[Collins] Meter sensitivity-312B-4 etc

[Dave Brown]

Finally got a few spare minutes and measured one of the meter 
movements in the 312B-4- as expected, 200 uA.
So no problems using the 100 uA movements in the JRC cross-needle 
meter. Think I need to use an order or two of magnitude scaling down, 
though- foward power is scaled at 10 kW fsd!  Nothing a pot won't fix.

 Comments on CTs etc noted- yes, quite familiar with RF CTs now, 
Gerry.  I spent quite a few nights working on accurately measuring LF 
loop antenna current (~180 kHz) some year back- uses a CT with full 
scale on the meter at 50 amps- hadn't ever played around with RF CTs 
before then.  Didn't have the Tek current probe then either-would have 
made life a bit easier calibrating the darn thing!
73
Dave, ZL3FJ

----- Original Message ----- 
From: "Gerald" <geraldj at ispwest.com>
To: <collins at mailman.qth.net>
Sent: Wednesday, April 20, 2005 2:53 AM
Subject: Re: [Collins] Meter sensitivity-312B-4 etc


> 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
>
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