[ARC5] eBay Item - What Is It?

[Roy Morgan]

Device Puzzlers,

The listing is gone away as of this morning, so I can’t see the posted pictures. 

It looks like a General Radio device.  GR had a policy (actually a philosophy of business) that if the military or government wanted an instrument they made, that was fine.  If they wanted something all new or redesigned, no thank you.  So we have military style manuals for standard GR instruments that are the same as the normal production instruments.

There were exceptions however:

-The APR series of ECM receivers was evolved from a normal pre-WW-II GR instrument that was meant as an IF detector for use with bridges and slotted lines at VHF and UHF.  (I can’t recall the GR type number just now.). It detected the results of mixing the signal of interest, such as picked up by a probe moved along a slotted line to measure SWR, and a local oscillator - the mixing was done by a crystal such as the 1N23.  The detector had an indicator meter and switchable attenuator to provide a reasonably accurate measure of the detected signal strength.  The thing operated at 30 Mc.  

So along comes WW-II and the need for detecting and measuring radar signals in aircraft, and the GR instrument was evolved to include a set of plug-in front ends that covered from about 50 mc up to maybe 1000 mc. A separate rotary coax switch connected the desired antenna to the set.  I suspect that the Radiation Laboratory at MIT (“the Rad Lab”) was involved in the specification and development of the system. Later a single plugin appeared with four (or 5 ?) antenna connections and a switch to change the input in use and the range of the local oscillator.  The mixing was again done with a 1N32 type diode.  The antennas were narrow-waisted mushroom shaped and silver plated. The EA1C aircraft had a crew of two, pilot and ECM operator, a 3000 horsepower piston engine, and could fly for 10 or more hours across the pacific sniffing for any ship-born radars that might be out there.  The APR-4 may have been the latest evolution of the system and would run on 60 cycle and aircraft frequency power and used octal tubes, and the APR-4Y followed and accepted only 400 to 1000 cycle power and I think used miniature tubes.  Some (all?) of the plug-in front ends had a motor to scan back and forth over a selectable segment of the available frequency band to relieve the operator from manually cranking the knob.

-Aircraft fuel quantity gauge systems use a detector cell in the tank that exhibits changing capacitance depending on fuel level.  (This automatically compensates for fuel density changing with varying temperature.) Testing and presumably adjustment of the capacitance to standard values was extremely important to deliver accurate fuel levels to the pilots. GR adapted one of their capacitance bridges into a specialized fuel cell capacitance test set for the purpose.


Roy Morgan
k1lky68 at gmail.com <mailto:k1lky68 at gmail.com>



> On Jun 2, 2018, at 2:01 PM, Richard Knoppow <1oldlens1 at ix.netcom.com <mailto:1oldlens1 at ix.netcom.com>> wrote:
> 
>   A puzzle. The resistance knobs look like Leeds and Northrup and the others like pre-1950s General Radio. I wonder if its for telegraph or telephone cables. It appears to be direct reading rather than a substitution bridge sine there is nothing marked for initial balance. The X-S switch might be Unknown-Standard.
> 
> On 6/2/2018 4:30 AM, Bob kb8tq wrote:
>> Hi
>> The standard resistance goes all the way up to 110 ohms. That sounds a bit low for antennas.
>> It is some sort of balanced bridge test set. No labels on it.   Post connectors for the generator
>> and the device under test. It is not going to be for VHF use for sure ….
>> Bob 
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