[Hammarlund] Drift of Newer Hammarlunds? was Re: Old Hammarlunds

[Kenneth G. Gordon]

On 8 May 2011 at 8:20, Glen Zook wrote:

> For AM use the Hammarlund receivers were fine.

And for CW. My BC-779 was OK with RTTY too, once throughly warmed up.

>  It was when SSB became
> popular that the drift in a number of the receivers started becoming
> problematic.  The series starting with the HQ-120X, then the HQ-129X,
> HQ-140X, and the HQ-150 were all generally very stable after between a
> 30 minute and 1 hour warm up period.

I suspect that part of the reason for this was that, in the HQ-129X at least, 
the HF oscillator was included in the pentagrid mixer, a 6K8.

All the others mentioned used a single 6C4 triode as the HF oscillator, and, 
in addition, temperature compensation was not well-thought-out nor 
implemented.

6C4s, especially those remaining to us today, can be very variable as to 
quality, and proper choice of a "good" 6C4 can do much to correct serious 
instability. Changing 6C4s will not do much to help the normal instability 
however.

According to my reading on the matter, proper temperature compensation, 
and the addition of either a cathode-follower following the 6C4, or a complete 
replacement of the 6C4 triode oscillator circuit with a true ECO circuit makes 
a substantial difference in stability.

One of the biggest problems for us today is the availability of TC caps. They 
are very difficult to find in any quantity or variety now.

Ken W7EKB