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Old 31st October 2003, 11:41 PM   #1
Thunau is offline Thunau  United States
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Default Frying noise in one channel of a 6ch amp

Hi,
I have a question for the amp experts. I have a 6 channel B&K amplifier that I bought used off eBay. I use it for tri-amplifying a set of home made speakers. One of the channels recently started to make a frying noise.
I do know that sound from an experience with a recording board (I used to run a recording studio). That same sound would creep in my console and I would temporarily kill it by turning the console on and off a couple of times. One time I had to have some work done on the external power supply. Afterwards when I moved the trim pot that set the 18V DC supply to the board I heard the same noise coming from the console. I know it was stupid to have the console connected and the speakers powered up while servicing the PS but it turned out to be a fruitfull mistake. I sprayed the trim pot with Caig CaiLube and the noise stopped.
I assume that a trim pot needs cleaning in my amp. The problem is that first, the noise is only in one chanel, so it must be a trim pot on that channel, right? There are two trim pots per channel board. They look sealed, and even if I manage to get some cleaner inside I would like to know how to adjust the pots afterwars.
Any ideas what they can be doing? The amp is a B&K VMR 6.5.4. I don't have a schematic, my enquiery to B&K about a service manual or a schematic went unanswered.
Can anybody point me the right way?
Thanks in advance.
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Old 1st November 2003, 12:44 AM   #2
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Default Trim Pots.

Dirty or faulty trim pots could cause noise like you describe. But this is not the only possible source. It could be defective transistors ( usually towards the input side ) or dry solder joints.

Regarding the presets. Measure the resistance ( in circuit) between two terminals with a digital multimeter. Then clean it and rotate the slider and then reset it to the same resistance with the DMM. Don't power up at any time till you finish the work.

For the dry solder joint. Resolder every single joint on the board with a little extra solder . Use good solder and ensure it dries to a shiny finish. The noisy transistor is more difficult to solve. You may have to heat or cool the small signal transistors while the amp is on to see if there is any change in noise. The one that produces a significant change in the noise 'might' be the one.

Just my two bits !
Cheers.
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Old 1st November 2003, 01:07 AM   #3
Thunau is offline Thunau  United States
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Default Wow, that's a lot of possibilities

I just used "air in a can" and cooled one output transistor briefly. The noise seemed to go away for a while.
I also noticed that the noise increases with the input volume pot being turned up. I sprayed and cleaned the pot, but the noise is still the same, louder when input pot cranked up. Maybe I do have a bad output transistor after all. If that were the case, how do I determine that indeed it's that part?
BTW the channel seems to have stabilized after a few minutes of poking inside and spraying cold air on transistors while I was typing this message. Weird.
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Old 1st November 2003, 01:35 AM   #4
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Default Not weird.

It is possible that you have a bad connection somewhere. If poking around stopped it , it points to that. The movement would have "improved " the joint. That does not mean it will stay that way. So one method is to resolder every single joint on that board. Unless you can examine every joint carefully . Cold solders often are not visible externally.
Cheers.

I doubt the output transistor is bad. The volume pot should not affect it in that case.
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