[Hammarlund] B on SP-200 faceplate?

[Kenneth G. Gordon]

On 27 Mar 2013 at 19:56, Richard Knoppow wrote:

>      The BFO injection on mine was reasonable and I used it 
> for both CW and SSB.

I now suspect that the reason the BFO injection was so low on my original 
BC-779 was that there was a defective part, probably the BFO coupling cap.

>  A product detector would help

The one I added didn't just help: it made a simply amazing difference.

> but I 
> think you can improve injection by increasing the value of 
> the coupling cap.

Well, athough I am sure that would help, what I did made a far greater 
difference.

>      The AVC system is such that the BFO is pretty well 
> isolated from the AVC.

Yes. It has its own IF transformer and IF amp.

>      The Geisler mods resulted in low gain. I don't remember 
> what else. Unfortunately, the RF and mixer tubes used in the 
> Super-Pro are very noisy especially the mixer.

Well, yes, but there are two RF amps, at least in the BC-779. After I 
incorporated my product detector, that BC-779 became unusually quiet. The 
two RF amp stages masked the mixer noise almost perfectly. I could hear 
signals on that receiver that I didn't even know were in there before the mod. 
The BC-779 sounded better on 20 meters than my hot-rodded SB-101. The 
BC-779 was quieter and just plain more transparent than the SB-101.

The increase in apparent sensitivity would have to be heard to be believed.  I 
was truly amazed.

I have always wished I had been able to do a before and after comparison of 
sensistivity and signal-to-noise ratio. At the time, I simply didn't have the 
equipment.

> It taks some 
> care if you want to re-design the front end that you don't 
> wind up with overload problems and spurs.

Yes, but it can be done.

>  Somewhere I have 
> a Signal Corps technical manual (don't remember the number) 
> which has details of all the receivers used at the time (mid 
> 1940s). One set of charts shows image and spurious responses 
> of the Super-Pro and SX-28. The Super Pro has only one 
> image, quite low, while the SX-28 looks like a corn field. 
> I've been trying to find this book in my stored stuff with 
> no success so far, but its there!

Well, when you find it, I am sure that there are many of us out here who 
would LOVE to read it.

>      FWIW, the RF amplifiers in the BC-1004, the 40 mhz 
> version of the Super Pro, are shunt connected instead of 
> series connected as in the other models. I think this was 
> done to improve the Q of the coils by getting any DC 
> magnetization off of the iron cores.

That IS interesting, all right.

>  The Super Pro is also 
> one of the few receivers that have genuine Faraday shields 
> on the antenna coils. They are woven metallic cloth between 
> primary and secondary of the coils. You can see them if you 
> take the bottom cover off the coil compartment.

Yes. I have seen them.

>      The broadcast band and LF bands are also broad banded 
> using resistors. Otherwise the bandwidth is not as wide as 
> the IF.

16 KHz... :-)

>      The Super Pro seems to be the most expensive receiver 
> on the general market but the RCA AR-88 was probably double 
> the price. Most, if not all, AR-88s were sold to the 
> government.

A large percentage were shipped to England. I have never had the 
opportunity to use an AR-88, but have always wanted to.
 
Ken W7EKB