CR-6A Radio Receiver

By Ray Robinson
VK2ILV
E-Mail robinson@srsuna.shlrc.mq.edu.au

Introduction
The CR-6A is a valve communications receiver made by A.W.A. (Amalgamated Wireless Australasia) from 1958 to 1962. It is a 12 valve double conversion super heterodyne receiver covering the frequency range 2 to 30 mcs in 6 bands. The bands are band spread and each band covers 5 mcs (except for the lowest one). It is a 19" rack mounting receiver but was also available in a metal desktop case, painted in Hammer tone enamel. It requires an external speaker. The receiver runs from 240 VAC.

All the controls are on the front panel, with a distinctive black V shaped escutcheon that encompasses the tuning scale, tuning knob, the S (Signal strength) meter and the band switch. All the other knobs are arranged symmetrically below this. It has a large tuning scale which shows a different range of frequencies when a band is selected. The tuning knob has nice feel and has a flywheel with adjustable drag. The receiver has selectable IF bandwidths, and a product detector for SSB reception. It is sensitive, is easy to use and heavy.

The receiver has a number of interesting features. The second IF is at 100 kcs and uses a block filter with selectable pass bands. The second IF amplifier is resistor /capacitor (RC) coupled. The first IF is at 1800 kcs and also uses a block  filter. The receiver has 2 detectors, one for AM and one for SSB.

Performance
The receiver is quite sensitive, even though it only has one RF stage. The controls are easy to use, especially the tuning knob with the flywheel and adjustable drag. The knob is coupled to the tuning capacitor with dial cord. If there is zero drag, the tuning feels a little springy, and it will move slightly after you let go of the knob! A tiny amount of drag eliminates this. Then you can spin up and down the band, and tune a signal easily. More drag makes it stiffer, and you can continue increasing the drag until the dial is locked. The band spread linear dial is very easy to read. The S meter is the best I've ever seen. Resolution of SSB signals is very easy with the product detector. Altogether, it is a very nice receiver, and reflects unconventional but well thought out design, at the end of the valve era.

Different Models
A low frequency model called the CR-6B was introduced for Flight Service Units to monitor long wave beacons. This receiver has an LF range of 200 to 540 kcs, and does not have the 25 to 30 mcs band.

CR-6A Radio Receiver

Controls The CR-6 has a black V shaped dial bezel that holds the main controls. Along the top is the almost linear frequency scale. When you change bands with the right hand side TUNING RANGE MC/S knob, this scale rotates away, and a new one is shown . The scale can be moved a little with a DIAL CORRECTOR knob. Turn the OFF/STD.BY/ON/CAL switch to CAL which switches on the crystal calibrator. Tune to a harmonic at a megacycle marking, and set the scale exactly with the DIAL CORRECTOR knob.

On the left hand side is the S meter, and below this is the ANT. TRIMMER knob, used to tune the aerial. At the bottom of the V is the main TUNING knob, with a semi circular logging scale above it. There are 2 dial lamps to light the scale.

On the left-hand side of the metal front panel is the AM-MCW/CW-SSB knob, which selects between the 2 detectors and switches in the BFO. Below this is the B.F.O. KC/S knob which adjusts the BFO beat to 3 kcs above or below the signal. To the right of this is the A.C. OFF /A.F. GAIN control, to adjust the volume, and to turn the receiver OFF. Next to that is the A.G.C. switch, which has OFF, FAST and SLOW positions. In the centre of the receiver metal front panel is the DIAL DRAG adjustment, and the PHONES jack. To the right of the centre is the STD.BY/ON/CAL switch. In the STD.BY position, only the valve heaters are powered. The next control is used to set the R.F. GAIN. At the extreme right hand side is the BANDWIDTH switch which selects IF pass bands of 6, 3, 1.5, and 0.7 kcs. Above this is the NOISE LIMITER control which changes the clipping level and is also used to switch it OFF. An optional control, which is fitted to this receiver is the FREQ. CONTROL. This may be covered by a plate on some receivers. The knob has 2 concentric controls. The outer one called CRYSTAL is a 7 position switch that selects one of 6 crystals for fixed channel reception, or a MANUAL position for normal reception.  The centre knob is a trimmer which provides fine control of the optional oscillator frequency.

The back of the receiver has the mains cord entry, and some tag strips for, aerial, earth, speaker, line, and a break in facility. There is also a BNC connector for the IF output.

CR-6A Radio Receiver Internal View

Design The receiver consists of one main 19" rack chassis  housed in a large steel case, with  3 smaller units (or sub chassis). The chassis parts are mild steel and cadmium plated. One is the RF section, another is the IF filter, and a third is the optional crystal oscillator unit. The smaller units are easily removable for replacement, or service. The RF section is a self contained rigid assembly and includes the 3 gang tuning capacitor, the band change switch, the RF coils and trimmers, and the RF and mixer valves. The removable IF filter section consists of 5 tuned inductors which use ferrite extensively, and a multi wafer bandwidth switch.

This filter can be removed and sent back to the factory for realignment. The crystal oscillator unit allows the receiver to be crystal locked to 6 frequencies. It consists of a small chassis with a valve, 6 crystal sockets, trimmer, and the selection switch.

The power supply consists of 4 semiconductor diodes in a bridge, feeding a pi filter, consisting of a choke and 2 capacitors, and providing 150 VDC high tension. This is fairly low compared to most receivers, and has probably been chosen to reduce internal receiver noise. There is an OB2 voltage regulator valve which supplies 105 VDC for the main oscillator for stability. The audio section consists of a 6AU6 preamplifier and a 6AQ5 power amplifier driving an output transformer, which has a 3 ohm secondary for a speaker and a 600 ohm secondary to drive a line. There is a phone jack on the front panel for headphones which can silence the speaker.

A noise limiter consisting of 2 semiconductor diodes, takes the detector output and clips both positive and negative peaks, dependent on the NOISE LIMITER control. The noise limiter output is then applied to the A.F. GAIN control and then to the audio amplifier.

There receiver has 2 detectors. There is a normal semiconductor diode detector for AM-MCW and a product detector for CW-SSB signals. The product detector is a 12AU7. Either can be selected with the AM-MCW and CW-SSB switch. The BFO runs at 100 kcs and uses a 6AU6 which is injected into the product detector. The BFO has a variable capacitor which allows the frequency to be varied 3 kcs above and below the 100 kcs IF frequency.

The AGC voltage is derived from the anode of the last IF and uses a semiconductor diode and a resistive divider to provide delayed AGC.  A switch is provided to switch in an extra capacitor to provide slow AGC recovery. The AGC is applied to 2 of the IF stages and the RF stage. The S meter is a conventional left to right reading meter and measures the RF signal level directly in dB. The meter is connected between a voltage divider and the cathode of the second IF. There are zero and a maximum pre-set potentiometers for adjustment.

The electrical design of the IF is interesting. It has an RC coupled IF amplifier with a 100 kcs filter in a block, consisting of 5 tuned circuits. The IF amplifier uses two 6BA6 valves with A.G.C. applied and a 6AU6 valve which are all RC coupled. The output is transformer coupled to the detectors. The 6AU6 has negative feedback to control the output impedance and broaden the response of the output transformer. The input to the IF amplifier comes from a 100 kcs passive filter consisting of 5 tuned circuits which make extensive use of ferrites, both as slug tuning and as an encapsulation. Under the filter is a long rotary switch which carries many capacitors. These series and shunt elements are switched in and out to provide a variable pass band. The filter bandwidths of 6, 3, 1.5, and 0.7 kcs.

The input to the 100 kcs filter and IF comes from the second mixer which uses a 6AJ8. The triode section is used as the 1700 kcs crystal controlled oscillator and the other half being the 1.8 mcs first IF and mixer. There are 3 tuned circuits used as the 1.8 mcs first IF filter. This takes the output from the RF section.

The RF section on the separate removable chassis has a 6AJ8 mixer and oscillator. The oscillator operates on the high side of the received frequency for bands 1, 2 and 3 and on the low side for bands 4, 5 and 6. The band switch selects different oscillator and RF coils, as well as resistors to ensure the sensitivity is the same for all bands. There are 2 terminals on the chassis which allows a 100 uA meter to be connected to, measure mixer oscillator injection, which should be between 15 and 40 uA. The RF amplifier is a 6BY7 with an ANT. TRIMMER control for aerial tuning. A 3 section variable capacitor tunes the coils. There is an optional oscillator unit that allows the receiver to operate as a fixed frequency receiver. A front panel switch turns the mixer oscillator off and switches in one of 6 crystals. This uses half of a 12AU7 as the local oscillator.  A crystal calibrator using a 6AU6 injects harmonics into the RF amplifier for scale calibration.

Restoration
I replaced a pilot lamp, lubricated the moving parts, and gave it a clean.

The receiver was working when I purchased it, but the sensitivity was not very good and the scale readings were way off. I opened it and did a visual inspection. There are 3 different types of resistors used, which is strange. There is evidence of several hands having been here before, including sloppy soldering. The A.F. GAIN control had been changed, and a dual concentric one had been fitted. No need for this, so I assume it must have been all that was available at the time of repair. Definitely not a factory repair.   The power supply voltages were OK, and the frequency response of the audio amplifier was checked and found to be good.

The B.F.O. was off frequency. I padded the tuning capacitor, as the tuning slug could not be moved at all. The second IF gain was OK. I attempted to tune the IF filter, but could get no obvious peaking. It only appeared to work on the broad band setting.  I dragged out the sweep generator, and set it to 100 kcs. The pass band consisted of 3 peaks at 104.7, 105.5, and 108.6 kcs. No wonder the B.F.O. was off! On the other band pass settings, nothing was getting through at all.

When I worked at the AWA factory in 1970, I was told that the IF block filter was the strength and weakness of this receiver. The low IF frequency made the selectable bandwidths possible. The weakness was that it was difficult to align, and had to be removed and returned to the factory.

One of the slugs in the IF was a slot head, the others were hex heads. One tuned circuit thread was partially stripped, and occasionally, if is was set too high, when I upended the chassis, it fell out! It was obvious that here was the receivers problem, and several heavy hands had attempted to sort it out. I began by finding out which slugs controlled which peak. It was not always obvious, and there were interactions. I could not get any of them back to 100 kcs and I could not get them all to coincide at any frequency. I used a variable tuning gang (30-450 pf) to get the coil on frequency, then measured it with a capacitance meter, and made up the value in high stability Styroseal capacitors.   This method was partially successful, but with interactions. After a whole day (!) I ended up with one peak, but it was too low, at 98 kcs. I tried another method. I disconnected them all and individually padded them to 100 kcs. This only took half a day, but when they were reconnected, they were all over the place, due to the interactions. I gave up and went away. The next day, I tried a third method. I padded the input and output tuned circuits using the variable capacitor till they were on frequency. There was no interaction here. Then I started from the input end and did them one by one, and each time, checking the previous one to ensure it had not moved, and readjusting it, if it had. I used the narrowest bandwidth setting. If they worked on this, then they would be fine on the others, but I checked each time to be sure, and that the slug was tuning in the centre of its range. After another day, I had them all on frequency (actually I was 700 cycles too high), and with an almost constant output from each bandwidth setting. The narrowest one was a little lower in output. The shape was good on the 3 narrow settings (only one peak), and almost flat on the broad setting (2 small hills on a plateau). It was a vast improvement. I decided this was enough, and I locked the slugs with wax. Fixing this filter without a sweep generator would be impossible, and even with one, it required over 2 days of effort. I then had to redo the B.F.O. to get the beat correct. The first IF was straight forward and aligned easily.

The RF frequencies were off and one band was dead. I began by aligning the lower band. It and the next one required a lot of shifting to get it correct. The third band was dead, because the optional crystal oscillator was installed incorrectly. There are 2 coax leads that come from the oscillator and are in series with the local oscillator grid circuit. Normally the grid goes straight to the appropriate coil. With the option installed, it should go via the optional oscillator switch, using these 2 coax leads. During fixed frequency operation, the optional oscillator feeds the crystal frequency down via the coax. The braid of the coax should have been soldered to ground. Instead it was soldered to a lug on one of the coils, which was assumed to be an earth point. It was the hot end. Hence the dead band. I connected it correctly, tidied up the sloppy wiring and soldering, and the dead band came alive, and was very near the correct alignment. I checked the 2 bands I had just done and they were now way off. I realigned these. Had I located this fault first, I would have saved myself a lot of effort. The rest of the bands aligned easily.

The crystal calibrator was not working and required a new valve. The S meter adjusted easily. With no signal input I adjusted the zero set potentiometer for zero on the meter. I set the signal generator to 0.5 micro volts at 7 mcs and increased the level until the pointer just began to move. This is the start of A.G.C. I then increased the signal input by 100 dB and set the maximum potentiometer for FSD. I reduced the level in 10 dB steps and verified that the readings were correct.

A very nice receiver but difficult to align.

References
Instruction Book No. 60600R, General Purpose Communication Receiver CR-6, Series C60600, Amalgamated Wireless (Australasia) Limited, 47 York Street, Sydney, Australia.

Short Wave Receivers, Past and Present, Fred Osterman, 3rd Edition, Universal Radio Research, Reynoldsburgh, Ohio, U.S.A., 1997, ISBN 1-882123-07-7

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