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Amplifier interface buffer

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Manuel Marques

New Member
Hi There
I´m working to repair a interface Buffer that uses TWO 2n6740 one 2n3906 and one MPSA92
One LM358n
After checking,,, all diodes are good
I was supected one of 2N6740 was bad ... I was wrong
This interface was damage after the Tube Amplifier went in schok
any sugestion ?
Its possible that 2N3906 or MPSA92
Thanking in advance

This Amplifier buffer is to activate a big relay inside of the amplifier
Operate like a soft circuit
Input J2 RADIO ....Output J1 AL amplifier (voltage come directly from radio
or outside PSU 12v)
I think that when the amplifer went in schock maybe trhout the relay cable
was return to much current ( or maybe voltage?) and damage the buffer
I took one leg of D6 1n4007 and show me on Multimeter that still good.
My point of view is that MPSA92 is very sensitive also Q3 2N6740 maybe also dead.

Thanking in advance for any idea wher might be the fault.

Last edited:


Well-Known Member
Most Helpful Member
1) You forgot to post the schematic showing good input and output signals and their levels.
2) You forgot to say what the defective buffer is doing.
3) Interface? From what to what?
4) Is the tube amplifier at the output of the buffer?
5) Why is a buffer needed?
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