AKAI GX-F91 REC DIFFERENTIAL AMPLIFIER OFF-SET

[Sergei A]

Any suggestion. I can’t adjust the offset setting in the left channel of the rec. differential amplifier. In SM should be up to `+/-10mV, but in fact the voltage in TP1 fluctuates after fine tuning in a couple of minutes from 5mV to 60mV? Was checked transistors TR,25,26 replaced TR24 and electrolytes C6,7.

 

[jan.didden]

Well, all the voltages are penciled in the schematic so it should be straight forward to verify them and find the problem and/or where it drifts

Jan

 

[Sergei A]

All voltages in both channels are the same as the in diagram. Defect start after several minutes after power on.

 

[MarcelvdG]

Dirty trimming potmeter?

 

[Osvaldo de Banfield]

Try a termal issue in a semiconductor using freezing fluid.

 

[Sergei A]

Freezing fluid didn't help.

 

[Osvaldo de Banfield]

This seems to be a lind of opamp discretely built. What is the DC return to the right JFET from the output, if any?

The termal derivative in the other channel is +/- the same? Is it really important for the entire rig?

 

[Sergei A]

Sorry I'm not understood. Can you tell me more about DC return? The thermal derivative in the other channel is normal , off-set is stable. Will replace potentiometer.

 

[Osvaldo de Banfield]

Perhaps there is a conductive path from TP1 and right JFET's gate. (Say, a resistance or a coil not shown in the piece of schematic above depicted).

 

[Sergei A]

Perhaps there is a conductive path from TP1 and right JFET's gate. (Say, a resistance or a coil not shown in the piece of schematic above depicted).

Can you suggest possible reasons

 

[Sergei A]

On D8 R CH 2.6V , L CH 1.7V ?? May be D8 fault ?

 

[Osvaldo de Banfield]

Check R89. As its high valued, it may be deffective.

 

[Sergei A]

Check R89. As its high valued, it may be deffective.

R89 91 is ok

 

[Osvaldo de Banfield]

TR 27 & 28? Seems they are using them as switches. D8 is a varicap an is DC isolated from TP1 node.

 

[Sergei A]

TR 27 & 28? Seems they are using them as switches. D8 is a varicap an is DC isolated from TP1 node.

BINGO !!! C9 defective ! Capacitance decrease !

 

[Osvaldo de Banfield]

Is it leacky? A decreasd cap value can't modify any DC value...

 

[Sergei A]

I replaced C9 6800pf (in fact was 4700pf) by 10000pf

 

[Sergei A]

Is it leacky? A decreasd cap value can't modify any DC value...

Many thanks for the help !!!

 

[Osvaldo de Banfield]

Does the drift dissapear?

 

[Sergei A]

Does the drift dissapear?

Yes the drift disappeared , now all voltages in both channels are stable.