[Collins] Meter sensitivity-312B-4 etc

[Gerald]

On Tue, 2005-04-19 at 16:49 +1200, Dave Brown wrote:
<SNIP>
> And the meter in the 302E-2 (in the 32RS-1) is 200 uA as 
> well-according to the circuit. The rest of the cct is again identical 
> to the others, apart from the diode DC return resistors which are 21.5 
> ohms, as opposed to 10 ohms in the later versions.
>  This all started because I want to use a pair of identical meters (a 
> JRC cross needle dual 100 uA meter in fact) driven from one of the 
> Collins couplers for a temporary test setup-hence the need to find out 
> what meter sensitivity was needed. Thanks anyway, Bill!
> 73
>  Dave, ZL3FJ

Technically those low value (21.5 or 10 ohm) resistors are the loads for
the current transformer. The lower their value, the better. They develop
a bit of RF voltage in phase with the current in the transmission line.
The RF line voltage sample applied through the capacitive dividers lets
the diode do the phase sensitive voltage multiplication to make power.

The 100 microamp meter will work fine, just will take less power for
full scale.

For relative SWR, put a dual gang pot, say 50K per section, one gang for
each meter and set the forward one for full scale, the reverse one will
follow. Or back off on the transmitter output.

That schematic showing 290 microamps was lettered with a Leroy lettering
set template. Might be the draftsman missed and got 9 instead of zero
for the middle digit. I don't remember the order of the digits on the
lettering templates they used. Though it could be that the engineer
scribbled the value and it was easily interpreted as a 9. We weren't
using typewriters for parts lists, though I did for wire charts because
it was faster and more legible. I had to supply they typewriter, Collins
didn't think engineers should type. Now I suppose they do at the
computers on every desk.
-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer