[TMC] SBE-3 & SBE-6 on AM & hum

[Duncan Brown]

Just an update on my SBE-6 saga.

I did try the "Carrier Insert Phase Shift Network" from the SBE-8 
schematic. There is a transformer in the circuit (looks like a balun?). 
Thanks to K4OZY & K4NYW for the specs on the transformer, however the 
spec says there are three windings with a ratio of 1:5:1 but doesn't 
define which winding is which (the network only uses two of the windings).

For simplicity's sake I just wound a 1:1 transformer: 33 T on an -06 
core (but I think a step-up would be good as there is some loss in the 
network).  The circuit worked in that I could see an improvement in the 
AM modulated RF waveform.  The adjustable resistor ended up at one end,  
but it was an indication that it was doing something. We had decided not 
to put this mod in our SBE-6, but it could help others if you want to 
improve the AM on your SBE-2, -3, or -6.


I did make an improvement on the hum problem by removing the filament 
returns from chassis and running separate wires to chassis at the power 
connector J109. (I had already tightened all the tube socket screws 
without much effect.) The tube sockets I changes were V122& V123 (audio 
amps) and V124, V125, V126 (250Khz amps). V122& V123 seemed to make the 
most difference, but I did them last and did not go back and undo others 
to see if that was all that was necessary.  Originally hum sidebands 
were down 40-50db. Now 60 hz sidebands are down 55-60 db and all others 
are down >60db.


Thanks for everyone's help & comments.

73 & have fun,

Duncan Brown, K2OEQ
USASA 31J
Antique Wireless Association Museum Asst. Curator, Commercial Equipment
(also Chief TTY operator & repairman)
http://www.antiquewireless.org/



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