Testing amp shut down

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I have an ALTO PBM8.500 Powered mixer. I want to repair it for a friend. Apparently it has been shutting down after prolonged high output use. I do not have the facilities to run this amp at full load to replicate problem. The amp is rated at 500 Wpc @4 Ohms, 300 Wpc@8 Ohms. Can I run a household incandescent from outputs or similar to cause shut down?
Thanks
 
I have service manual for the amp. It contains a couple of suggestions.
Bias adjustment being one of them
A couple of capacitor changes recommended as well.

Could this be the cause of shut down?
 

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If bias were incorrect the amp would run hotter than it should under no signal conditions. It would have to be a long way out to cause overheating at full drive only.

The cap shown (C9) is just the reset generator for that IC (holds rst low for an instant at power on). After that initial time it's job is done 'till the next power off and on sequence so it won't be anything to do with the cap.
 
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Could you please clarify for me how to adjust bias? I really love electronics work but have no education and very little experience.
Thank you

You will normally be measuring the voltdrop across some resistors in the output stage. By using ohms law you can then work out what the current actually is.

The procedure in your picture tells you to adjust for 130 millivolts across resistor R1-17 in one channel and R2-17 in the other by using presets R1-16 and R2-16.

The first part of the procedure seems to be to confirm some fixed voltages... strange but there we go.
 
I have an ALTO PBM8.500 Powered mixer. I want to repair it for a friend. Apparently it has been shutting down after prolonged high output use. I do not have the facilities to run this amp at full load to replicate problem. The amp is rated at 500 Wpc @4 Ohms, 300 Wpc@8 Ohms. Can I run a household incandescent from outputs or similar to cause shut down?
Thanks

Judging from the pictures it very likely has a cooling fan(s). Is it overheating because of dirt build up? Go after it with an air hose at 20 PSI (what is that in metric?) and if you get a small mushroom cloud you might be done.

 
I took your advice to check the fan. Seemed like a good place to start without getting overwhelmed. Fan and heatsink was quite clean. I decided to check to make sure there was enough thermal compound and found this.
I don't know if it is baked thermal coumpound, some type of glue or the vital fluids of the fets. It seemed to be focused only around two of them.
 

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If the FET were shorted it wouldn't work at all.

tbh I think you are going to struggle with this. You would need to know first hand how what the amp is driving, how hot its getting etc. Does this fan/s cut in correctly ? Have you seen them run.

It sounds as though the amp is shutting down in response to some overload sensing circuitry...
 
OOPS. I am not sure what happened!
I cleaned surfaces and replaced output ICs
When I turned it on, it seemed to be running fine. Fan was running at normal speed.
There was a small spark yet everything seemed fine.
I plugged a speaker in and got a full 60hz hum. Maybe cooked speaker but that's OK
Feeling discouraged but not ready to back down.
 
I would have wagered that there was nothing wrong with the amp to begin with. Probably just overloaded by a "friend" who doesn't know any better. Now this amp that worked fine is broken to be sure!

I don't mean to be rude but your in too deep. Take the thing in and have it serviced by a professional. Unless, of course, your friend doesn't mind you using it as a learning tool.
 
I will never improve my skills by taking it to be serviced. Just need some help along the way
There was definitely something wrong with the amp. 30 seconds of operation and it was running fan full speed.
I have replaced the ICs perfectly with brand new direct replacements not a drop of excess solder.
New ICs are doing something that the old ones didn't do.
Now I am hoping to find out what that is.
There is more skill, knowledge and experience on this forum than in most service centers.
Unfortunately I don't have it!
Thanks
 
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