Why bias stays zero, 0.00v and doesnt react to the bias trimmer?

Hello,

I trying repair an amplifier

When i got it amp unit was short circuited, burning transistors

- changed many parts and got it running except bias reads ZERO!

So bias is 0.00v and doesnt react to the trimmer

- bias trimmer pot is tested and ok, resistors attached to bias circuit are ok, capasitor in bias circuit was changed

- dc offset is within limits and reacts to trimmer

- transistors were changed and tested too (with cheap component tester)

What could be the main causes for bias to stay at zero volts?
 

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music soothes the savage beast
Joined 2004
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I see, ok.

In that case if you have many, 12, at least few should be working perfectly. I suggest you select best working and measure voltages in all important spots. And write it into schematic and post it. As reference.

Than you do the same for amp with bias issue, and post. Same for amp with dc on output issue.
It should be easy to spot the differences.

If you can fix the transistor symbols, please do. Hard to look at, it burns my eyes.
 
Administrator
Joined 2007
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What could be the main causes for bias to stay at zero volts?
Two things I can think of:

1/ The replacement (modern) transistors have slightly higher vbe due to manufacturing process differences. You just need to tweak the resistors around the vbe multiplier. Try reducing R39 to a 1k8. I would advise using a bulb tester.

2/ It looked that old I wondered if the originals were actually germaniums.
 
Administrator
Joined 2007
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That doesn't really tell us much tbh.

You need around 0.6v across the b/e of each output transistor so if you measure between the base of each output (from base to base) you need to see the voltage rise to around 1.2 volts as you turn the preset before the output transistor will conduct.
 
Yes they are original transistor types

More info on the attachment

This is a three way active speaker and the treble amplifier has always the problems as they age

Thats why the attachment speaks about treble amplifier
 

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