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Old 10th February 2010, 09:16 PM   #1
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Join Date: Feb 2010
Default Volume dropout

Hi - first posting, be nice

I've got a Carlsbro Colt 45watt keyboard amp - so about 20yrs old. Not had a lot of use, but always been reliable. 2 channel input. And I'm not averse to soldering up pedals or the odd patch bay - so a self-fix is an option here.

Just recently, I've had sudden volume drop outs which affects both channels (so a common circuit issue?).

I can temporarily get the volume back where it should be in 3 ways:
1) power cycle the amp (ie flick the switch off, then back on again a few seconds later), and it comes back (usually to drop out again a few minutes later).
2) hit the input with a louder volume note (eg, hard piano hit, when it's dropped out thru playing constant organ volumes). Again - tends to die again after 5mins or so.
3) have the amp running for an hour or so which seems to make it more reliable - and then not a problem for then next few hours (until the gig finishes).

My question is: what could be the culprit/where do I start looking?
The big 40V 2200muF caps on the power side? The smaller caps by the master volume knob? The speaker relay? Anything else ?

Cheers - because it's a good enough bit of kit, and I can't afford 240 for a new 45W Roland amp just at the mo...
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Old 10th February 2010, 09:26 PM   #2
tinitus is offline tinitus  Europe
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Carlsbro ? and its not a tubeamp ?
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Old 10th February 2010, 09:37 PM   #3
mjf is offline mjf  Austria
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........a common circuit issue:
i would begin to search in the psu unit.......perhaps only a bad soldering point,or a bad wire contact.....
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Old 11th February 2010, 06:57 AM   #4
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@tinitus - from about 1990. deffo no tubes in there

@mjf - have been having a good look, and can't see anything obvious yet. There are a couple of transistors mounted to a heat sink, and the solder under them is a bit brown in places. Is this a likely suspect?

My current thoughts have switched from the PSU caps to the speaker output relay. If it's got old and cruddy, then that would make it intermittent. But then after an hour, with lots of voltage across it/current through it it might loosen up?
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Old 11th February 2010, 03:58 PM   #5
cbdb is offline cbdb  Canada
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Join Date: Oct 2008
Location: Vancouver
It sounds like a bad (dirty,corroded) connection somewhere, but both channels are doing it together (is the source stereo?) so this is unlikely (unlikely that both channels of the relay kick back in at the same time) unless the relay coil has a problem but volume shouldnt effect this. Look to single devices that effect both channels, like in the power supply or a protection circuit that effects both channels.
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Old 11th February 2010, 06:27 PM   #6
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Originally Posted by cbdb View Post
Look to single devices that effect both channels, like in the power supply or a protection circuit that effects both channels.
or the effects send/return sockets.... Thank heavens for google caching sites - and in particular the now defunkt carlsbro forum.

Turns out its a relatively common problem with the 1980s amps that the effects jack sockets are cheap connectors that corrode, and even the slightest movement changes the resistance and hence cuts the volume.

Solution one: patch lead between the send and return.
Solution two: bit of a clean.

Currently it seems OK, but about to run it again from cold for half an hour to see what happens.

Thanks for the suggestions folks. I may well be back when the slight hum gets worse for advice on how to change the caps...

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