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a5002 high side bias on channel 1

Bias problem

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#1 SAJ

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Posted 06 June 2013 - 04:55 PM

I have a 5002, with a high bias problem on ch 1.  It can only adjust fom 164 to 179 mv.  Everyhing else is good. Anyone?
Thanks

#2 joust

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Posted 11 June 2013 - 07:30 AM

How dirty was the module? Are there any indication that 'fumes' may have contaminated the boards?
If so, I can suggest that you 'wash' thoroughly the boards and refurbish the output and intra-heatsink devices (bias and sensor).
A good way of removing the excess water is via an air compressor. Or, bake at 125°F.
It is possible you may have stray electrical interference caused by board contaminants.
Of course, verify continuity of devices and components!

Alain

#3 SAJ

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Posted 11 June 2013 - 04:32 PM

i will try washing the board. What do you normally use? All the components on the bias string tested good. I still replaced the bias transistors. Same problem. Are you suggesting it might be more serious like drivers and outputs?
Thanks.

#4 joust

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Posted 13 June 2013 - 07:36 AM

Whenever I receive a 500x, it's usually in bad condition: dirty, broken solder joints on o/p devices, compound turned to dust. If the modules were located where fog machines have been in use, the boards are covered with s**t that can only be removed by a good washing. Yes, I literally take the module apart, board by board, and wash with soap and water. What a difference this makes! I use an air compressor to remove excess water from the boards and from components. If no compressor is available then I use an oven at low heat. I've had boards that were damaged by corrosion caused by the fog: traces  were eaten away, bit by bit by, causing misreadings and odd faults. Luckily the faults are usually located at the edges of the cards. The last thing is to clean out the compound from the output devices (including intra-heatsink devices: bias and sensor) and apply a little compound to the device and heatsink and reset the devices.
I do remember a few odd faults similar to yours: there are some (very) high voltage caps, brown in color, 400 or 500V tolerance, that in circuit may appear normal, has high resistance, because in parallel to other components. When removed, the caps were resistive!
Ah! now to determine if the fault is on the output module or main board: Just swap cards with the other side! Since you have a good side, swap modules until you determine where the fault lies. Then it is just a matter of time and experience to locate the defective component.
As for output devices, I rarely have intermittent faults; If a transistor is dead, then it will be dead. Though, I rarely had to replace drivers or bias transistors even when a major fault occurs on the outputs. If replacing outputs try to replace all the transistors that are located on the same base trace. Try to use graded devices or you 500x will return!
Good luck!

Alain

#5 SAJ

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Posted 14 June 2013 - 10:03 PM

Jt,
   It turned out to be an intermittent high side bias 500 ohm pot that my new apprentice missed.  It would sometimes test good when a little pressure is applied to it from the test leads. D517 and 516 were also defective.
Thanks for your reply.

#6 joust

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Posted 17 June 2013 - 03:29 PM

Great! At least the amp is in working condition!

Alain