Forte F44 preamp, DC Voltage leak

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terfyo

terfyo

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#1
Hi all,

Hopefully I'm able to get some help from the pros out there.

Been meddling and trying to solve this problem for a long time.
I have a Forte F44 preamp and i measured the DC output of both channels on the XLR pins to ground as reference. One side is giving me 31.7mV constantly and the other varies sometimes when i play with the volume control. The range that it varies is around 30-80mV.

Which part of the pre amplifier usually causes this problem?

Or, if somebody send me the service manual please.

My email address is lim.tim.mx@gmail.com

Thanks!
 
#2
terfyo said:
Hi all,

Hopefully I'm able to get some help from the pros out there.

Been meddling and trying to solve this problem for a long time.
I have a Forte F44 preamp and i measured the DC output of both channels on the XLR pins to ground as reference. One side is giving me 31.7mV constantly and the other varies sometimes when i play with the volume control. The range that it varies is around 30-80mV.

Which part of the pre amplifier usually causes this problem?

Or, if somebody send me the service manual please.

My email address is lim.tim.mx@gmail.com

Thanks!
Click to expand...

When you say "one side" has a variable D.C. offset, do you mean one of the channels, or one of the XLR output phases on both channels?

If the answer to that question is the variable offset appears on only one of the channels, then there may be a D.C. offset at the input to that channel. The F44 is D.C. coupled, so if there is a D.C. offset at the input it will appear at the output and will vary with the volume control setting.
 
#3
Ken Newton said:
When you say "one side" has a variable D.C. offset, do you mean one of the channels, or one of the XLR output phases on both channels?

If the answer to that question is the variable offset appears on only one of the channels, then there may be a D.C. offset at the input to that channel. The F44 is D.C. coupled, so if there is a D.C. offset at the input it will appear at the output and will vary with the volume control setting.
Click to expand...

Hi Ken,

Thanks for the quick reply.

One side = One Channel (Left).

I did a pin 2 "Hot" on one crocodile clip and the other clip to ground.

Latest update, the DC spike only comes once, then it'll settle down at around 40 +- mV.

There are 2 trimmers inside the pre, one for each channel and i suppose they're for Class A bias tuning. Though I don't really know what exactly they are for.
 
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