LSR&D Leach Superamp bias problem - diyAudio
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Old 19th December 2014, 09:52 PM   #1
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Default LSR&D Leach Superamp bias problem

Hoping someone can help me here. I'm not able to bias up or down with the 500R pot in one of my Superamp monos. I can but only in a narrow 60 to 90ma range. I can't get it out of there, up or down. I've removed and checked the pot. Can someone please point me in the right direction? All I can think of are the bias diodes/resistors. Thanks in advance.
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Old 19th December 2014, 09:57 PM   #2
AndrewT is online now AndrewT  Scotland
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What is the bias voltage across the Vbe multiplier?
Does that bias voltage change as you adjust the VR trimpot?
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Old 20th December 2014, 12:23 AM   #3
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Thank you for responding. Voltage is steady .976 regardless.
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Old 20th December 2014, 09:17 AM   #4
AndrewT is online now AndrewT  Scotland
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thoroughly check the Vbe transistor, noting type (NPN, or PNP) and the orientation.
Check the VR is actually changing.
Check the currents flowing through the Vbe resistors.
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Old 20th December 2014, 02:10 PM   #5
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Check loop connection of thermal compensation diodes or possibly defective pot.
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Old 20th December 2014, 04:06 PM   #6
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I have 1.9mV through the VR and .64v through the fixed resistor. The transistor is a PNP MPS8093. The VR is definitely changing. I can hear it. In any case I replaced it but the old one functions identically so I know the pot is good.
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Old 20th December 2014, 08:25 PM   #7
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Correction. It's NPN functioning as it should. Conduction from base to emmitter/collector.Collector to fixed resistor, emitter to a zener diode.

Last edited by Discopete; 20th December 2014 at 08:34 PM.
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Old 21st December 2014, 03:43 PM   #8
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bump
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Old 21st December 2014, 04:53 PM   #9
llwhtt is offline llwhtt  United States
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Can you post a schematic? Maybe more people would chime in.

Craig

I found the schematic but no zener on the emitter of Q13, we need YOUR schematic.

Last edited by llwhtt; 21st December 2014 at 05:00 PM.
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Old 22nd December 2014, 02:08 PM   #10
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Dr. Leach made it clear there isn't a schematic for this amp. I've taken photos of the board, front and back but can't upload them.
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