Collins 51S mods
The 51S receiver doesn't have
proper AGC operation. I noted soon that my Drake 2-B performed
very much better and I started to wonder what could be done.
While 2-B is constructed around 1961, the 51S was designed a few
years earlier, when AM and CW were the dominant modes for
professional users. SSB wasn't important, and at coast stations
the RF gain is normally turned down so much that AGC isn't
operating. Considering this fact, it wasn't important to have
good SSB performance, and it wasn't any desires to improve this,
a compromise was good enough. But for radio amateurs, when ssb is
the most interesting mode, and when one want AGC to follow the
signal strength it is a quite different need.
AGC performance is just too bad! But amateurs who have used all sorts of bad receivers it may be more important to own a high reputation equipment than high performance, still it is a lot of rubbish in use by amateurs, and still it is more important to give report '59' than it is to hear what the opposite stations say.
Fig 1. Modification to optimize AGC for ssb reception - fast attack and slow decay.
Remove R73, R125, C176, CR17. Mount R73=2.2MW, and install a 22kW resistor in series with C192 (=0.47µF)
Remove R43 (to remove AGC from the valve which is after the point IF voltage to AGC amplifier is derived). Install a 100ohm resistor from cathode to ground (no bypass capacitor needed), and rewire the suppressor grid to go direct to ground.
AGC IF amplifier and detector (shown as reference only), it is interesting to note that it is in principle the same as for the German WWII receivers KwEa (pg 22b) and Spez 801 (pg. 17t).
Fig.4.) 51S AGC improvement, this is only necessary when you want to have different time-constants dependent on the mode. It is a problem to have long time-constant for AM, for cw it is not very important, somebody want short timeconstant while most don't really care.