RA.121A/B INDEPENDENT  SIDEBAND  ADAPTOR

The RA.121A and RA.121B adaptors are self-contained having a built-in power unit, and are designed to operate from the 100 kc/s i.f. output of the receiver. The only differences between the A and B models are in the mains connection and adjustment arrangements and the input socket.

• RA.121A model

  The "A" model is for the British market and has the following visual recognizable items.

  • Mains connection is a socket with a free lead.
  • Mains voltage selection and adjustment is a panel accessible at the rear side as you can see in the right lower corner in the picture.
  • The 100 kc/s input socket is a British type.
    However, I have seen USA versions (RA.121B) with the typical British Burndept socket.
  Click the image for a larger picture  (700 x 577 pixels, 93 kb) .
  Note that the fine tuning dial of this RA.121A does not have the split white/orange lower/upper sideband section. I have never seen this version before (for what's it worth...).

• RA.121B model

  The "B" model is for the North American market. Here is a description of this unit.

  • Mains connection has a permanently fixed mains connection lead.
  • Mains voltage selection and adjustment is not accessible at the outside.
    The adjustment must be made at the transformer input terminals underneath the chassis.
  • The 100 kc/s input socket is an American type, SO239.
    Same remark here, this RA.121B actually has that typical British Burndept socket!
  Click the image for a larger picture  (710 x 569 pixels, 94 kb) .
  This unit has the more common split white/orange lower/upper sideband fine tuning dial.
  Note the switch in the middle at the rear side with the text markings "RA17" and "RA117".
  With this switch you change the connections to the lower and upper sideband filters to correct the upper/lower sideband inversion that occurs in the RA.117 receiver. So, this RA.121B can be connected to an RA.117 and the text markings on the front like "LOWER S.B." and "UPPER S.B." for the "A.F. GAIN" controls are indeed what they describe.

• RA.121D model

  The "D" model has a crystal controlled oscillator instead of an LC oscillator for increased stability.

  • The most prominent difference is of course the crystal controlled oscillator. You can see it in the middle of the unit on the picture.
  • Mains connection is a socket with a free lead.
  • Mains voltage selection and adjustment is a panel accessible at the rear side as you can see in the right lower corner in the picture.
  • The 100 kc/s input socket is a British type.
  Click the image for a larger picture  (710 x 579 pixels, 95 kb) .
  

The i.f. input signal of 100 kc/s to the RA.121, fed via a preset potentiometer, is mixed with the output of a stable 118 kc/s oscillator. This oscillator is variable over a range of +/- 2 kHz and is fitted with a slow motion drive which provides an accurate control of tuning essential to take full advantage of the carrier and unwanted sideband rejection characteristics of the unit, and to avoide pitch disdortion. A further control in the form of a three position switch alters the frequency by +/- 3 kc/s.

The mixer output, centred at 18 kc/s, is fed via a 12 to 24 kc/s (18 kc/s +/- 6 kc/s) band-apss filter, carrier rejection bridge stage and an amplifier stage to both upper and lower sideband filters. The filters provide a high degree of rejection to all signals other than the wanted sideband.
Each sideband filter output is mixed in a product detector, with a signal at 18 kc/s, from a fixed frequency oscillator, giving products including an audio component. Unwanted signal frequencies are subsequently removed by means of further filter network. The resultant output therefore consists only of the audio frequencies.
The audio frequency on each channel is fed to an a.f. output stage with a gain control and having three independent outputs, one of which is provided with a switch for sideband selection.

A Cathode Ray Tube (C.R.T.) functions as a tuning indicator. The 18 kc/s carrier re-insertion oscillator output is applied to the X plates and the input carrier signal taken from the band-pass filter is applied to the Y plates through a two-stage tuned amplifier. This gives a Lissajous presentation showing a stationary ellipse when the input carrier frequency, after the first mixer, is the same as the frequency of the carrier re-insertion oscillator, (18 kc/s), that is when tuning of the input signal is correct.

A simple form of a.f.c. is provided for optional use to maintain correct tuning over a small range of drift of either the input signal or the internal oscillators. H.T. and heater supplies are included in the equipment, making it entirely self-contained.

The adaptor is designed for rack mounting or for fitting in a suitable cabinet above the RA.17 receiver.
All electrical connections with the exception of the headphones are made at the rear side of the unit.
A dust cover is fitted which provides adequate ventilation and at the same time ensures ample protection from dust and dirt.

Tubes

• Independent Sideband Adaptor Type RA.121 A & B