Racal RA37 LF Converter.
Detailed Technical Information.
Version 1.00
RA37 Circuit Diagram: GIF 142K
RA37 Top View: GIF 222K
RA37 Underside View: GIF 216K
RA37 Technical Specification:
Freq Range: 12.5 to 980KHz.
Nominal Input Impedance: 75Ω
Aerial attenuator ranges: 0, 10, 20, 30, 40dB.
Preselector ranges: Wideband, 12.5-37KHz, 37-110KHz, 110-330KHz,
330-980KHz.
Sensitivity, A1, 3KHz Bandwidth: 1μV for 20dB s/n.
Sensitivity, A3 30% mod, 3KHz bandwidth: 3μV for 20dB 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.
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 1U (1.75" max.).
Electrical Differences between RA37A and RA37B
|
Component |
Function |
RA37A (UK) |
RA37B (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) |
|
|
|
|
|
|
L18 |
2MHz Filter |
Racal AA7845 |
Racal AA8313 |
|
|
|
|
|
|
R18 |
V2 grid 2 |
33KΩ |
47KΩ |
|
R25 |
V3/V4 cath. |
82Ω |
33Ω |
|
R33 |
V6 Cath. |
82Ω |
39Ω |
|
R34 |
V6 grid 2 |
4.7KΩ |
15KΩ |
|
R35 |
V3 grid 1. |
180Ω |
deleted |
|
R36 |
V3 grid 3 |
180Ω |
deleted |
|
R37 |
V4 grid 3 |
180Ω |
deleted |
|
R38 |
V4 grid 1 |
180Ω |
deleted |
*6F33 and 6AS6 have different base connections. Pins 6 and 7 (G2
and G3/D) are swapped.
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.
Preselector Tracking Checks and Adjustments.
|
Range / KHz |
LF end |
HF end |
|
End Point |
Adjust |
End Point |
|
12.5-37 |
12KHz |
(L2)* |
40KHz |
|
37-110 |
34KHz |
L4 |
120KHz |
|
110-330 |
105KHz |
(L6)* |
350KHz |
|
330-980 |
315KHz |
L8 |
990KHz |
* Fixed inductors are used on these ranges.
Preselector adjustments are limited. Check that the tuning range
end points are approximately as above, or adjust accordingly if
possible.
1MHz LPF.
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. R16 and R23 are
terminating resistors 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 the
input selector set to 'Wide-Band' and the Ae attenuator set to
'Min'. 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 L13 to obtain minimum
output at 1030KHz. Adjust L9 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 12.5KHz
to 980KHz should be within the limits +1 and -3dB relative to
the level at 500KHz. Adjust L10, L11, and L12 as necessary to
correct the response in the region of 900-980KHz 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 L20 and L21.
The 2MHz drive level at grid 3 of V3 and V4 should be at least
0.5V RMS (ie, the 2MHz output accross the balanced winding L21,
when loaded by R24 + R26 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 L16, L17, and C27 in turn for maximum output.
Balanced Mixer.
British versions of the adapter use 6F33 mixer pentodes for V3
and V4. North American versions (RA37B) use 6AS6 mixer pentodes
and omit the grid-stopper resistors R35, R36, R37, R38. 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 (L19//R27)
is broadly tuned to 2.7MHz, which is the logarithmic centre of
the 2 to 3 MHz output range. There is no resonating capacitor
because the coil is tuned by self and stray capacitances. To measure
the frequency respons 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 L19 for maximum output at 2.7MHz.
Mixer balance adjustment is by RV1 (front panel preset) and C28,
set by tuning the RA17 interpolation receiver to 2MHz (0 on the
red scale) and minimising the carrier level.
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