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Old 17th August 2011, 02:49 PM   #1
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Default Testing amp shut down

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?
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Old 17th August 2011, 03:01 PM   #2
Mooly is offline Mooly  United Kingdom
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If we are talking "proper watts" thats 45 volts RMS into 4 ohms.

Bulbs won't draw the current required... how about an old electric fire element etc and use enough of the length to give 4 ohms.
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Old 17th August 2011, 03:06 PM   #3
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The resistance of the element will change as it heats.
A couple of 225 watt power resistors wired in series or parallel would probably do a better job.
They can take a little more power than their ratings indicate.
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Old 18th August 2011, 04:28 PM   #4
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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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File Type: png bias adj.PNG (70.4 KB, 97 views)
File Type: png pb8m.PNG (142.1 KB, 95 views)
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Old 18th August 2011, 05:27 PM   #5
Mooly is offline Mooly  United Kingdom
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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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Old 18th August 2011, 05:37 PM   #6
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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.
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Old 18th August 2011, 06:11 PM   #7
johnr66 is offline johnr66  United States
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Ask your friend what speaker(s) and how they are connected. He may simply be connecting too low of impedance to it.
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Old 18th August 2011, 06:23 PM   #8
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by argonrepublic View Post
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.
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Old 18th August 2011, 06:51 PM   #9
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Quote:
Originally Posted by argonrepublic View Post
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.

G
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Old 18th August 2011, 07:37 PM   #10
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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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