[time-nuts] Racal 1992 input amplifier offset problem

[Joop]

Hi,

Sorry if this question is a bit out of place. It is not about timing, but
more an electronics thing. But searching for Racal 1992 seems to get a lot
of hits here on this list.
I have just received a used Racal 1992 counter and are checking it for
proper operation. All seem fine except the channel A only triggers properly
on small signals with DC input or when in "auto-trigger" mode selecting AC.
Checking the trigger level in this mode shows -1.08 (Volts I guess)

I went through the protocol for channel-A adjustment with R192/R149 and
channel-B adjustment with R193/R150.
Channel B seems to work fine and needed only minor adjustment. The offset
can be adjusted to 0V and the specified sensitivity levels of 13mV (count)
and 7mV (gate off) are fine. Changing from AC to DC has no effect, both
work fine here.

However channel A seems to have a problem. It is not possible to adjust it
to 0V in AC mode. Only with DC it can be adjusted to 0. Sensitivity is also
a bit out of range. I get 17mV (count is steady) and 7mV (gate off).

The trigger output levels at the back seem ok for both channels (about
+5.1 to -5.1) So I guess the DAC is still OK.

Looking at the schematics I suspect IC34 (CA3140E) might be having a
problem, but I did not dare to open the module yet. Is it placed in a
socket?
Has anybody experienced a similar problem and fixed it? Are some
capacitors known to go bad? Any help would be appreciated.

Cheers,
Joop