[Hammarlund] Send receive swtich

[Richard Knoppow]

    The send/receive switch on most receivers is connected to 
terminals on the back. Its purpose is to desensitize the receiver 
when transmitting. Usually it is connected to a pair of contacts 
on the transmitter which also operate the antenna relay.
    On some receivers, such as the HQ-129-X, the switch cuts off 
the B+, usually by opening the center tap of the power supply 
transformer, in other receivers, such as the Super-Pro series and 
SP-600- it cuts off the screen or plate supply to the RF stages. 
The Super-Pro cuts off the plate supply, the SP-600 cuts off the 
screens (much better way to do it). The external relay contacts 
go across the S/R  switch on the receiver.
     The SP-600, except for some very early receivers, does not 
have terminals at the back, it was meant mostly for receive only 
use. However, one can attach wires to the switch and bring them 
out to the S/R relay.
    Some receivers have a bias to cut off the RF section, for 
instance one version of the R-388 (Collins 51J-3) has an internal 
relay for "break in" which controls the bias on several of the 
tubes. It also has a B+ connection on the back.
    I used a BC-779 (Super-Pro) with minor modifications for my 
station receiver and used its send/receive terminals connected to 
my antenna relay. I also had a small auxiliary relay right across 
the antenna terminals to short them when transmitting.
    Almost all tube communications receivers have this sort of 
set up.


On 12/5/2019 1:03 PM, ve4feb at gmx.ca wrote:
> Hi allNow that my hammarlund has become my main receiver i have some curiosity questionsSend/receive swtich - back in the day how did people use this? Seems to exist since super pro series so must have been useful.RegardsFernan
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-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL