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StompBoxZone • View topic - LPB-2 low volume

LPB-2 low volume

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LPB-2 low volume

Postby 944cabby » Tue May 15, 2012 3:35 pm

I just picked up an original 1970's EHX LPB-2 linear power booster off fleabay.
Now, am i right in thinking that the knob on this should act as a volume?
When i press on the footswitch you can hear a slight increase in sound/clarity, but as you turn the knob clockwise there's no increase at all.
I found a schematic which shows a 100k pot - is it likely that the pot needs changing or is it likely that one of the other components is faulty?
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Re: LPB-2 low volume

Postby Laurie » Tue May 15, 2012 3:39 pm

yes, the pot should act as a volume.

As you are getting audio out this tells us that the pot is OK.

have a look at this - an excellent resource:
http://www.beavisaudio.com/techpages/HIW/hiw1.gif
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Re: LPB-2 low volume

Postby 944cabby » Tue May 15, 2012 4:00 pm

Thanks Laurie! :thumbs:
Looking at that,I'm now thinking maybe the transistor - but if this or one of the caps/resistors was faulty surely there'd be no signal at all?
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Re: LPB-2 low volume

Postby Laurie » Tue May 15, 2012 4:19 pm

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Re: LPB-2 low volume

Postby ranjam » Wed May 16, 2012 4:16 pm

If you can operate a meter and a scope, you should have this thing nailed in ten minutes. It's a simple circuit. Wait, you'll need a signal generator, too. So it gets slightly complicated, but it's still a simple pedal.
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Re: LPB-2 low volume

Postby 944cabby » Wed May 16, 2012 4:40 pm

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Re: LPB-2 low volume

Postby Laurie » Wed May 16, 2012 5:40 pm

if the pot was dead it probably wouldn't cause distortion.
But.
Because the signal disappears then comes back that does usually indicate pot.

If it was on my bench I'd probably throw a new pot into it and see what changed.
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Re: LPB-2 low volume

Postby 944cabby » Thu May 17, 2012 5:55 am

Well, changed the pot over today for a spare one i had and it does exactly the same thing.
Think i'll swap the transistor next?
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Re: LPB-2 low volume

Postby DeFrag » Thu May 17, 2012 8:22 am

& some more.
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Re: LPB-2 low volume

Postby Laurie » Thu May 17, 2012 9:19 am

This is such a simple pedal the approach of just changing components until you find the dead one is valid.

Normally I'd ask "Do you have an audio probe?" ... You could trace the signal through the pedal and see where it went wrong...
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