[Test-Equipment] AN/PRM-10 Question

[Richard Knoppow]

----- Original Message ----- 
From: "Richard Knoppow" <1oldlens1 at ix.netcom.com>
To: <k2cby at optonline.net>; "Discussion of Electronic Test 
Equipment" <test-equipment at mailman.qth.net>
Sent: Wednesday, November 02, 2011 2:04 PM
Subject: Re: [Test-Equipment] AN/PRM-10 Question

     One last time. I fixed the internal oscillator, not 
sure what I did but think it may have been a dirty normal on 
the external modulation jack. Maybe something else because I 
resoldered a couple of things. Also, the 1 meg resistors I 
thought were half-value are effectively in parallel for the 
ohm meter, they are fine when lifted at one end.
     Now, the main problem is the brush contact that 
connects the tuning capacitor rotor to ground through a 470 
ohm resistor. The brush is a spring contact with a button on 
the end. Its held in place on a small Bakelite insulator 
which is screwed to the side of the capacitor frame and 
designed so that the button projects though a small hole in 
the frame where it is supposed to contact the geared drive 
wheel for the capacitor. The problem is that it does not 
reach and there is no adjustment. If I try to push it 
further the spring contacts the frame. The rotor is properly 
centered so adjusting its bearings is not possible. I don't 
know why the contact is so far off, it does not appear to be 
wear and there is nothing else that can move much. The only 
solution I can think of is to enlarge the hole it projects 
through enough to allow bending the spring so that the 
button rides on the gear as its supposed to. I can imagine 
that there might be something missing but I can't figure out 
what it could be. The whole arrangement is not very well 
designed; I can think of a couple of better ways to connect 
the rotor and still allow it to move freely. The designer 
used a similiar brush to ground the brass intermediate gear 
that drives the variable resistor used to equalize the 
output, probably to eliminate static build up. This gear 
drives the capacitor gear through plastic gears. At any 
rate, unless I hear from someone who knows for certain that 
there is some adjustment for the mechanical alignment I will 
haul out the hand drill and enlarge the hole. The dead spots 
in the output are due to the rotor floating so its necessary 
to fix this. Once done, assuming my cure actually works, the 
thing should be complete and working properly.
     I am just home from work and need some sleep so will 
check the list later.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com