[Milsurplus] Re: Milsurplus digest, Vol 1 #806 - 9 msgs

Sat, 27 Dec 2003 16:49:45 EST

In a message dated 12/27/2003 1:05:57 AM Pacific Standard Time, 
[email protected] writes:

> [email protected]

wrote: '433 survived into 90s as p/o ARN-7* as many here know

R 5 ARN 7 is VERY similar but not identical to BC 433. The 7 has an 
additional band 100-200 KHz.  The plugs are all the same, wonder if you could swap a R 
5 ARN 7 with a BC 433 and not fry anything?

Never saw a 3 loop system on any plane and I have looked at LOTS being an 
aircraft history nut. Two is good enough for a fix although 3 is certainly 
better. Those LP 21 Loops added a fair amount of drag, so much so that Bendix 
designed a very low (almost flat) profile replacement loop using crossed ferrite 
bars. This new loop was designed for post war airliners using the MN 62 ADF which 
was an ARN 7 with new plugs. DC 7s used them, Martin 404s and perhaps Super 
Connies as well. There is even FAA paperwork approving conversions of surplus 
ARN 7s to MN 62 rcvrs. MN 62s were also made new by Bendix according to what I 
have heard from old propliner avionics techs.

Those 2050 Thyratron loop motor drivers were really vacuum tube SCRs (silicon 
controlled rectifiers)  that controlled the 400 hz motor voltage for 
proportional loop steering. As a kid I interfaced these to control the DC autopilot 
steering motor on my dad's commercial fishing boat. I didn't have enough 
knowledge to figure out how to do proportional motor control so I just did L-R 
"hunting" , but it worked just fine. You could freeze the loop for a straight ahead 
null and then switch to controlling the pilot. It would take the boat directly 
to a radio beacon. Alternatively, you could set the loop for a side null and 
have the boat circle an island beacon, although that didn't work as well as 
steering straight to a beacon since it did not keep a constant distance from the 
island, just a constant relative bearing.

Those old WW 2 ADFs were astoundingly accurate, especially if you used the 
ingenious variable cam shape azimuth compensator that coupled the loop to the 
direction selsyn. This cam set up could be calibrated to compensate for all the 
distortion from acft or boat structure and rigging. When you were close, were 
in the open ocean and had a narrow stable null, the ID 82 needle would point 
EXACTLY at the antenna of the xmtr, not even one degree off.

On fishing boats, Loran A made the old ADFs less essential for NAV, LORAN C 
made them obsolete and GPS relegated them to scrap. The BC 433 was very popular 
as a comm ADF with band 4 converted to 2-3 MHz marine AM band, but when AM 
was outlawed they were viewed as power hungry space wasters. 

The selsyn system in the WW 2 ADFs was adopted into many post war commercial 
marine ADFs. I have seen I 82 indicators with the RADIO COMPASS lettering 
removed and replaced by RAYTHEON. It was dirt cheap to buy these components 
surplus and very expensive to but them as new commercial items.

Interestingly, the Russians seemed to have copied the BC 433. I have seen 
control boxes from MIG 15s that look just like the 433 boxes. Guess if they could 
copy a whole B-29 acft (TU-4 Russian version), then an ADF was easy.

Mark

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