[Hammarlund] Re: HQ-170A S meter problems

[Roy Morgan]

At 03:47 PM 2/25/02 -0500, Joe W2KJ wrote:
> > Have been chasing my tail trying to solve a problem with my
> > HQ-170A S meter.
> >
> > After warm-up the meter pointer is pinned to the left side
... Interestingly, when I swap the 12AU7A for a 12AT7A, the
>meter  unpins and the pointer settles to about mid-range.


Joe,

I suggest that the next step is to understand how the S-meter circuit works 
and to measure the voltages in it.  (Does the manual give normal tube 
voltages?  Something is unbalanced in the circuit with no AGC 
voltage.  Suspect the following:

  - there IS an agc voltage when there should not be
  - the AGC voltage is right but the sensing and meter driver circuit is 
unbalanced somehow.

Separate the problem into two parts:
-first, get it to zero properly with no signal in the receiver (pull out 
the last IF tube, and check the state of the AGC line to be sure it is right)
-second, get the thing to indicate correctly once the zero condition is 
established.

You may need to feed a "fictitious" agc voltage to the thing (a 9 volt 
battery an audio volume control and 4 clip leads will do the trick).  You 
may have to measure voltages at high impedance circuits: the VTVM is good 
for this..your 4-digit VOM or trusty Simpson Model 360 may be useless.

Good luck,
Roy


- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
7130 Panorama Drive, Derwood MD 20855
Home: 301-330-8828 Work: Voice: 301-975-3254,  Fax: 301-948-6213
roy.morgan@nist.gov --