momentarily closed that would trigger the 555 ic as it is wired as a monostable circuit that is " one shot"
mode and not as astable mode (oscillator).This one shot lasts 4-5 secs. as
the manual says.Do you get the picture now? so it's not suppose to oscillate but ONLY last 4-5 secs. once and stay there but what if S1 is faulty or stuck? If I am wrong then replace 555 ic ( also check C11 ie. leaky or not) and if it still not work then check the input all the way to the driver transistor with the voltages supplied in the schematic.I am sure you will find the fault.
Singa