[Milsurplus] BC-375: Unusual TU-10 Tuning Unit

[WA5CAB at cs.com]

Tom,

All of the original tuning units designed for the BC-191-A have -A suffixes, 
TU-1-A through TU-10-A plus TU-22-A.  Tuning units for the earlier BC-AA-191 
are completely different in mechanical design and have nomenclature TU-AA-3 
through TU-AA-10 inclusive.  The more common "B" suffix models including TU-26-B 
(there does not appear to have ever been a TU-26-A) appeared circa 1940/41.  I 
have a TU-5-B made on a low number NY-41 Order, which means no later than 30 
June 1941.

The -A suffix tuning units were issued with the BC-375 at least through the 
-C model in 1939.  I'm supposed to have a BC-375-D manual but have apparently 
mis-filed it.

According to TM 11-800 of November 12, 1942 (referring to the -A and -B 
suffix tuning units), "The only difference between the A and B types is that the B 
type does not have the two insulated binding posts found in the upper 
right-hand corner of the front panel of the A type.  These posts are nor required for 
operation of the equipment but are a convenience in some servicing operations. 
 They are connected to the tapped secondary of the power amplifier tank 
circuit through control ANT. COUPLING SWITCH D.".

In a message dated 5/21/2006 10:22:59 PM Central Daylight Time, 
quixote2 at ix.netcom.com writes: 
> I have a TU-10A (coverage = 10,000-12,500 KHz). It sort
> of stands out immediately because it has, on it's front
> panel, two spring loaded binding posts which are, I can 
> only assume, going to an antenna WITHOUT going thru the
> big antenna matching network included in the transmitter;
> ie, it appears they go directly to the output link coil 
> for the PA!
> 
> The backs of the binding posts include couplers that allow
> the output link to simultaneously feed the transmitter's
> antenna tuning circuits...  so it's possible to either go
> thru the matching network or couple the link directly to
> an antenna at the operator's choice.
> 
> The binding posts DO appear to have been installed at the
> factory; this isn't a field modification. They're not
> labeled. However, they're rather badly placed; when you tune
> the PA plate variable cap, your fingers get AWFUL close to
> them, risking an RF burn if you operate it with a hand not
> encased in a leather flying glove.
> 
> The nomenclature plate on the TU gives a construction date
> of 1936...  quite a bit earlier than your common garden
> variety BC-375 tuning unit. 
> 
> What am I dealing with here? Is this perhaps a BC-191 TU?
> If not, did aircraft installations commonly use a seperate
> antenna for that tuning range?
> 
> Anything that anybody here knows about this oddball would be
> of assistance re. what's going on here.

73
Robert Downs - Houston
<http://www.wa5cab.com> (Web Store)
MVPA 9480
<wa5cab at cs.com> (Primary email)
<wa5cab at houston.rr.com> (Backup email)