[Hammarlund] Old Hammarlunds

[Kenneth G. Gordon]

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?

> They have a long warm up drift 
> time but eventually stablize.

I left mine on and never turned it off for several years. It was also rack-
mounted. Long-term drift was never a problem after the first week or so. My 
basement ham shack at the time had a very stable temperature.

> The back and forth drift may 
> have come from the AVC working because there is no voltage 
> stabilization on the oscillator and it changes with RF gain. 

Yes, it could have, all right. 

Well, in this case, I remember it doing it pretty noticeably after I had added 
the product detector mod, and when listening to a weak signal on 20. As I 
said, it wasn't unpleasant. In fact, I rather liked the sound. The back-and-
forth drift was probably not more than 20, or maybe even 10 Hz. It did not 
make it at all difficult to copy the signal, even for RTTY.

BTW, the product detector mod I made to my first '779 made an amazing 
difference in the receiver. I was VERY impressed. I intend to do that same 
thing to the two I have to restore.

> These receivers can be made much more stable by adding a 
> 150V regulator tube for the oscillator plate. The VR tube 
> should have about an 8k dropping resistor and the 12K 
> oscillator plate decoupling resistor is eliminated.

Excellent suggestion. Unlike some folks, I LIKE VR-tubes, when they are 
connected correctly and working properly. I like the colors for one thing. :-)

> This 
> 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