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volume loss from time to time

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2/3/2006 3:23 AM

volume loss from time to time

Hi all

Regarding an amp that isn't in my shop yet:

German Marshall clone, new tubes and e-caps. The amp was working greatly till it was sold to another guy (don't know if that guy used it without speakers, he did use it with a wrong impedance-setting for some time). The amp does loose volume from time to time, loose input-jack already replaced.

Any ideas?

tia,

Chris

2/3/2006 7:19 AM

Chris,

I had this problem in a Marshall Major, it turned out to be a partially-shorted OT secondary. It could also be a bunch of other things, but the "wrong impedance setting" in your post made me think of the Marshall. This was a hard problem to find, as all the voltages and currents were within the normal range except the output power, which would either be correct & steady or about half power & steady, depending on whether the OT turns were shorted at the time of measurement.

Ray

2/3/2006 7:26 AM

Dirty/contaminated effects loop
Failing power rectifier lowering B+ from time to time
Failing choke making lower B+ on preamp tubes
Failing resistor in pi-filters making lower B+ to preamp tubes
Bad contact in AC power line making intermittent power loss
Bad fuse in AC line making intermittent power loss
Intermittent contact on tube socket
Intermittent crack in signal coupling capacitor/signal wire/resistor/etc.
Intermittent short/loading on OPT winding

this just goes on forever.

What you have to do is poke around until you find out how to make it fail reliably, or catch it while it's failing, as noted in the TADP.

Does it fail suddenly? Probably a crack or short. Does it fade in/out? Probably a thermal problem with a a part. Does banging on the chassis make it work/fail? Probably a mechanical contact.

First, find out how to get it to fail. That's probably 90% of finding a fix.

2/3/2006 8:12 PM
Enzo	Or to paraphrase, the most valuable troubleshooting tool you have is the symptom itself.

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