[Hammarlund] HQ-110...rats...

[Richard Knoppow]

----- Original Message ----- 
From: "Kenneth G. Gordon" <kgordon2006 at frontier.com>
To: "David Langley" <dave at daveandsue.com>
Cc: <Hammarlund at mailman.qth.net>
Sent: Saturday, July 09, 2011 9:43 AM
Subject: Re: [Hammarlund] HQ-110...rats...


> On 9 Jul 2011 at 8:50, David Langley wrote:
>
>> Well I haven't had drift problems with my HQ-110 but hear 
>> about this
>> from others like your self.
>
> I have heard of drift from most Hammarlunds which use the 
> 6C4 or
> equivalent as an HFO. Many don't drift, however.
>
> My HQ-110 didn't have this problem shortly after I 
> "restored" it. It
> popped up some time later.
>
>> Might be something other than the 6C4,
>
> Agreed, but I have always felt that a simple triode HFO 
> was a problem
> waiting to happen. Other oscillator circuits are 
> inherently more
> stable; the so-called ECO, for one.
>
>> such as line voltage variations,
>
> not the problem here,
>
>> resistor or capacitor changes with
>> heat.
>
> perhaps, here, but unlikely: it happened too quickly
>
>> If a tube change doesn't solve the problem, I'd suspect 
>> something
>> else before I change tube sockets etc.
>
> Yes. While I certainly agree with you, and while I intend 
> to, FIRST,
> substitute tubes, I also want to play a bit with the 
> circuit to see
> if I can reduce both the instability AND the necessity for 
> using
> "perfect" tubes to get that stability in the HFO.
>
> I would like to make the circuit less "touchy" and more 
> reliable in
> that regard. I think it is really unnecessary to need 
> especially good
> tubes to make the circuit work properly. To me, that 
> requirement
> proves a poor design.
>
> Any really good circuit should be as independent of the 
> quality of
> the active device as possible.
>
>> Just an idea. Let us know what
>> you do find that cures the problem. Dave -W5QWX
>
> Oh, I will, Dave, thanks.
>
> The main reason I enjoy forums like this is that it gives 
> me an
> opportunity to share ideas and experiences with other 
> like-minded
> curious individuals.
>
> By the way, the instability that suddenly showed up in my 
> HQ-110
> manifests itself, primarily, in really SEVERE pulling by 
> even
> moderately strong received signals, yet voltage checks 
> show no
> significant voltage changes with signal input.
>
> The regulator is regulating just fine too.
>
> At this point, although I am only "somewhat" suspicious of 
> the 6C4
> HFO, I am VERY suspicious of one or both of the 6BE6s. I 
> hate, loathe
> and despise pentagrid mixers, especially self-excited 
> ones.
>
> In fact, in this case, I am MOST suspicious of the "first 
> converter":
> the 6BE6 mixer which is also a crystal oscillator.
>
> Mine is the first version.
>
> Ken W7EKB


    I would be loath to mofify the receiver unless the cause 
is proved to be the tube type. 6C4's and its octal-base 
predecessor were very widely used as oscillators in all 
sorts of receivers. The simple circuits used with them tend 
to be sensitive to voltage fluctuation, both plate voltage 
and filament voltage. If you have drift when the line 
voltage is known to be steady you must look elsewhere. Other 
causes are temperature drift and unstable components. To 
isolate temperature drift you must change the temperature. 
One way is to let the receiver heat up for long enough to 
reach some sort of equilibrium, maybe a couple of days! The 
heat the chassis, or at least the parts that determine 
frequency with a hair dryer or even just a hot light like a 
reflector flood light. See what happens. Since some 
receivers seem to be stable and others do not I suspect a 
small component, perhaps a ceramic capacitor, rather than 
the main tuninig capacitors although they may be the problem 
(like a good detective you can't assume anyone is innocent 
without some evidence). Its common to use ceramic capacitors 
with high temperature coefficients to compensate for other 
thermal drift. If the compensating caps have changed they 
may exagerate drift instead of reducing it. Also, the 
temperature coefficient of ceramic caps varies over a very 
wide range depending on the exact type of dielectric used. 
Small value ones can be NPO, that is no change, but larger 
ones often have a very substantial change. The ceramic used 
in NPO caps has a relatively low dielectric constant so 
large value caps are physically large. They are still not 
big from the boat anchor point of view but there has been a 
trend to smaller ones. For many applications the thermal 
coefficent doesn't matter but it does in frequency selective 
circuits like oscillators. Also check for resistors that for 
some reason are temperature sensitive. Both of these can be 
checked with a heat gun capable of concentrating the heat in 
a controllable area or even just a soldering iron. Use 
Freeze Mist or canned air to cool components you are 
suspicious of.
     Also do some of the common voodoo, work any screws that 
may be used for grounding since flakey contacts can be 
temperature sensitive. Check for flakey solder joints. Poke 
the wires around with an insulated stick to see if here is 
anything unusually sensitive.
     It may well be that Hammarlund had problems with the 
tuning caps or coils. If all, or even most, of the receivers 
of a given type had the same problem I would be more likely 
to suspect this. I don't know how widespread the problem is 
with the HQ-110.
     Many really old receivers, the pre-war Super-Pro is a 
good example, had no temperature compensation and take at 
least 48 hours of continuous running to stabilize. They also 
have no voltage regulation on the oscillator. I used one as 
a station receiver for a long time. I modified mine with a 
VR tube on the oscillator, which keeps the frequency from 
varying around with RF gain. Once stabilized it was actually 
very stable.
     Note that if the trouble is due to poorly constructed 
coils or air capacitors that no circuit will cure the drift.


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