[Collins] 51S-1 "S-meter" sticking at extreme right hand side of meter during receiver mute

[Dr. Gerald N. Johnson]

On Tue, 2006-12-12 at 07:47 -0600, Dave, K2DP wrote:
> Gentlemen:
> 
> My 51S-1 (mid-1960's winged emblem) has developed a minor problem. The S-meter now sticks in the extreme right end edge of the meter each time I mute the receiver to put the KWM-2 into transmit. A simple tap on the meter glass will return the meter back to the left hand edge when I put the KWM-2 back into the receive mode. Is there any way to limit the right hand deflection of the 51S-1 meter when putting the receiver into mute ???????
> 
> Many thanks for your help,
> 73,
> Dave, K2DP, St. Louis

I have no information on the 51S-1 meter circuit. If there is some
resistance in series with the meter there may be enough voltage drop to
allow limiting the meter deflection with a silicon diode or better a
zener. If the voltage is above several volts a plain zener diode may
work though the knee is gentle for either the silicon diode (conducts
forward at about .6 volt) or the ordinary zener. For a much sharper
knee, the TL430C programmable zener is best and works fine with a square
knee at currents well under a milliamp. The data sheet says 1 ma minimum
but G3SEK reports it works well at 100 microamps. For 2.5 volts it takes
no external parts, though one has to be careful of external capacitance.
There are ranges of capacitance that will cause it to oscillate.

Mechanically, one might add a bumper at the movement (or adjust the
existing one) so that the tip of the pointer can't wedge against the
case. Many movements had bumpers near the zero adjust hair spring for
just such purposes in both directions of deflection. There used to be a
good meter man at Scherrer Instruments on Manchester but he's probably
long gone. With a good touch that adjustment should take a few seconds
of time.
-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer