[Hammarlund] HQ180A S Meter Not Tracking RF Level Properly

Tue Feb 1 15:55:18 EST 2022

Wade,

The S-meter circuit in the HQ-180A I'm restoring had a bizarre problem which led me to do a deep-dive into how the circuit actually works.  The problem I was experiencing was that if you zeroed the meter as instructed in the manual and then turned the AVC to the Off position, the needle would pin negative.  Zeroing the meter with the AVC Off stopped that behavior, but then the needle wasn't at zero when the AVC was on and the RF gain at minimum.  Furthermore, the needle would shift to radically different positions on the scale when you switched between Slow-Med-Fast (with the RF gain at zero).

To make a long story short, the S-meter circuit operates like a VTVM, using a bridge circuit.  V13 and R88 are on one side, and R20, 21, and 22 on the other.  The zero adjust "balances" the bridge when no signal is present and the needle goes to zero.  I replaced every resistor in the entire circuit, including the resistors at the switch, and it STILL wouldn't behave.  The problem?  Age, most likely in the wiring.  That bridge is very sensitive and it only take a tiny amount of imbalance to affect the meter operation.  The S-meter wiring, as well as the wiring to the AVC switch, are all buried in harnesses and have been subject to decades of degradation.  

Once I learned how the meter circuit actually operated, a fix was found:  it involved experimenting with resistances between the R20 zero adjust pot and R22 until the circuit would balance again.  It ended up being around 800 ohms.

Problems with those resistors, and the aging circuit, can absolutely affect how the S-meter works.  Check the resistors at the switch as well.  For maximum improvement, however, you might want to run new wiring to the S-meter and the switch -- I think it could be done without too much trouble.

73 - Steve, KW4H

On 2/1/22, 1:11 PM, "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:

    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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