[sdiy] Semi-OT: bass-amp problem

[epeasant at telusplanet.net]

It sounds like maybe a bad connection in the power supply before the main filter
capacitors. When you turn it on it connects and gives the capacitors a hit of
power which causes thermal stress to open the bad connection. The power slowly
drains from the caps and the dropping voltage lowers the gain of the amp. After
a while it cools down and reconnects, starting the process again. You are probably
right about it being in the preamp, the output section with it's high current
draw would drain it's caps quite quickly, but the preamp would not.

The problem in this case could be as simple as a defective fuse. I was once
called out to repair a microprocessor-controlled piece of equipment back when
they were just putting imbedded computers in medical equipment and hadn't learned
how to deal with line noise yet. To protect the computer in this machine the
manufacturer had installed a power monitor module that would shut down the power
if there were any line disturbances and keep it off until the power had returned
to normal for 10 seconds. When I arrived, the unit would flash on and then immediately
off again for ten seconds, and then it would repeat. I found the that main fuse
would allow the machine to switch on but would then heat up enough to break
connection. This would turn off the machine, the fuse would cool down and reconnect,
and ten seconds later the monitor would turn the power back on. A new fuse fixed
it, even though the old one measured good with the low current draw of my meter.


Take care,
Doug

______________________
The Electronic Peasant

www.electronicpeasant.com

>Hi Folks,
>
>Sorry for the semi off-topic post but I have this question concerning my 
>Laney DP 150 bassamp. This is the problem:
>
>On powering up it works fine, but after a while the volume decreases until

>you can't hear anything. Then suddenly, without warning, it bursts into 
>full volume again (which is of course is much louder than in the beginning

>because of the dumb-ass volume-tweaking). This cycle keeps on repeating 
>itself over and over. I think it must be something in the capacitors but I

>have no idea which ones... As far as I could see the problem lies in the 
>preamp.
>
>As I need the amplifier for a gig on october the 7th any help would be 
>greatly appreciated!!
>
>Thanks!
>
>Corneel.
>
>PS: Due to the off-topic nature of this mail you could choose to respond to

>me privately....
>
>
>
>
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>"the modern day composer refuses to die" - E. Varese
>
>            "real music never will" - K.S.
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