[Hammarlund] Old Hammarlunds

[Richard Knoppow]

----- Original Message ----- 
From: "Kenneth G. Gordon" <kgordon2006 at frontier.com>
To: "Richard Knoppow" <1oldlens1 at ix.netcom.com>
Cc: <Hammarlund at mailman.qth.net>
Sent: Saturday, April 09, 2011 2:50 PM
Subject: Re: [Hammarlund] Old Hammarlunds


> On 9 Apr 2011 at 13:30, Richard Knoppow wrote:
>
>>     The BC-779 in common with other Super-Pro receivers, 
>> has
>> no temperature compensation.
>
> Yes. I know. Interesting, isn't it?
>> will stop the change in frequency when the RF gain is
>> changed. The VR tube can be mounted under the chassis.
>
> If you do that, though, you need to put a pilot light 
> somewhere nearby so that
> the light can "encourage" the tube's firing. They don't 
> work in the dark.
> Besides, when below deck, I can't see the colors.
>
>>      I have never understood why Hammarlund did not adopt
>> both temperature compensation and voltage regulation for
>> this RX since they certainly knew how to do it and had 
>> both
>> on the HQ-120X.
>
> Yes. I never understood that either, and always wondered 
> about it.
>
>>      Note also that even with HV regulation there may 
>> still
>> be drift from the filament voltage varying with changing
>> line voltage.
>
> That is actually pretty easy to compensate for: there are 
> two methods that I
> know of, one of which the U.S. Navy used to replace the 
> ballast tube in the
> SRR-11/12/13 receivers.
>
> It is a pair of back-to-back Zeners. I can share the 
> schematic with you and
> others sometime. It works like gangbusters and doesn't 
> require DC filament
> voltage. One Zener regulates one "side" of the AC voltage, 
> and the other
> Zener regulates the other "side". It turns the sine-wave 
> into a sort of flat-
> topped sine-wave. It works very well, indeed.
>
> The other method is to use a three-terminal SS regulator 
> as shown in the
> latest issue of ER Magazine in the article on the National 
> NC-400. However,
> this one requires DC filament voltage. Besides, I don't 
> care for SS regulators
> of this type in this service.
>
>> Even the SP-600-JX has this problem. There are
>> oscillator designs that are much more stable with voltage
>> changes than the simple ones used in most receiveers.
>
> OH yes! Franklin, Vackar, other two-terminal oscillators, 
> even an ECO with
> the correct relationship of screen and plate voltages. The 
> main trouble with
> any of them is getting them to track with the RF stages.
>
> I have a couple of BC-779s in the queue for restoration, 
> and an SP-600-
> JX1...among about 15 others...
>
> I have a "thing" for receivers. Every one of the various 
> receivers I have ever
> used had something unique about them which I enjoyed.
>
> Thanks for the info, Richard.
>
> vy 73,
>
> Ken Gordon W7EKB

    I used a modified BC-779 as my station receiver for 
several years. I had changed the oscillator to an ECO using 
a circuit from a General Radio frequency meter. It worked 
quite well but, as you say, it shifted the calibration a 
little. I had the VR tube under the chassis and never had 
any problem with it firing.
    I eventually returned the receiver to original and was 
astonished at how much it drifted with voltage. Hoever, it 
brought the dial calibration exactly on all over.
    I still have it and need to get it working again. I also 
let it run continuously and it was quite stable. I used it 
for SSB and never had a problem with drift. OTOH, it 
probably needs about 48 hours of continuous running to 
stabilize.
    I had thought of installing a product detector and slow 
AVC but never did. That may be a future project. This 
receiver was modified when I got it. The mod followed on 
which appeared in an early edition of CQ magazine and used 
cathode coupled 6SL7's for RF and mixer stages. It may have 
been quieter than the original pentodes but was not really a 
good mod. This RX has evidently had a fixed frequency 
crystal oscillator in it at one time. It has two holes in 
the front panel over the main tuning dial, that would have 
been for the frequency selector and fine tuning control as 
in the SP-600-JX, I've seen pictures of receivers with this 
mod but have never seen it documented. It makes me wonder 
why, if Hammarlund knew about how these receivers were 
employeed they didn't build in a crystal oscillator in the 
SP-600-JX instead of having the obvious add on. Perhaps they 
thought it would be stable enough not to need fixed 
frequency crystals. However, there are a lot of mysteries 
about the SP-600-JX. From the early advertising it seems it 
was either entirely re-designed or, perhaps, the early specs 
were wishful thinking and no prototype was built.
     A note about the SP-600-JX. I've found that the stators 
of the tuning capacitor often are out of square with the 
rotor. I think they creep with time. Its not difficult to 
get them centered and correctly alighned vertically. DO NOT 
EVER BEND PLATES!!!, its not necessar and will ruin the cap. 
I know there is one site that shows complex instructions for 
plate bending but they guy didn't understand how the cap 
works, don't do it. When adjusted correctly the frequency 
calibration is very accurate and RF tracking is excellent. 
If you get the cap right and there is still one or two bands 
that are off its probably the padders on the individual 
oscillator modules. Silvered mica cap can go bad and I've 
found a few in these RX.


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