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Topic: Starts clean, ends distorted |
Tony Palmer
From:
St Augustine,FL
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Posted 10 Nov 2006 10:48 am |
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This is an annoying problem I have with an old Session 400.
It sounds clean for about 10-15 minutes.
Then it starts to have what I'd call about a 25% distorted sound.
Next day, turn it on and it's ok again...for 10 minutes.
Obviously, something is warming up and causing the distortion.
Any ideas? |
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Jack Stoner
From:
Kansas City, MO
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Posted 10 Nov 2006 11:28 am
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First thing I can think of is filter caps in the power supply if they have never been changed.
I had a Session 500 that would do that at times and the filter caps was the cause. |
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Darvin Willhoite
From:
Roxton, Tx. USA
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Posted 10 Nov 2006 11:45 am
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I am having exactly the same problem with an old Session 400. I had Brad Sarno do a cap job on it so I think that can be ruled out. I cleaned and tightened all the molex connectors, no change. I have the circuit boards out now to inspect for bad solder joints. If that doesn't work, its off to Peavey, where it probably should have went a long time ago. Boy does it sound good for those few minutes though. LOL
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Darvin Willhoite
Riva Ridge Recording
[This message was edited by Darvin Willhoite on 10 November 2006 at 11:46 AM.] |
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Bill Terry
From:
Bastrop, TX
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Posted 10 Nov 2006 11:48 am
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| Try cold spray maybe? particularly the discrete transistors. |
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john lemay
From:
Ainsworth NE
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Posted 10 Nov 2006 12:42 pm
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| I don't have a schematic in front of me but it sounds like a leaky bias diode. When the diode goes bad it changes the bias point of the following transistor stage and the amp starts to sound like a fuzz tone as the note dies off. Sound familiar? Easy cure. Change out the diode. It shows up as a dual diode on the schematic. Very common failure mode for these amps. |
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Mike Wheeler
From:
Delaware, Ohio, USA
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Posted 10 Nov 2006 1:36 pm
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Darvin, cold solder joints are sometimes "invisible". They can look good, but are intermittent under the "skin" of the connection. Bill Terry has the right idea. Use some freeze spray, put a low volume signal thru it, and once the distortion starts, move thru the board a little at a time till freezing makes it stop. Then resolder that area...or replace the bad component.
CAVEAT:
I think you know to be VERY, VERY careful in there when it's powered up. If you don't know EXACTLY what I mean, don't get in there.[This message was edited by Mike Wheeler on 10 November 2006 at 01:37 PM.] |
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Tony Palmer
From:
St Augustine,FL
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Posted 10 Nov 2006 1:41 pm
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John, you say "Very common failure mode for these amps"
Do you mean the old Session 400 or all Peavey amps?
Just curious. |
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john lemay
From:
Ainsworth NE
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Posted 10 Nov 2006 2:23 pm
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I've replaced several dozen over the past 14 years in all versions of the amps. The diode gets old and starts to leak. Not a design problem, just a component failure. I'm speculating from a distance. It's a cheap part.
Then again it could be a bad solder trace or even the screws holding the power transistors in. They get loose over time and cause problems. Only tighten them after the the power has been off for a while. Hope this helps.[This message was edited by john lemay on 10 November 2006 at 02:27 PM.] |
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