[Hammarlund] HQ180A S Meter Not Tracking RF Level Properly

[Steven Reed]

I'm restoring a HQ-180A right now and had to deal with S-meter issues, which required learning how the circuit operates.  Got to head out to run some errands but will respond with details when I get back.

Steve, KW4H

On 2/1/22, 9:29 AM, "hammarlund-bounces at mailman.qth.net on behalf of wmacleod2 at sympatico.ca" <hammarlund-bounces at mailman.qth.net on behalf of wmacleod2 at sympatico.ca> wrote:

    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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