[Hammarlund] HQ180A S Meter Not Tracking RF Level Properly

[wmacleod2 at sympatico.ca]

Thank-you very much Les! Hollow State News seems a wealth of information. I wonder if it is searchable online somewhere 😊

This should be fun determining what vintage of S meter design I have and how to fix it to scale in a linear fashion.

Apparently Steve has some data as well so I look forward to hearing from him.

Many Thanks and I will let you know how it goes
Wade
VA3HM

-----Original Message-----
From: Les Locklear <leslocklear at hotmail.com> 
Sent: Tuesday, February 1, 2022 1:42 PM
To: wmacleod2 at sympatico.ca; Hammarlund at mailman.qth.net
Subject: Re: [Hammarlund] HQ180A S Meter Not Tracking RF Level Properly

Here are a couple of old issues of Hollow State News Issue 17 page 5 and issues 24&25 (double issue) page 10. That should get you in the right direction.

Les Locklear


----- Original Message -----
From: <wmacleod2 at sympatico.ca>
To: <Hammarlund at mailman.qth.net>
Sent: 2/1/2022 10:28:41 AM
Subject: [Hammarlund] HQ180A S Meter Not Tracking RF Level Properly ________________________________________________________________________________


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

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