OK I have done some further testing and found the following:
With the output of U1311 (pin 6) disconnected and the inverting
input at -13V and the non-inverting input at -4.5V the out still
remains at -13V.
Disconnecting pins 1 and 8 offset null adjustment does not change
this.
Disconnecting the frequency compensation components R1301 and C1301
allows the output to swing to 14V (what it should be).
Addition of external frequency components of the same value applied
to pin 5 does not force the output back to -13V
Conclusion one of the frequency compensation components is faulty
replacing C1301 has so far confirmed this as it is now working
properly.
This has been a really hard problem to find!
Has anyone encountered this issue before with the external frequency
components forcing the output of an OP-amp to go the wrong way?
I can't seem to measure anything that indicates that the capacitor
is stuffed either. Has anyone found any issues with the green axial
capacitors that Tek used in this vintage stuff?
--- In TekScopes@..., "cmbath" <cmbath@...> wrote:
I have 1 CG551AP which is having issues with its Low SAC that I
can't seem to understand. The output of the Low SAC is being
driven
to -12.5V, after some stuffing around I have found that the output
of U1311 which provides offset adjustment for the Low SAC has is
output at about -13V despite the inverting input (roughly -13V)
sitting well below the non-inverting input (roughly -4.5V). I have
tried removing U1311 which allows the Low SAC to function normally
apart from there being an offset which is unable to be corrected
for. I have also tried replacing both U1311 (LM725) and U1211
(LF356) which did not help at all.
The big mystery for me is why the output of U1311 is not at
positive
saturation rather than what appears to be negative saturation
given
that the voltage of the inverting input is below that of the non
inverting input. The input voltages appear to be in the specified
range for the supply voltage being used i.e. +/-13.5V (worst case)
for a supply of +/-15V. I am even considering the possibility that
the circuit may be going unstable due to the frequency
compensation
components going out of spec.
Has anyone experienced this problem before with the Low SAC in the
CG551AP or had similar issues with OP-Amps?
