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rosario 15th February 2013 05:19 PM

F4 dc offset problem
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 15th February 2013 05:32 PM

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 18th February 2013 07:46 PM

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 19th February 2013 01:49 PM

Thx for advice , i will try these days.

gomorracamorra 20th February 2013 01:22 PM

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

rosario 20th February 2013 02:45 PM

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

rosario 21st February 2013 08:41 AM

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

Zen Mod 21st February 2013 09:25 AM


rosario 27th February 2013 07:45 PM

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

biju55 27th February 2013 09:43 PM

thx zen mod

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