R216 Receiver

 

 The R216 is a portable "Intercept Receiver" used in the 1950s for example for eavesdropping on VHF radio signals emanating from East Germany. Several types of receiver were used for this general purpose including various Eddystone models but, comparing the two makes, the R216 (made by Ekco in the early 1950s) I think is superior to the Eddystone types (and which anyway do not offer the portability of the R216)and looks more professionally made... no doubt to an unrestricted "cost plus" budget. Frequency coverage is pretty wide and the receiver can demodulate AM, MCW and FM or CW with selectivity at 30 or 120KHz. (The R216 looks similar to the R210, a short-wave receiver). During 1965 the R216 receiver was fitted in a new Canberra variant, the B Mk6 which was effectively a sort of GCHQ outstation (ref RAF Strike Command 1968-2007: Aircraft, Men and Action).

 Band

 From

To

 1

19MHz

30.5MHz

2

29.5MHz

47MHz

3

 45MHz

 69MHz

4

67MHz

103MHz

5

100MHz

157MHz
 

 

 As you can see in the above picture the set uses a 4-section turret tuner, seen below from a couple of views.

 

 

The use of a turret tuner allows the coils for the different wavebands to offered up to the RF valves without the need for extraneous wiring (like that used in the R208). 

 

Above is a view showing the chassis of the IF/AF unit (note V12 is missing ... perhaps used to replace another in a more important role). V12 is used to provide an IF output (at 4.86MHz) for connection to alternative demodulation equipment.

Below right is the RF chassis which includes the front-end plus the crystal calibrator (top) and left the IF/AF chassis. The curved aluminium panel marked "Serial 593" encloses the rear of the dial. Should the serial number originally have matched that of the receiver which is 597? Perhaps an inadvertent switch was made when mods 1 & 2 were being incorporated (see mod record panel below)?

 

 

 

 
 The valve line up is slightly odd as it uses a mixture of both battery and mains valves possibly because VHF battery valves were relatively uncommon, although one might query the use of the EF91 as crystal calibrator and 1st IF amplifier (the simple explanation may have been to just keep the valve heater types common to each of the two chassis, but then... why use the EB91.. was there no suitable 1.4 volt equivalent?). Note that the valve listing doesn't include an RF mixer. For some odd reason the designer chose MR1, referred to as a "Westinghouse silicon diode rated at 19 volts and 80uA" which carries the code "KMSA" which I'm assuming was the manufacturer's part number:-

 Valve Designation

 Valve Type

 Equivalent

Function 

 V1

 CV138

EF91

1st RF Amplifier

 V2

CV138

EF91

2nd RF Amplifier

 V3

CV138

EF91

Local Oscillator

V4

CV287

QS150/15

Voltage Stabilser

V5

CV138

EF91

Crystal Calibrator

V6

CV131

EF92

1st IF Amplifier

V7

CV785

DF91

2nd IF Amplifier

V8

CV785

DF91

3rd IF Amplifier

V9

CV1758

DF92

BFO

V10

CV1758

DF92

4th IF Amplifier

V11

CV1758

DF92

Limiter/AM Detector

V12

CV1758

DF92

IF Output buffer

V13

CV140

EB91

FM Discriminator

V14

CV785

DF91

AM AF Amplifier

V15

CV140

EB91

Noise Limiter 

V16

CV1758

DF92

AF Output

 MR1

 CV291

 KMSA

 Microwave silicon diode
 
 

Above and below... RF and IF/AF circuits of the R216. Click to see a larger image complete with component values.
 

Of course you'll have noticed the R216 uses an external power supply: Two versions were used, AC and DC connecting via the multi-way "Plessey" connector PLE on the front panel (and SKE on the PSU). Below is an enlarged view of the plug with its connections.

Note that the external DC power supply includes a second battery switch made by a link in the receiver across pins M and L which is seen once the lead from the DC power supply is plugged into the receiver. M is fed by the fuse following the power supply main on/off switch connected to battery positive terminal and L is the input to the battery power supply main circuit. Maybe a method of saving battery life when the DC power supply is left switched on when a receiver isn't plugged in?

 

 

 You'll note that the wiring takes account of sharing currents to minimise cable losses and the resultant voltage drops.

 

 PIN

 VOLTAGE

 CURRENT

 NOTES

 A

 0V

400mA DC 900mA AC
Ground return for V6, V7, V8, V9, V10, V11, V12, V13, V14, V15, V16

 B

 1.4VDC

 400mA DC
 Filament supply for V7, V8, V9, V10, V11, V12, V14, V16

 C

 6.3VAC

900mA AC max
 Heater for V3 + V5 + dial lamp return

 D

 250VDC

43mA DC
 HT supply for valves V1, V2, V3, V5, V6

 E

 100VDC

26mA DC
 HT supply for valves V7, V8, V9, V10, V11, V12, V14, V16

 F

 -19VDC 

15uA DC
 Grid bias supply for valves V6, V7, V8, V14, V16

 G

 6.3VAC

900mA AC
 Heater for V1, V2, V6, V13, V15

 H

 0V

1.2A AC
 Ground return for V1, V2, V3, V5

 J

 19VAC

300mA AC max
 Dial lamp supply voltage (return via Pin C)

 K

 6.3VAC

 -
 Not shown in circuit diagram, not connected

 L

-

 -
 DC power supply battery positive linked to Pin M

 M

 -

 -
 DC power supply battery feed linked to Pin L
 
 

 This is a close up of the R216 power connector. You'll note that there are two polarising techniques used. The plastic insert has a couple of protruding pips and the outer aluminium shell has five slots cut in the inner section. Within the assembly there are also six slots allowing the inner plastic part to be located correctly. The five slots give six available orientations.

These connectors were made originally by Plessey and their code number for the "Free Socket" mating connector to this is 508-1-40009-320. The corresponding NATO code is 5821-99-638-1648.

Weald Electronics now make these connectors under their LMF/LMG range. The code for the mating part for the R216 plug is LMF/1/40009/320 clearly lined up with the original Plessey code to avoid confusion.

The relationship between those five shell slots and orientations is given by the last digit in the final 3-digit part of the code number. So you can have 320/321/322/323/324/325. The picture opposite has the polarisation pip between Pin A and Pin B symmetrical between the two pairs of shell slots. This is given the 320 orientation number. As the pip rotates through 60/120/180/240/300 degrees the number increases by one 321/322/323/324/325.

For those unfamiliar with battery valves I'll explain the need for a grid bias supply. Most valves, battery or those with indirectly heated cathodes ("mains" valves) draw anode current when an HT supply is connected. The value of this current is determined by the valve characteristics which call for a voltage between control grid and cathode. In the case of "mains" valves the addition of a cathode resistor will result in a negative control grid voltage (bias). The higher the resistor, the greater the bias and the less the anode current. This method of biasing is usually termed automatic bias and roughly maintains a constant average anode current. With battery valves the use of a cathode resistor isn't practical so each valve may need a separate grid bias voltage to set its anode current. The designer will work out the optimum value for bias voltages. One could just ground the bias lines but this would result in increased anode currents, and although may not affect operation much (it might produce distortion), it will reduce the life of the HT battery. Another point to mention is the decoupling capacitor used in auto-bias. One can tailor the effect of auto-bias on the frequency of the signal being amplified by altering the cathode resistor decoupling capacitor, making it anything from nothing to several tens of microfarads. Although the anode current of the valve is roughly governed by the cathode resistor (say 600 ohms) working at DC, a 25uF capacitor will have an impedance of about 6 ohms at 1KHz thus shunting the 600 ohm cathode resistor and much lessening the instantaneous biasing of the control grid and producing a 1KHz current at the valve anode in excess of the DC value.

A bias voltage can not only modify the valve anode current, but with some specially designed valves called variable mu valves, increasing the bias will reduce the gain of the valve. This technique is very useful in receivers which might have to deal with signals ranging from 1 microvolt to hundreds of millivolts. To handle this range of signals an AGC or AVC circuit is employed. The amplitude of the received signal is converted to a negative control voltage and applied as feedback to the amplifying circuits. Their gain is reduced, the amplitude drops and the amount of feedback is reduced. Very rapidly the receiver will establish a more or less constant output no matter what the received signal strength. This protects an operators ears but there should ideally be an S-Meter as all signals will sound much the same. It depends on exactly what the receiver was designed for as to whether an S-Meter was included or not.

In the R216 you'll see CV138, EF91 valves which are variable mu types, as are the CV785, DF91. Type CV1758, DF92 is not variable mu and if you examine the circuit diagram you'll see that these are operated under zero-bias condition (control grids DC-wise tied to chassis).

The first step in getting the receiver working is to build a power supply. As I already have a general purpose power unit (see below) for running an R1155 separately from the T1154, I decided to modify this so it would be suitable for the R216 whilst still enabling it to be used with the R1155 (this means isolating the HT negative from ground because the R1155 uses this to develop an extensive network of bias voltages.. perhaps via a front panel switch which can be used to ground or open HT negative from common ground).

  

The R216 requires several voltages which I'll summarise...

1.4 volts DC @ 400mA. This supply is extremely critical as it's used to feed the filaments of the battery valves. I chose to use a linear voltage regulator type LMS1585ACT-1.5 whose output is a nominal 1.5 volts (1.47 to 1.53 volts) at up to 5 Amps and which requires an input voltage of 2.9 volts to 14.5 volts. The device requires only input and output smoothing capacitors plus a "minimum drain" resistor of about 150 ohms across the output. The circuit is fitted to the left of the right hand mains transformer above and is driven from the 6.3 volt winding of the transformer on the left and comprises a full wave bridge rectifier, the LMS1585ACT-1.5/NOPB, two 2200uF smoothing capacitors and a small 47uF output capacitor (needed for stability). A 150 ohm load resistor is fitted across the choc block terminals carrying the output voltage. Because of the low voltage it's important to minimise the output wiring length to avoid voltage loss when current is drawn. Using four 3.3 ohm resistors wired in parallel this supply drew 1.8 Amps with the output voltage maintaining over 1.47 volts.

100 volts DC @ 26mA. This is the HT voltage for the battery valves and is supplied from the basic HT line of 300 volts. In the centre of the baseboard you'll see a tagstrip carrying two plastic power transistors type BUT11. The left transistor is a series pass regulator with its base connected to a pair of 51 volt zener diodes connected in series and fed with a 22Kohm feed resistor from the 300 volt HT line. The emitter of the BUT11 is maintained at 100 volts. I tested this supply using a 1.1Kohm load resistor drawing 88mA with the output dropping from 100 to 99.8 volts.

250 volts DC @ 43mA. Using the same technique as the 100 volt stabiliser, the base of the second BUT11 is fed with a chain of five 51 volt zener diodes and a 8.2Kohm resistor to provide an output of 250 volts at its emitter.

6.3 volts AC @ 3A. This is supplied directly by the right hand transformer, above.

-19 volts DC @ 15uA. This voltage needs to be perfectly hum-free. I fitted a third mains transformer to the baseboard for the bias supply (and also the dial lamp supply). The total power drain for this circuit is pretty low so the transformer needs a rating of no more than say 10VA. To ensure compatibility with the dial lamp AC supply (as in the official R216 power supply) the DC output will be provided by a half-wave rectifier followed by a twin pi-filter using two 22 Kohm resistors and three 2200uF capacitors.

19 volts AC @ 300mA max. In the official R216 power supply this voltage is supplied by the same winding as the bias supply and I used a small standard transformer with twin windings to match the requirement.

  Metering of the various voltages is carried out via the rotary switch in the centre of the front panel connected to a voltmeter and to monitor HT current I used a milliammeter. This indicates the drain of the main 300 volt HT supply, but because the HT supply also provides feeds to the 100V and 250V stabilisers I arranged the wiring to take the feed for the zener diodes from the input of the milliammeter rather than the output. This ensures that only the HT consumption of external equipment will be monitored. The latest part of construction is the addition of stabilized voltages minus 19 volts, 100 volts and 250 volts (left to right above). I used an 18 volt zener for the 19 volt supply (a PNP series pass transistor) and 51 volt zener diodes using two for the 100 volt supply and five for the 250 volt supply (both with NPN series pass transistors).

A permanent R1155 supply cable is fitted with a Jones plug and the supply to the R216 is fitted with a 12-way Plessey plug.

The right hand transformer (top picture) carries twin 6.3 volt 3 Amp output windings tapped at 4 volts and 5 volts. The twin outputs will be wired so that they can supply either 6.3 volts 6 Amp or 12.6 volts 3 Amps. The main HT rail is wired for the R1155 to supply outputs from before and after the HT choke. Here's a circuit diagram (liable to change).
 

 Note the need to have dual switches for the voltmeter because of the need to monitor one negative supply as well as the four positive supplies. The meter has a 0 to 3 scale which is read as 300V for the HT supplies, -30V for the grid bias voltage and 3V for the filament voltage. The movement required 300Kohm for each HT range, 30Kohm for the grid bias and 3Kohm for the filament voltage.

Because the R1155 has an isolated HT requirement (+250V including -30 volts bias) as well as needing a little extra voltage (not necessary for the R216) I used a 2-pole switch and a relay, the relay is activated by the switch when R1155 operation is required and disconnects the 6.3V heater winding from HT negative (that connection is required for the R216) with the second pole of the switch adding a small reservoir capacitor to the swinging choke circuit used for the R216. I used the swinging choke circuit to keep the HT voltage down as the transformer secondary winding was a little high for the task, probably capable of a maximum, off load, of around 370 volts with a typical reservoir capacitor. The position of the switch in the circuit is important as there is a risk of damaging the HT regulator circuits if their HT return is removed, so I placed the switch in the 6.3 volt grounding connection (Black) as shown above. The R1155 heater circuit wiring connects one side of the 6.3 volt supply to the R1155 chassis which is some tens of volts higher than HT negative so disconnecting the PSU heater ground connection in this power unit eliminates the risk of the R1155 bias circuits shorting out (note: when a T1154 is used in conjunction with the R1155 the heater supply has to be DC rather than AC because that same circuit operates the T1154 send-receive relay which has a 6 volt DC coil). Unless I add this 2-pole switch the cabling carrying power to the R216 might need to be disconnected from the PSU terminals before plugging in an R1155 because R216 Plug Pin A carries HT ground and a grounded heater connection as well as the filament return and the dial lamp supply. The relay (which may not have been strictly necessary but gives some flexibility for later modifications) is driven from the 30 volt supply used for the -19V R216 grid bias. I added a 220 ohm resistor to reduce the coil voltage to exactly 24 volts.
 
 The power supply is built on a piece of chipboard and the front metal panel is isolated, so circuit ground hasn't any real meaning, other than the interconnection circuitry between the various output supply voltages. If the PSU used a metal chassis and this was connected to HT negative it would have a potential difference of some tens of volts compared with the R1155. A significant advantage of using wood is that parts can be moved around easily as design changes are made. Some more work is required to fit the R1155/R216 switch and the addition of the voltmeter wiring for monitoring the bias supply. Once everything is working I can tidy up the rats nest of wiring.

 Here's a picture of the R216 plug wired up. I found a drum of cable with 4-cores plus screen, so two lengths of cable will do the job as the R216 needs 10 connections with two being duplicates of earth return for HT and LT. I used the screens of the two cables connected together giving me enough wires for carrying the various power connections. The ends of the cable were soldered to tails secured in the choc bloc.

Once completed I decided to test the receiver.....

 

 After checking over as much as I could I plugged in the receiver to the new power supply. There was a slight crackling sound and smoke rose from somewhere inside one of the two chassis. Clearly something was amiss but I couldn't work out what component was burning so I disconnected the various tails and measured the current for each. The 1.4 volt supply was drawing 360mA and as one valve is missing (it's only the IF output buffer so not critical) this current is correct. Next I connected back the 6.3 volt supply and this stayed at the same level so I judged it to be OK. Next the bias supply which measured 53uA and the voltage remained constant. The 19 volt AC supply drew 0.9A which is too high but it didn't change when the illumination control was turned. I suspect the phase is wrong. Looking at the proper power supply circuit, the 19 volt supply is derived from the same winding as the 6.3 volt supply, but the dial lamps are supplied by the voltage between the 19 volt and 6.3 volt windings (ie. 12.7 volts). I should therefore modify my PSU which presumably has the opposite phases for the two voltages resulting in something odd happening. As there's a spare 12 volt winding on the bias transformer, I'll use this.

Next, I checked the HT currents. 100 volts drew 12mA and 250 volts drew 9mA which seems OK as these measurement were taken with the valves cold.

As all the measurements appeared to be fine I plugged in the receiver once more... no smoke and the combined HT current measured around half scale on the milliammeter. As the total HT current is 69mA I reckoned this was OK. Plugging in a pair of headphones gave me a loud hiss on the FM settings, dropping to about half the volume on the narrower bandwidth setting. AM and CW gave me a loud hum which dropped off a lot when I reduced the AF gain. Connecting a length of wire to the aerial socket did crackle slightly but absolutely no signals were present even from the crystal calibrator. Apparently the IF module is working but not the front end.... Maybe the problem is associated with the smoke? A short-circuit condenser may have open-circuited something? That would explain why the smoke didn't reappear.

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