[Hammarlund] update HQ-180 issues

[Richard Knoppow]

----- Original Message ----- 
From: "Roy Morgan" <k1lky68 at gmail.com>
To: "GRANT YOUNGMAN" <nq5t at tx.rr.com>; "Hammarlund Mail 
List" <Hammarlund at mailman.qth.net>
Sent: Tuesday, December 16, 2014 9:44 PM
Subject: Re: [Hammarlund] update HQ-180 issues



On Dec 17, 2014, at 12:10 AM, GRANT YOUNGMAN 
<nq5t at tx.rr.com> wrote:

>
>>>
>>  The HQ-180 has an "auto response" function that varies 
>> the audio bandwidth according to gain….
> This circuit actually causes problems even when it’s 
> working correctly.  It pretty well screws up HQ-series 
> audio, IMO.  There’s a simple fix for it here:
>
> http://www.amwindow.org/tech/htm/hqaudiomods.htm 
> <http://www.amwindow.org/tech/htm/hqaudiomods.htm>

There is also a mod by Dallas Lankford.  HIs mod is simpler 
and may accomplish about the same things.
Called HQ-180 Audio Fidelity Mod.jpg
I don’t know where I got it.

There was a thread entitled "HQ-180 sweep alignment” on 
antiqueradios.com:
 http://www.antiqueradios.com/forums/viewtopic.php?p=991731
That talks in part about the audio of these radios (and also 
about differences in manuals and that the HQ-170 manual is 
of value in aligning the HQ-180).  This thread includes the 
mods by Larry - W9MDX cited in the am window link above.

Roy


Roy Morgan
RoyMorgan at alum.mit.edu
K1LKY Since 1958

    I don't think I quite like the W9MDX mod. He has 
disabled the feedback resulting is a narrower response and 
higher distortion. I think its possible to keep the feedback 
but make it constant. I have not quite worked out how to do 
this in this receiver.  There are a number of receivers with 
feedback around a single pentode stage to improve response 
and lower distortion, among them are the TMC GPR-90 and the 
RCA AR-88.  Both use feedback from transformer to the 
cathode of the driver stage.
    Also, the author has misunderstood the purpose of C-132, 
the 0.005 uf cap, it does not go from plate to screen grid 
but from plate to ground and is to prevent parasitic 
oscillations in the output stage and damage to the output 
transformer. It should be left in place. Capacitors for this 
purpose are also found in amplifiers without feedback.
    He also recommends changing C-47 from 0.01 to 0.1 uf. 
This is the coupling cap between the detector and the volume 
control. It works in a very high impedance circuit and I 
think such an increase is unnecessary. I think he has done 
it to increase bass but removing the feedback has probably 
decreased the bass. In any case, the time constant can be 
calculated to find out where it is actually rolling off. In 
most single-ended output stages like this one the output 
transformer is skimped and trying to push a lot of bass 
through it is not a good idea.
    I don't know what Dallas Lankford has done.  I do think 
that investigating the "couplate" is a good idea. I don't 
know how stable they are. I think the caps are ceramic, 
which are very long lived but the resistors may have 
changed. In any case its worth checking.

    The handbook at BAMA does not have a procedure for 
aligning the 60Khz IF. Hammarlund may not have published 
one. Its not a circuit I would try adjusting by guesswork or 
touch even with the proper procedure unless there is good 
reason to think its mis-tuned.
     This is not a receiver intended for high-fidelity 
broadcast reception. To have any sort of fidelity from AM 
you need a very broad IF with, ideally, a rather flat top. 
Not many receivers have such an IF. A couple of exceptions 
are the RCA AR-88, the old series Super-Pro and the 
SP-600-JX. A few receivers have combination over and 
critically coupled IF stages that have fixed bandwidth, for 
instance the HQ-120-X and 120-X and the Collins 51J series. 
Collins provides a procedure using a detuning circuit for 
alignment but a sweeper is necessary for an HQ-120/129 or 
for the AR-88. The Super Pro SP-600-JX and older 
SP-100/200/400 series have the IF transformers critically 
coupled at minimum non-crystal bandwidth so they can be 
simply peaked at the crystal frequency. Some of the 
transformers in the others are overcoupled with 
double-humped response and can not be adjusted in this 
simple way.  BTW, this is sometimes referred to as 
"staggered tuning" but is is not, its expansion of the 
bandwith by adjustment of mutual coupling. Stagger tuning is 
the use of a multiplicity of critically coupled transformers 
tuned to different frequencies. This is a very common 
arrangement in vacuum tube television sets and in other 
wide-band gear such as some radar sets. While each 
transformer can be tuned to a given frequency usually a 
sweeper is necessary for the best adjustment.
    Another BTW, there are several methods of changing 
coupling in IF transformers. The best is the use of 
mechanical adjustment to vary the distance and thus the 
mutual inductance of the primary and secondary.  AFAIK the 
only receiver to have employed this method is the Super-Pro 
SP-100/200/400.  The SP-600 and RCA AR-88 use tapped 
auxilliary coils and Hallicrafters in several of its 
receivers uses capacitive coupling.  The difference is in 
the way the band expands. The mechanical method is the only 
one that provides a change in pure mutual inducance with no 
shift in center frequency. The tapped coil method is very 
nearly as good. The capacitance coupling method depending on 
the way the capacitors are connected usually provides 
expansion to one side so that the if tunes either higher or 
lower than the center frequency.
    Enough already, I am recovering from a cold and have a 
writing jag.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com