F4 dc offset problem

[rosario]

Hello everybody , i have a problem with my f4 amplifier.My right chanel the dc offset when i turn on i have -70mv and after 3 hours is near 0mv , but with my left chanel i have some problems when i turn on everytime i have different value and and goes up and down very fast and the las value was from 86mv to 120mv in 10 minutes.

Thx for help, greetings from Romania.

 

[Zen Mod]

as I said in PM - post pictures

double check mounting of outputs on heatsink - are they tightly & evenly screwed on heatsink , check all solder joints (including all resistors in biasing network)

replace TL431 - it's cheap anyway

luckily - your problem isn't connected to input Jfets in any way

 

[Dzius]

I had similar problem caused by P2 trimpot (500 ohm). Change it then. Besides I noticed that it's very easy to set DC offset very close to zero within range of -3 to +3mV or smaller. If problem still exist replace TL431 as Zen Mod said.

 

[rosario]

Thx for advice , i will try these days.

 

[gomorracamorra]

I put an 500 ohm trimpot and the lowest voltaje was 1,516 volt. What can i do?

 

[rosario]

And with an 2 kohm the lowest voltage was 478 mv, i will try with an 5 kohm tomorrow.

 

[rosario]

After a new 5 kohm trimpot nothing change, i changed also the TL431 same problem.
What can i do , other changes?

 

[Zen Mod]

pics

 

[rosario]

The problem was TL431 , i change it and is working now. Thx people.

 

[biju55]

thx zen mod