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

[Kenneth G. Gordon]

On 9 Jul 2011 at 10:50, Richard Knoppow wrote:

>     I would be loath to modify 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.

Yes. Which, nonetheless, did not make them superior for the 
purpose...

> 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.

All of what you offer above is excellent advice, Richard, and it is 
helpful to be reminded of all those matters.

However, in the case of my particular HQ-110, and as I mentioned in 
one of my previous e-mails, the instability I am suffering with is 
NOT the long-term version we are talking about above:

Mine manifests itself by extreme, and I mean EXTREME, "pulling" of 
the received frequency by strong signals.

Furthermore, this came on rather suddenly.

For instance, while I am listening to a fairly strong CW signal, I 
can hear the frequency of adjacent signals being pulled back and 
forth in time with its keying, AND in many cases, chirp on the signal 
I am listening to, when the same signal in another receiver has no 
chirp.

Yet voltage checks show no apparent DC or AC voltage changes.

As I said, I suspect the second converter (6BE6+crystal) stage at 
this point.

Ken W7EKB