Racal RA137 LF Converter.
Detailed Tecnical Information.
Version 1.00

RA137 Circuit Diagram: GIF 163K
RA137 Top View: GIF 221K
RA137 Underside View: 239K

RA137 Technical Specification:
Freq Range: 10 to 980KHz.
Nominal Input Impedance: 75Ω
Aerial attenuator ranges: 0, 10, 20, 30, 40dB.
Preselector ranges: Wideband, Wideband with 500KHz LPF, 10-20KHz, 20-40KHz, 40-90KHz, 90-210KHz, 210-500KHz, 500-980KHz.
Sensitivity, A1, 3KHz Bandwidth: 1μV for 15dB s/n.
Sensitivity, A2 30% mod, 3KHz bandwidth: 3μV for 15dB s/n.
Image Rejection (tuned input): better than 60dB.
Spurious responses: less than 1dB above noise.
Mains Input: 100-130V, 200-250V (adjustable in 5V steps), 45-65Hz, approx 11W.
HT Supply: 220V DC nominal.
1MHz Reference Input: Greater than 2V RMS, high impedance.
RF Output: 2 - 3MHz (Input freq + 2MHz).
AVC Input Control Range: 0V (max gain), -25V (min gain).
19" Rack mounting, panel height 2U (3.5" max.).

Electrical Differences between RA137 and RA137B
Component Function RA137 (UK) RA137B (N America)
V1 RF Amp. EF93 (CV454) 6BA6 (EF93)
V2 Freq. doubler EF91 (CV138) 6AU6 (EF94)
V3 Bal. Mix. 6F33 (CV2209) 6AS6
V4 Bal. Mix 6F33 (CV2209) 6AS6
V5 Volt. Stab. 150C2 (CV1832) 0A2 (150C2)
V6 RF Out. EF91 (CV138) 6AU6 (EF94)
R25 V3 grid 1. 180Ω deleted
R26 V3 grid 3 180Ω deleted
R28 V3/V4 cath. 82Ω 33Ω
R30 V4 grid 3 180Ω deleted
R32 V4 grid 1 180Ω deleted
R38 V6 Cath.  82Ω 39Ω
R39 V6 grid 2 4.7KΩ 15KΩ

North American versions have an SO239 aerial connector and 75Ω BNC sockets for 1MHz in and RF out. British versions have obsolete NSF and Burndep connectors, which can be replaced without affecting the alignment of the unit.

500KHz LPF.
THe RA137 is equipped with a 500KHz LPF which is in circuit on ranges 10 - 500KHz and wideband with filter. This is to reduce interference from strong medium-wave stations. Rejection frequencies of the filter are at 560 and 680KHz, but should rejection be required at specific frequencies close to these (within ±5%), the appropriate cores can be re-adjusted. L2//C4 and L4//C6 reject 680KHz, and L3//C5 rejects 560KHz.

Preselector Tracking Adjustments.
The preselector uses a two-point tracking scheme, with the adjustment points at the band edges. Tune the interpolation receiver to the LF end of the band and adjust the coils for maximum signal with the preselector dial set to the receiving frequency. Then tune to the HF end of the band and likewise adjust the trimmer capacitors for maximum signal. Repeat the procedure over and over again until the required adjustments become insignificant.
Range / KHz

LF end

HF end
Set Point Adjust Set Point Adjust
10-20 10KHz L5*, L16 20KHz C7*, C24
20-40 20KHz L7, L17 40KHz C8, C19
40-90 40KHz L9, L18 90KHz C9, C20
90-210 90KHz L11, L19 210KHz C10, C21
210-500 210KHz L13, L20 500KHz C11, C22
500-980 500KHz L15, L21 980KHz C12, C23

*Adjustment of L5 and C7 is very flat and consequently not critical.

Adjustment locations:

Image rejection is accomplished by feeding the output from the RF amplifier (V1) through a low-pass filter consisting of three constant K sections with m-derived end sections. R17 and R24 are equal to the characteristic impedance of the filter. The filter cut-off is at 980KHz.

To set up the LPF, apply a signal to the antenna socket with all input filters and attenuators out of circuit. Measure the signal arriving at V3 pin 1, with V3, V4, and V5 removed. To avoid overloading the RF stage, keep the signal generator output below 800mV RMS. Adjust L26 to obtain minimum output at 1030KHz. Adjust L22 to obtain minimum output at 1080KHz. Apply a 100mV signal at 500KHz and check that the signal at V3 pin 1 is not less than 800mV. The frequency response from 10KHz to 980KHz should be within the limits +1 and -3dB relative to the level at 500KHz. Adjust L23, L24, and L25 as necessary to correct the response in the region of 980KHz and ensure that there are no output responses above 1020KHz..

Harmonic generator (frequency doubler).
V2 is biased for non-linear operation. The output bandpass filter is tuned to 2MHz, and has a bandwidth of 50KHz. An electrostatic screen is placed between L33 and L34.

The 2MHz drive level at pin 6 of 6F33 V3 and V4 (pin 7 of 6AS6) should be at least 0.5V RMS (ie, the 2MHz output accross the balanced winding L34, when loaded by R27 + R29 should be greater than 1V RMS.

Align the frequency doubler with its screening covers in place. Check that there is ≥2V RMS of 1MHz drive at PL2. Measure the doubler output at grid 3 of V3 (pin 6 if 6F33, Pin 7 if 6AS6). Adjust L29, L30, and C50 in turn for maximum output.

Balanced Mixer.
British versions of the adapter use 6F33 mixer pentodes for V3 and V4. North American versions (RA137B) use 6AS6 mixer pentodes and omit the grid-stopper resistors R25, R26, R30, R32. Mixer pentodes, also called 'dual control' or 'short suppressor base' pentodes, differ from normal pentodes in that grid 3 has a close wire spacing and is internally connected to a diode (between grid 3 and the cathode) which prevents grid-current. The 6F33 and 6AS6 are not pin compatible (g2 and g3 connections are swapped) and different cathode resistor values are used (see electrical differences table above).

The balanced mixer has a broadband output. The anode load (L32//R31) is broadly tuned to 2.7MHz, which is the logarithmic centre of the 2 to 3 MHz output range. To measure the frequency response of the stage, inject a signal into V3 grid 1 ((pin 1) with the antenna disconnected, and measure the output across a 330Ω terminating resistor connected across the RF output socket. Adjust L32 for maximum output at 2.7MHz.

Mixer balance adjustment is by RV1 (front panel preset) and C52, set by tuning the RA17 interpolation receiver to 2MHz (0 on the red scale) and minimising the carrier level.

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