Re: FG 504 Offset problem


Thank you for your replies.
First, I have to correct my first post:
I pulled out Q1150 and Q1160. Q1170 is still on the board.
Also, I forgot to mention that the +/- 25V are also OK.
According to the suggestions, I checked R1160 and R1180 and they are fine.
The voltage on Pin 2 varies from -17mV to about +3.0V, at the two extremes of offset adjustment,R1130.
I have not checked Q1170 and Q1180 because the negative offset is working fine, even with Q1150 and Q 1180 out of circuit.
I have been thinking about trying to replace U1145 from the top of the board, using my dessoldering gun (vaccum type).
I used that to remove Q1150 and Q1160 and it worked fine, but transistors are much more easier to dessolder that IC's.
Does anybody ever tried that?
I am afraid of damaging the board or get stuck with a overheated IC still stuck in place.

--- In TekScopes@..., "Tom Miller" <tmiller@...> wrote:

Do you have the +/- 25 volts? What is on pin 2 of the amp? Can you set that to 0 volts
with R1140? The +7 at pin 6 sounds bad if pin 2 is near 0 volts. What about Q1160
and 1180. Are they ok? Check R1160 and 1180.


----- Original Message -----
From: gehp01
To: TekScopes@...
Sent: Saturday, August 28, 2010 5:16 PM
Subject: [TekScopes] FG 504 Offset problem

Dear All,
The positive side of the DC offset of my FG-504 is not working, but negative is working fine.
I thought this would be an easy fix, but I am about to quit.
In case someone gets interested, I have uploaded this section of the schematics.
First, I checked the + and - 15V and the +/- 4V input and they are ok.
Next, I pulled out Q-1150 and Q-1170 and, to my surprize, both are ok also.
With these transistors out of circuit, the output of U-1145 goes from
-13V to + 7.0V.
I ran out of ideas.
I would not venture to replace U1145 since this would mean to disassemble almost the entire unit.
Any suggestions?
Thank you

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