[ARC5] BC-454-B listed in THAT place.

[Mike Morrow]

> OK on the taps, but what determined if the lo-z was used or not?

Every NIB SCR-274-N version B receiver or BC-348-* receiver that I've ever owned came out of the box connected for high impedance, which matched the impedance of headphones like the HS-23 that were commonly-used by the USAAC/F before LOW-Z phones like the HS-33 became common after early WWII.

An aircraft that used low impedance AF output in the SCR-274-N command set required the AF tap to be changed to LOW-Z as part of the installation process on each receiver output and on the modulator sidetone output.  The same had to be done for the BC-348-* receiver and the BC-375-* transmitter sidetone adjust switch in the SCR-287-A, the ADF receiver in the SCR-269-*, the receiver in the SCR-522-A VHF command set, the marker beacon receiver, the BC-733-* VHF ILS localizer, etc.  A sticker in red ink that warns of the LOW-Z connection was affixed on the outside of each unit with wiring changes.

The USN had historically used LOW-Z headphones, although RU-* receivers prior to the 1934 RU-3A could be connected for HIGH-Z or LOW-Z.  Therefore, all ARA and AN/ARC-5 receivers and modulators have only LOW-Z AF output.

LOW-Z became the standard everywhere.  If one has a receiver connected for HIGH-Z, that's a pretty good indication it was never actually used before being surplused.

WRT SCR-274-N receiver nomenclature end-letters, there are only A, B, and BM possibilities (not including units radically modified for the UK's Standard Beam Approach landing system).  

The broadcast band receiver was added late to SCR-274-N for use with the AN/ARR-1 VHF homing adapter, and exists only as BC-946-B.  There was never a BC-946-A.

A few BC-454-B and BC-455-B receivers were modified for use in the AN/CRW-3 remote radio control system for the GB-4 glide bomb.  Those receivers have the frequency dial removed and have a paper tag with BC-454-BM or BC-455-BM nomenclature.

Mike / KK5F