Siemens Sitral "a212" needs help

[alex0terry]

Hi I stumbled uppon a pair of Siemens Sitral a212 fixed gain mic pres with really nice haufe trannies
I got them to work, but cannot solve the fixed gain issue.
Could find any info or shematics on those candies, so maybe there is a saviour here who know something about the circuit?
Or just overall, how to aproach this issue

Thanks
Alex.

 

[jensenmann]

If you post pictures then we can give you a more educated guess than without.

...and welcome aboard!

 

[alex0terry]

 

[alex0terry]

Has anyone modified those?

 

[jensenmann]

Without any trimmers on the board the gain must be defined in a feedback loop through one of the fixed resistors. Without a schematic it´s barely possible to find that one. You´ll have to trace the schematic down from the PCB. If that particular resistor is known it can be replaced with a pot+resistor or gainswitch.

 

[alex0terry]

Without any trimmers on the board the gain must be defined in a feedback loop through one of the fixed resistors. Without a schematic it´s barely possible to find that one. You´ll have to trace the schematic down from the PCB. If that particular resistor is known it can be replaced with a pot+resistor or gainswitch.

Yeah you are right, I guess there is no shortcuts on this one
Thanks for the advice !