[MRCA] T-195 Slow Semi-Break In Turnaround Time on CW

[Christopher Bowne]

When in doubt, RTFM!  Downloaded a copy of TM 11-806 and sure enough there is an 
antenna relay delay circuit that causes a hang time to prevent excessive cycling 
of the antenna relay.  Works using a 4 uF cap across of the grid of one half of 
a 12AT7 that controls a pilot relay that in turn switches the antenna relay and 
associated T/R contacts.  The pilot relay in the plate circuit of the 12AT7 is 
normally off due to a blocking bias voltage applied to the tube grid.   Keying 
the radio either by closing either the PTT switch, tune switch, or CW key 
immediately removes the blocking bias, switches the tube into conduction, and 
causes the relay to pick up.  On key up, the cap across the grid holds the 
blocking bias low until it is recharged from the negative bias supply through a 
nominal 2 meg resistor.  When the cap charges up, the tube conduction is 
eventually cut off, and the delay relay turns off. Looks like the hang time of 
the circuit in my T-195 is about three seconds.  There is no delay time 
specified in the TM, no built in means to adjust it, nor is there any specific 
troubleshooting direction for a long delay time.  What is odd about the way my 
set is behaving  is that there is no observed delay in antenna switching to the 
receiver or receiver de-muting (which is also controlled by the relay) in voice 
mode PTT operation when releasing either the test key or mic PTT switch after a 
transmission.  The circuit as described, should operate the same no matter what 
mode is used, and and I have not found any mention of any differences in T/R 
switching that are mode dependent.  I suspect that the grid block supply 
resistor has likely drifted high in value, extending the hang time.  An obvious 
fix will be to  replace the resistor with a pot, or add in a pot in parallel 
across the resistor to provide a means to adjust the time constant of the RC 
circuit supplying the grid.  In the mean time, I can just cycle the mode switch 
to standby to speed things up when needed!  Curious as to what other people have 
for hang times on their T-195s.




________________________________
From: Christopher Bowne <aj1g at sbcglobal.net>
To: mrca at mailman.qth.net
Sent: Mon, October 22, 2012 6:37:12 AM
Subject: [MRCA] T-195 Slow Semi-Break In Turnaround Time on CW


I ran my T-195 this weekend on the AM and CW Old Military Radio Nets for the 
first time after at least a year. Although the problem did not occur on push to 
talk voice, I noticed that the receiver would be deaf for about 5 seconds after 
I stopped keying on CW.  
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