Author Topic: Adjustment of bias and other VR's, portable amplifier.  (Read 546 times)

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Offline Ashhhhh

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Adjustment of bias and other VR's, portable amplifier.
« on: October 05, 2018, 09:34:57 pm »
Hi guys,

I'm trying to resurrect a Lectrosonics Maxi Mouse amplifier.
It was a bit of a basket case and is now up and running, I would like to adjust the bias next.

Unfortunately the manufacture doesn't provide any details but I was able to search out a schematic.

Would someone be kind enough to cast their eyes over it and give thoughts on the bias adjustment?
In addition there are two other unlabelled VR's, one near the bias and the other near the charger input. It would be great to know their role and adjustment.

Thanks!
Ash

http://cloud.tapatalk.com/s/5bb7d8f7a3717/maximouse__17049f_sch.pdf

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Offline cvanc

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Re: Adjustment of bias and other VR's, portable amplifier.
« Reply #1 on: October 06, 2018, 12:26:51 pm »
The 5K pot at the LM301 sets your output stage bias

The 25K pot near the loudspeaker controls how hard the audio compressor works (note the CLM50 optocoupler in 2 locations on the schematic - this is the compression control device)

And the 25K pot in the power supply sets the power supply output voltage.  BUT... are those batteries rechargeable while in the unit?  If so you might have to set the 25K power supply pot to provide some specific charging current that the batteries would be happy with - you might do this by pulling the battery fuses and putting your meter (in amps mode) across the open fuseholder.

Good luck.
 

Offline commongrounder

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Re: Adjustment of bias and other VR's, portable amplifier.
« Reply #2 on: October 06, 2018, 04:14:04 pm »
On a battery powered amp like this, there is often no emitter resistors on the output transistors to refer a set bias value to.  If I were adjusting this unit, I would put a scope across the output, and feed in a mid to low audio frequency sine or triangle wave at a low volume. Then I would adjust the bias until the crossover notch in the waveform just disappears.  That should give the best balance between distortion and power consumption(battery life).  The service manual might have had a spec for current measured across one of the (empty) fuse holders as a way to arrive at a typical bias setting to avoid the need for a scope.
Best of luck to you!
 

Offline Ashhhhh

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Re: Adjustment of bias and other VR's, portable amplifier.
« Reply #3 on: October 06, 2018, 08:59:34 pm »
Thanks to you both.
Excellent, I'll use a scope and do exactly that.

Any thoughts on adjustment of the compression pot?

Yes, the batteries are charged within the unit, I just need to source a 20VAC wall wart.
On a battery powered amp like this, there is often no emitter resistors on the output transistors to refer a set bias value to.  If I were adjusting this unit, I would put a scope across the output, and feed in a mid to low audio frequency sine or triangle wave at a low volume. Then I would adjust the bias until the crossover notch in the waveform just disappears.  That should give the best balance between distortion and power consumption(battery life).  The service manual might have had a spec for current measured across one of the (empty) fuse holders as a way to arrive at a typical bias setting to avoid the need for a scope.
Best of luck to you!

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