Bias Problems;

[starbender]

Hello,

I'm trying to repair old grundig amplifier. Previous owner messed up with it pretty bad. In fact beyond imagination. So many resistors were replaced with wrong values, output transistors not up to spec as amp suggests, driver transistors shorted, pcb traces destroyed etc etc..

I fixed one channel, and repaired most of the parts. Replaced resistors according to schematic. Replaced output transistors and drivers. Fixed pcb traces.

SM calls for 30mv bias for each channel. I can adjust left channel to 30mv. But right channel jumps from zero to 50mv suddenly. Minimum I can dial bias pot is around 45mv and its wondering. I couldn't find the reason for this behaviour.

I attached schematic for right channel. Red circled parts are replaced by me. GPSA05 replaced with bc546b. Other parts same as originals..

Any idea where to look next?

 

[audiopro]

Replace the bias pot R414 even if it seems to test okay.
Does the amp work otherwise?

 

[starbender]

I forgot to mention;
I already replaced r414 trimpot.

Amp works. I didn't test with load yet. But other than this unstable bias it seems fine..

 

[jwilhelm]

Your unstable bias may be the amplifier going into oscillation. Can you watch the output with a scope? Is there a zobel network on the output? If so is the resistor getting hot?

 

[starbender]

Unfortunately I don't have a scope. I attached another -larger- schematic.
Can you please check?

 

[jwilhelm]

R446 and R447 are the zobel resistors I was referring to. The resistor and capacitor in series form a low pass filter. Any really high frequency (like usually produced when an amp oscillates) is shorted to ground through this filter. The high current usually makes the resistor get very hot or burn.

 

[starbender]

Thanks. I'll check them when I get home..

 

[starbender]

Zobel network checks fine.
I think I found the problem. As I said most of the pcb traces damaged. I checked closer and saw one 47uf cap has cracked trace. When I repaired that trace bias settled around 30mv. At last

 

[jwilhelm]

Good the hear! It sounds like the amps a mess!

 

[nigelwright7557]

Your unstable bias may be the amplifier going into oscillation. Can you watch the output with a scope? Is there a zobel network on the output? If so is the resistor getting hot?

I had an amp become DC unstable.
It turned out to be the feedback circuit to ground electrolytic capacitor was reverse biased which caused the DC offset to be amplified by AC gain !
I reversed the capacitor and it was then very stable.