[Hammarlund] HQ180A S Meter Not Tracking RF Level Properly

[wmacleod2 at sympatico.ca]

Hello Hammarlund Group

Does anyone out there have a description how the S-Meter circuit in the
HQ180A  functions? I have been troubleshooting an issue but am not sure
where to go next. 
I would appreciate any suggestions. Here is the issue.

Description of Issue
The HQ180A was restored and realigned by me in 2004 and has been working
great until now. My issue is that when calibrated to 50 uV for S9, the meter
will not increase much beyond S9 with larger signals even with 1000 uV! It
is not tracking the incoming signal level . It is still extremely sensitive
for low signal levels and handles large levels without distortion. 

Testing

The S Meter was first calibrated to S9 with a 50 uV 10 MHz source. S meter
level gradually increases to S9 when RF gain is increased as expected.
However RF levels >50uV show no increase in S meter reading.
With a 10 MHz Strong Signal (1000 uV) input and using a VTVM to monitor L8
&C44, I gradually increased the RF gain to see what happens. With RF gain at
12 O'Clock the S meter showed S9 VTVM negative 5V (as described in the
manual). As I increased the RF gain to full CW the voltage increased to
negative 10V.so there was indeed extra signal that the S meter was not
reflecting. The S meter did not increase with the voltage. 

I then went to the grid input pin2 of V13B (Meter Tube) and observed the
same result.but different voltages of course. In both cases there was about
double the voltage present when the RF gain was increased after the S9
saturation. It was just not showing any increase in the meter.

I then adjusted the sensitivity of the meter to determine if the  meter
could be calibrated at a high setting and confirmed that I could increase
sensitivity to maximum and push the meeting to 40 dB over the S9. This
confirmed the meter is not frozen and can move to higher levels if driven
properly.

I then replaced the V13  tube with a NOS tube with great GM and Emission
measured on an AT1000. Although the meter set points changed, the results
were the same.

Question:

Where do I go next?
1. Should I measure the actual meter current and see if it is increasing
with RF Gain increases beyond S9 50 uV levels?
2. Is there something in the meter circuit which might limit the meter at S9
despite a higher signal input to V13B? 
3. When the RF gain increases and the grid signal at V13B increases, should
it be linear or is it logarithmic? Perhaps doubling the voltage after the S9
point is  not sufficient to drive the meter higher?
4. What voltages would you expect to see at the V13B Grid and the output to
the meter?

I would appreciate any suggestions from the group.

Regards

Wade
VA3HM