R216 Receiver

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 usually 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. For some reason the R216 does not use AGC, instead it has an RF gain control. This sets the overall bias level for those valves arranged to be controllable.

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).

  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).

 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 might be OK as these measurement were taken with the valves cold. On the other hand, only the regulator should draw HT current, and that from the 250 volt rail so what explains the 100 volt drain? I can see there's two resistors across the 100 volt rail at V11. These add up to 55K so should draw 1.8mA leaving 10.2mA unaccounted for. The RF chassis doesn't use the 100 volt rail so the drain must be in the IF chassis.

As all the measurements, bar that small 100 volt drain, 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 wasn't too critical. 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 a resistor? That would explain why the smoke didn't reappear a second time. What would smell hot... well a drain through a 10K resistor from 250 volts would be 25mA and dissipate 6 watts and if the resistor turned into a charred mass of say 50K this would dissipate around a watt or so and get pretty hot. The 5mA HT current wouldn't really make much difference in the sets consumption.

A second test... this time I fitted a spare 1T4 into the unoccupied DF92 V12 socket in case I decided to monitor the IF output. Turning on the power supply gave me a quarter scale current reading which slowly rose to half scale (something like 35mA). I guess this is because the battery valves have a very short warm-up time and the mains valves need up to 20 seconds or so. After a couple of minutes there was a hot smell but I couldn't easily figure out from where it was coming. I also turned on my spectrum analyser to see if any RF signals were present but drew a blank. This implies the RF chassis has a problem, perhaps a short-circuit condenser decoupling the HT line? If this had burned out it's associated resistor (the smoke when first turning on the receiver) it would all make sense. R15 and R21 are prime candidates especially the latter. A simple check is of course to simply remove the can from V4 and see if the voltage stabilizer is lit (but it's in an awkward position.. and I noticed it lighting up when power is applied to the receiver).

Another test could be to remove all the mains valves and see what current if any is drawn from the 250 volt supply. Unless I've missed something, only the QS150/15 would draw any HT current... I unplugged the interconnecting cable from the RF chassis and disconnected the filament supply. I measured 9mA from the 250 volt supply which disappeared when V6 was unplugged which seems fine, but I noticed that the HT rail wasn't too high, being only 230 volts dropping to 205 volts with the RF chassis plugged in, when the HT draw is 33mA (which equates to an additional 24mA from the RF chassis). Clearly the power supply isn't producing enough voltage so I'll have to add extra reservoir capacity, or remove the 250 volt stabilizer because it's not necessary. I'll do this once I've fixed the fault.

I had noticed something slightly odd when I carried out the initial checks. Rotating the RF gain control dramatically increased the HT current. This is a combination of the 100volt and 250 volt rails. Some of the change is current drawn from the 100 volt rail because two IF valves are controlled by the pot although only V6 uses the 250 volt supply. Of course I did hear a lot of audio noise and this would drive the audio stages to draw more current.

I then did another test with the filament supply disconnected. This would of course prevent the battery valves from drawing current. In fact I measured the 100 volt draw and it measured 1.8mA (this is the current drawn by the resistors at V11 as calculated above) and remained at this no matter what setting was the RF gain control which is correct. With the RF gain control set at full the 250 volt draw was 33mA, but if I set the RF gain at zero the 250 volt HT current dropped to 10mA. V6 drawing 9mA at full gain will drop to maybe 2mA at minimum gain. But that leaves 8mA from the RF valves rising to 24mA at max gain. This is because the RF valves are fed from a bias supply which is combination of the RF gain pot setting plus a potential divider formed by R43, R44 and R46 so their HT current will change with the setting of the RF gain control. All this seems about right.

I suppose I can unplug the RF unit and test the IF chassis by injecting varieties of 4.86MHz signals. To make things easier I should first find a suitable 2-pin headphone plug and a suitable RF plug for the IF output. This I did, fitting two old fashioned wander plugs to a standard quarter inch jack socket and plugging in a loudspeaker. I turned on my signal generator and tried listening for signals. I did notice that using FM I could hear a bleep when the generator was tuned to 4.86MHz, the IF. Cranking up the FM deviation did however produce a good response using an input of 50mV to the aerial plug. Switching the receiver to AM I could hear the FM signal so both AM and FM are working at least at the IF of 4.86MHz. Nothing could be heard when using the various wavebands with the generator tuned to corresponding frequencies such as 28MHz or !00MHz so the local oscillator or possibly the mixer is not working. The RF gain smoothly changed the audio volume so my guess is the RF amplifier may be working. The stabilizer is lit but there's still that burnt smell coming that seems to be coming from the RF chassis.

Swapping V2 and V3, type EF91 should prove the oscillator valve, unless both are too tired... , but no that didn't work and it was a real problem replacing the oscillator valve. It's held in place by a metal clamp and difficult to see the holes in the socket for lining up the valve pins. Of course the mixer might be u/s as it's a CV291 a special, extremely sensitive device (see a sheet carrying data on this). Also here's some more data including a picture. I have a few spare ones in the workshop if mine has been damaged. I prodded the signal generator on the thin end of the diode and the IF amplifier responded but nothing heard when prodding the fat end. Duff mixer diode or then again that burning smell might be relevant?

Frequency of operation  6000MHz  Min back to forward resistance ratio  8:1  Min forward resistance  265 ohms  Max amount by which performance is below "standard best"  3.0db  Max noise temperature ratio  2.6  Nominal IF impedance  350 ohms

I measured some resistors and voltages. Two 15K each measured 18K so not important. V2 anode went from 158 to 79 volts, drawing 7.6mA through 18K to 216 volts (15K reading high) and V1 anode was 116 volts fed from 230 volts and 18K (15K reading high) drawing 6.3mA at max gain. V3 (triode-connected) anode was 88 volts fed from 141 volts so is drawing 5.8mA through 9.2K (the 8.2K was high). All these measurements seem to me to be OK. I wondered about the three coax cables which mix the calibrator output to the aerial but placing a signal on the output cable proved they were probably blameless as there was no improvement. I probed the oscillator with an oscilloscope probe but no RF could be seen.

 What about the faulty dial lamp? I fitted a new 12 volt lamp in place of the duff one (open filament) and not seen a recurrence of the overheating of R73. Maybe there's something other than a short-circuit to explain this (but see the broken tag strip above). It does remind me of bias shorts in the R1155... Could the burning resistor perhaps be due to a bad grounding connection in the home brew PSU?

Whilst the turret is out I checked the various resistors which would normally be inaccessible. All read high in value so it will be prudent to replace them.

 Having got EMER 384 which carries lots of alignment instructions, I can hopefully now more easily track down the reason for receiver deafness. After having carried out more testing, which confirmed the turret is working correctly, I can use the test data to confirm or otherwise the performance of the IF amplifier. A test signal of AM at 30% depth should give me 5mW at the 150 ohm output sockets. What sort of level should be input to the IF strip? The data doesn't tell me this in so many words, but it does specify that a 10uV 4.86MHz CW signal applied to the IF side of the mixer diode should give me a decent tone using the BFO. If this works out then the deafness must be attributable to the RF front end (including the mixer diode), otherwise the problem is in the IF strip. I've already noticed that all the IF coils tune nicely which is a good sign. Currently I can hear local FM broadcasts but no other broadcasts on any waveband. My signal generator can be heard but that needs to be at best 50uV and sometimes more than 10mW across the different bands.

Part of the receive problem is fixed. The IF strip is prone to instability when the maximum gain setting is used and retuning the stages upset the centre frequency. I found this out when I injected 4.86MHz directly into the IF input socket on the chassis instead of forcing it through the front end. I set the spectrum analyser to 4.86MHz and proved the calibration by adding a signal from my signal generator and found the IF amplifier was several KHz away from 4.86MHz. It was quite awkward to shift it back and I soon discovered that I needed to back off the RF gain control to get the amplifier to centre on 4.86MHz. Once the shape of the response curve was correct and dead centre on the correct frequency I gradually turned up the RF gain. Instead of the shape of the resonse just getting a higher peak as the gain increased, it flat-topped and got wider and wider as the control was turned up. Backing off the input signal and increasing the gain showed that the amplifier began to get unstable. This seemed to be due to interaction between input and output leads used for testing. Finally, after checking the amplifier was properly centred on 4.86MHz I disconnected the monitor lead from the grid of V11 and re-plugged the lead from the mixer. I was then able to complete the IF tuning by adjusting V11's input at TR3 and V6's input transformer TR1.

That flat-topping I'd noticed at V11 grid.... I wonder if it's the so-called "noise limiter" working its magic?

The FM adjustment method in the EMER sounded pedantic so I adopted a much easier method. First L37 is tuned for maximum noise then T39 tuned to almost minimum noise, then L37 retuned slightly for max noise and finally T39 tuned for minimum noise. This lets me listen to broadcast stations without distortion if wide bandwidth is selected and careful tuning employed.

Next, I'll need to align the turret tuner RF circuits. As there are 4 trimmable coils for each waveband the difference between best and nominal adjustment will be very significant, but before tackling this I decided to check the mixer diode. Testing these types of microwave diode is not easy because a normal test meter would probably destroy it so I looked in my spare parts box and spotted a couple of Schottky diodes marked "HP5082-2835". I bought these diodes from a local Tandy shop that was closing down, probably about 1985 and checking their datasheet they look ideal for a new R216 mixer. The new diode was easy to fit by just soldering across the terminals at the original diode socket (so if ever a new CV291 becomes available it can easily be fitted). I turned on the R216 and immediately heard the familar racket of a shortwave band. That was because of the last test I'd done... I'd switched to the lowest range with the BFO on and a long wire connected and heard precisely nothing... apart from broadcast FM stations on range 4 the receiver had refused to ackowledge anything on any band (apart from millivolts from my signal generator). So there's the answer to receiver deafness. Mixing was presumably taking place at the first IF amplifier but only if sufficiently strong signals were present... the duff diode was acting purely as a tiny capacitor and coupling the RF and local oscillator signals to the grid of the EF91 where slight non-linearity within the valve produced mixer products. Essentially the IF amplifier was a valve down and the substitute mixer only working on strong signals.

I attempted to measure the old diode now it's clearly u/s and I could see absolutely no trace of anything except possibly a minute capacitance. It seems the capacitance of the diode socket and local wiring must have been responsible for coupling the RF from the front end to the IF amplifier. No wonder the receiver was deaf... the previous owner probably tested the mixer diode with a multimeter.

 On the right you can see a pair of Yaxley switches incorporating an interlock so that mains voltage can't be changed when the receiver is turned on.

Also, another view of G8JAC's Araldited plate, top right. Once the plate had fallen away you could see there was just a large hole with no metal left for mounting the socket. The superglue repair worked fine and the mains cable fits.

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