[HBR] Restoring old receiver

[hbr@mailman.qth.net]

Stan:
> I recent acquired an old receiver from the late 1940s that is not
> collectable grade.

This can be argued in both directions.

*For* making modifications.   From what you say the receiver is in no 
sense a rare original item -- as it would be if a historic first, fewer 
than 10 known to still exist, etc.  More likely tens of thousands were 
produced. There are lots of other examples out there, and many of 
them are in better condition.   The modifications will improve your 
set's performance.   Modifying sets that work is an excellent way to 
gain experience -- I'm one of the great beneficiaries of that technique. 
 

*Against*.   A receiver of that price range and vintage has  
fundamental limitations that make modifying it to get close to modern 
standards or even tolerable SSB performance pretty nigh impossible. 
When you get the SSB detection and AGC issues resolved, there's 
stability -- *all* such sets drift at least several kcs/hour, more or less 
forever.   If you allow it to warm up 'forever' and it actually does 
stabilize to the point of copying a signal for 15 minutes at a time, 
then you've got the problem of bandspread (usually 100 kcs or more 
per knob revolution even on lower bands, tho some sets have a type 
of two-speed tuning) and backlash.   The power supply is unstable 
and if you install transformers it will be worse.   The oscillator will 
move with any change of line voltage or (because of the variation of 
plate voltage), AVC action with changes of signal strength.  
Oscillator isolation?  Hahahaha ... you've got at best a triode driving a 
pentode.   Voice peaks will pull the oscillator, causing distortion.

That's all I can think of, but I'll bet I just haven't thought long enough.  
By far the best way to use a receiver like this to develop a good HBR 
is to strip the chassis to a bare plate and cover it with a sheet of 
aluminum.   Discard all parts except the knobs/controls, dial lamp 
and S-meter, cover the panel with a sheet of aluminum; the cabinet 
may be restored and used as-is ... 

As long as you are realistic about the expected results from one-step-
at-a-time mods and see this mainly as an interesting project, I'd say 
go for it!   It *is* possible to improve the performance of such 
receivers on AM reception considerably beyond what they did when 
new and there's still AM on the ham bands ... if you score much 
better AM as a success, you might be well pleased.

No reason to be concerned about the AC/DC design, although it 
does somewhat constrain tube substitutions.   Just replace the cap 
from one side of the line to the chassis with a modern 1KV+ ceramic 
designed for 'line voltage' use (Radio Shack has them) and replace 
the cord with a 3-wire type so the chassis is properly grounded via 
the green wire.   White wire is cold (becomes B- in the set), 
bypassed to chassis.   Black wire is hot.  Install a fuse in the hot 
side.  Before plugging in, double-check ground continuity from 
chassis to ground pin on plug and also check that your outlets are 
wired correctly -- you find some with the hot and cold wires reversed!  
You can buy a tester for outlets at any place selling home mechanic 
electrical supplies.

I *build* AC/DC sets ... they have many advantages for low power 
rigs.   The R-8040 described a couple of days ago is an example.

Walt Hutchens
KJ4KV