[Hammarlund] Old Hammarlunds

[Kenneth G. Gordon]

On 12 Apr 2011 at 11:23, djed1 at aol.com wrote:

> I have both a BC-779 and a SP-400, so I'll offer my experiences. It's
> been a couple of years, so some issues are a bit hazy. I restored the
> BC-779 and was surprised at how bad it drifted- 75 KHz from a cold
> start at 20 MHz.

That sounds about right. An early QST says "...50 Kc...", but I think that was 
wishful thinking. 

However, from my experiences with an essentially new BC-779 back in 
about 1959 or so, after mine had been left turned on for about a week (rack-
mounted), in a temperature-stable environment (basement shack), drift was 
no longer a problem. On 20 meters, mine drifted back and forth a few 10s of 
Hz.

As I said, I found it quite pleasant to listen to...

> I finally found some TC caps, and put in two 10 pf in
> series. This reduced the drift to about 50 KHz.

According to a short article in QST magazine, adding 3.3 pfd of negative TC 
caps to the oscillator section of the tuning cap in the BC-779 reduced the 
drift to 200 cycles....but I find that difficult to believe...maybe in a 
temperature-controlled oven. ;-)

> This is with the dust
> cover on- it may be significantly better in open air, but nowhere
> close to anecdotal numbers. As for the SP-400, I got one that had TC
> caps installed, but I have no idea if they are original. The radio had
> been worked on, so they may have been added. I did get the original
> manual, and the schematic doesn't show any compensation.

The manuals I have downloaded don't show any either.

> I do remember
> that the SP-400 drift was much better than the BC-779 when on the
> bench- maybe 15 KHz. But after I put it in the cabinet it wasn't much
> better than the BC-779.

We were always told to prop the lid of the cabinet open, which reduced the 
drift noticeably.

> Just to show how the technology changed, my
> SP-600 drifted about 12 KHz from a cold start, and the R-390A drifted
> 0.4 KHz. Ed W2EMN

Yes. 

But in my experience, once most of these old receivers have been "heat 
soaked" in a temperature-stable environment, they're all right. Not perfect, 
mind you, but quite acceptable.

I suppose that one could, with considerable effort, and a handful of TC caps, 
reduce the drift on every band to much more reasonable proportions.

I have another article that proved that the Swan 350 drift could be reduced to 
VERY acceptable proportions with that sort of work.

Ken Gordon W7EKB