Introduction
Restoring and collecting old tube AM radios may be an interesting and rewarding hobby, but the problem is -- AM broadcasting is gradually being phased out, both on the medium waves and on the short wave bands. While AM broadcasting may be still available in the large cities, it is not so in the remote rural areas. Besides, audio quality of AM is not so good by the modern standards. So, unfortunately, vintage valve radios, from being useful devices, are becoming just "dust collecting" items.
The situation is even more drastic in Australia, where FM broadcasting practically began as late as 1975, when the tubes have been already squeezed out by the transistors, so Australian FM tube radios simply do not exist.
On the other hand, FM broadcasting is not going to disappear soon, as it is still popular and provides high quality audio.
In this situation it would be reasonable to add FM reception to AM vintage valve radios, making them listenable and not compromising their authentic appearance.
Integrated FM tuners
Fortunately, technological advances in the electronics allowed to greatly simplify FM tuner design. Modern single-chip tuners use frequency synthesized local oscillators, running of a watch crystal, and digital signal processing (DSP) for everything else.
Most of them (like Si4702 - Si4704, TEA5767) are tuned and controlled by loading data by I2C from a microcontroller, others (like RDA5807) are tuned up and down by "CH+ / CH-" buttons, yet others (like BK1198) can accept an analogue voltage from a tuning potentiometer, which is converted inside the chip into a digital format and used to program a frequency synthesizer.
The circuits for the single-chip FM tuners are easy to build and require minumum number of external components. The main issue -- is how to conveniently arrange tuning.
One can fit UP/DOWN buttons somewhere onthe vintage radio, but that would certainly ruin its authenticity.
One of the knobs of the radio, for example, tone control, can be sacrificed and a shaft encoder or a potentiometer for FM tuning fitted instead. That would make FM tuning "weird" and counter-intuitive.
Worse, in both cases there would be no visual feedback as to which frequency (even approximately) the radio is tuned in the FM band.
The bottom line -- ideally the same tuning knob as on the AM and the same dial of the vintage radio should be used for FM tuning, but how to do it without mechanically linking FM tuning to the existing dial cord arrangement?
An elegant solution is presented below -- use the local oscillator of a superheterodyne AM radio for FM band tuning.
Medium waves local oscillator for FM tuning
Most if not all superheterodyne vintage valve radios have intermediate frequency (IF) around 450...480kHz, 455kHz being the most common. AM broadcast band in different countries stretches from 520...540kHz to 1600...1700kHz. Local oscillator frequency is always above the signal frequency. Thus, a local oscillator of a typical vintage superheterodyne radio would typically cover 980...2100kHz range, and, considering different IF and slightly different MW band boundaries in different countries, at least 1020kHz to 1950kHz range.
Therefore, if the local oscillator keeps running, its frequency can be measured and translated into a tuning signal for a FM tuner module. The "normal" tunig knob will make tuning conventional and intuitive. The dial will give some idea of the frequency tuned into on the FM band, even though there will be no Megahertz graduations for the FM band on the dial of a vintage AM radio.
A simplified block diagram of a "digital" implementation of such FM receiver is shown in Fig. 1.
Fig. 1. Tuning of a FM receiver by microcontroller based frequency counter.
The example in Fig. 1 is based on a typical triode-hexode frequency changer (frequency converter) of an AM radio. A small "gimmick" capacitor picks up the local oscillator voltage, which is converted into a square wave by a comparator A1. A microcontroller can do frequency counting and conversion into apropriate control signals for the FM tuner chip.
If a single-chip FM tuner accepts analog tuning voltage, then the circuit changes accordingly, as per Fig. 2.
Fig. 2. Analog tuning of a single-chip FM receiver module.
Here a frequency-to-voltage converter is required, with some circuitry for scaling and linearisation of the tuning voltage, so that the tuning does become overly "crowded" at the high end of the band. The analog method may be not so precise and repeatable, but does not require microprocessor programming.
The "gimmick sniffer" mentioned above is a turn or two of an insulated wire around a wire to the oscillator section of a tuning gang capacitor (Fig. 3). Capacitance of the gimmick is a fraction of picofarad, so it does not appreciably affect aligning of the radio on AM bands. As the local oscillator amplitude is usually as high as 5...10V, such loose coupling is sufficient to drive a comparator.
FM tuner PCB module
FA-5 is the latest variant of the module with analog tuning. PCB size is 31mm x 52mm, mounting holes are 3.2mm, 44mm apart (Fig. 3).
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Fig. 3. Top and bottom view of FA-5 FM tuner module PCB.
Basic connection of the modules is dead easy -- place on metal standoffs for grounding, power from ~6.3VAC (pin 12) from the heater winding, connect any antenna to pin 3 ("H") or to capacitively decoupled pin 2 ("A2"), throw a "sniffer" gimmick to the oscillator section of a tuning gang and get about 1Vrms of audio at pin 9. (Alternatively, in a transistor radio, any 5...12Vdc 50mA supply can be used.) For better sensitivity, FA-5 has an additional RF amplifier based on a MOSFET tetrode. With 455kHz intermediate frequency, broadcast medium waves range frequencies from about 570...580kHz to 1500...1550kHz correspond to 87...108MHz of FM band.
Typical pinout of FA-5 is shown in Fig. 4.
Fig. 4. Simplified FA-5 pinout configuration.
FA-5 has a special circuit for sensing of the load (that is a volume control potentiometer of 0.5...1MOhm) connection. When the volume control potentiometer is connected to pin 9, the FA-5 module powers up automatically. When pin 9 is left open, the module powers down and will not create any potential cross-talk (leakage, bleeding) noise in the background of AM reception.
Pinout
Pin 1 ("A1") -- RF signal input directly connected to the antenna RF LC tank and the gate of the RF MOS-tetrode amplifier. Usually this pin is left unconnected, but might be useful for coupling of an external coaxial feeder via a capacitive "tap", which will be explained below.
Pin 2 ("A2") -- Connection to a random short wire or a small telescopic antenna via a 4.7pF capacitor.
Pin 3 ("H") -- Connected to the tap in the coil and recommended as input, if the antenna is a quarter wavelength rod (50...70cm) or a coaxial 75 Ohm feeder.
Pin 6 ("+3V") -- 3.3V regulated voltage output. It can be used for powering some external user specific circuitry, provided current draw is below 10mA.
Pin 7 ("LED") -- Tuning indicator LED connection (sourcing current). The LED lights up upon once signal strength and signal-to-noise ratio exceed certain thresholds. It is driven through a p-n-p transistor switch and current limiting resistor R1 of 330R.
Pin 9 ("OUT") -- Audio output, about 1Vrms. (On request, can be increased up to 2.5Vrms for driving high transconductance output tubes, like EL33, directly.) The series resistor R3 = 330 Ohm permits shorting the audio output to ground without stressing the operational amplifier. However, for FA-5 shorting is not needed due to automatic power-up / power-down function.
Pin 11 (+8V") -- Switched 8.5VDC internal rail which is activated when connection to the audio output pin 9 is detected, otherwise the module is powered down and there is no power on pin 11. It can be used to drive "FM band" indication LED (through a resistor of 1K...8.2K) and/or some circuitry to shut down AM reception in FM mode, which will be explained later.
Pin 12 ("6.3V") -- 6.3Vac supply voltage input. DC current draw is about 35...40mA.
Pin 13 ("+8VS" -- Rectified (unregulated) supply voltage output. It is about 8.5V if the module is powered by 6.3Vac. This pin is usually unconnected, but also can be used as DC power supply input of 5...12Vdc. 12VDC shall not be exceeded, as otherwise RF stage MOS-tetrode may be damaged.
Pin 1 on the other side of the PCB ("OSC") -- Oscillator signal "sniffer" pickup (from a gimmick).
Other pins ("GND" or "G"), as well as mounting holes, are ground. If the module is screwed to the chassis (e.g., on metal spacers), no other ground connections are needed.
If the module is to be powered from a 12V car battery in a vehicle, to protect from possible spikes and overvoltage, it is advisable to use a series resistor of 47...75 Ohm, 2W, followed by a "decent" 5W 12V Zener diode, and connect the later to pin 12 ("6.3V") of the module.
A way of connection of the module to an AM radio with a separate and shared FM antennae are shown in illustrated by Fig. 5A and Fig. 5B respectively.
Fig. 5A. FA-5 connection to a "generic" AM radio with a separate FM antenna.
Fig. 5B. FA-5 connection to a "generic" AM radio with a shared antenna.
A separate FM antenna -- a random surrogate wire (0.1...0.7m long) or a telescopic rod or a coaxial feeder -- should be connected to pin 3 ("H") for the best impedance matching (Fig. 5A). If the same antenna is to be shared between AM and FM reception, then it is to be connected to pin 2 ("A2"), as per Fig. 5B. A decoupling inductor L5 is optional, as usually a the distributed wiring inductance is sufficientnot to attenuate FM signal. The tuning indicator, as well as FM mode indicator LED are optional. "FM MODE" LED would light up as soon as volume control is connected and FA-5 powers up. Fitted behind a dial glass (Fig. 5C), such LEDs would look great and might further add to the perceived value of the radio.
Fig. 5C. Indication LEDs ("FM MODE" -- blue, "FM TUNE" -- green) in a HMV "Super-5" Australian radio.
AM / FM switchover and cross-talk prevention
Two goals were kept in mind through the design of this FM module -- convenience and elimination of cross-talk. And both are achieved by incorporating load sensing and automatic shutdown. A single SPDT switch not only steers the signal path, but at the same time serves as enable/disable control.
It is imperative to disable the FM signal in AM mode. Otherwise, as the impedance of a typical AM detector in a tube radio is quite high, stray capacitances are substantial, wiring might be messy, not adequately shielded, and FM signal is generally treble-rich, leakage of FM signal into AM path would have been unavoidable.
The opposite situation is more relaxed. As the FA-5 module has low output impedance, it is not susceptible to signal bleeding from the AM detector of the radio. Normally, muting of the AM chain in FM mode is not required.
Therefore, a single SPDT switch can do the AM/FM band switching. A volume control potentiometer is switched between the AM detector and the FA-5 output. A "floating" AGC line in FM mode is not a problem. A "RADIO/PICK-UP" switch, if it is present, can be naturally used for that purpose (Fig.6).
Due to low output impedance of FA-5, screening of the cable from FA-5 to the switch is not necessary.
Fig. 6. Simple AM/FM changeover by a single SPDT switch. (Antenna, LEDs and "sniffer" are not shown.)
Circuit as per Fig. 6 works particularly well if the AM/AGC detectors are integrated with the IF stage tube (usually 6N8, 6B8, 6G8G, etc.) or if the AM detector is separate (6AL5, 6H6, etc.). However, in majority of the vintage tube radios, AM/AGC detectors are integrated with the first audio stage (6AV6, 6Q7, 6SQ7, 6B6G, etc.). It may lead to a potential cross-talk issue in FM mode, though fortunately does not often happens.
Due to physical proximity of the AM detector diodes to the control grid wiring, on the tube socket or inside the tube, some IF voltage or rectified audio (which may reach 10V at strong stations) from these diode plates may be induced onto the triode grid through stray capacitance. It can be a direct audio injection or IF demodulation, as the triode might work as a parasitic grid-leak detector (or anode bend detector, depending on the bias method) and create an unpleasant cross-talk murmur. 6Q7 is the least susceptible due to the diodes enclosed in a shielded "box" and the grid connected to the top cap. Hovever, a quite common 6AV6 tube is one of the worst in this respect. In some cheap variants (Fig. 7) grid support rods are literally sandwitched between the diode plates, conducive for such leakage.
Fig. 7. Simple 12AV6 construction lacking grid shielding.
If you encounter such cross-talk problem, the following may be tried:
(a) choose a different 1-st audio stage tube specimen with more shielding elements around the grid sipport rods;
(b) rearrange grid wiring and improve shielding -- remove all the components, soldered directly to the grid lug of the tube socket (usually a 10MOhm grid leak resistor and a 0.01uF coupling capacitor), move them away from the tube and the IF transformer, and from there run a shielded cable right up to the grid lug of the tube socket;
(c) free the 1-st audio stage tube from its detector function -- use a separate Schottky diode, e.g., 1N5711, BAT85, BAT42 for an AM detector and 1N4148 as an AGC detector, if it is present in the radio, and connect the now unused diode plates of the tube to the cathode. At nano-currents, the Schottky diodes virtually have no conduction threshold ("knee"), and linearity of the AM detector will in fact improve, compared to the original vacuum diodes. This approach would also allow to lift the triode cathode from ground and apply a general negative feedback to the audio amplifier to reduce distortion and speaker "boom", due to better speaker damping.
These modifications may feel quite extensive and complicated, so alternatively or pre-emptively, from the onset, it might be easier to mute the AM chain of the radio in FM mode. There are several ways of muting outlined below.
Mutuing AM reception in FM mode
If the radio does not have a convenient "RADIO/ PICK-UP" control knob, and a separate switch has to be installed somewhere, then it is reasonable to fit a DPDT switch and use its second section for muting the AM chain altogether in the FM mode. It can be done, in general, in three ways:
- applying a substantial negative voltage to the AGC line;
- disconnecting ("lifting") cathode of the IF amplifier stage from the ground;
- disconnecting or shorting to ground screen grid supply to the IF amplifier stage.
The following self explanatory schematic diagrams (Fig. 9, Fig. 10) illustrate the above methods. Only the muting related section of the switch is shown, the other section, steering the audio path, is omitted for brevity.
Fig. 9A. Using the heater voltage to mute the radio in AM mode. Rectifier D1 can deliver about --8.7V to the AGC line. It substantially, but possibly not completely, mutes AM reception.
Fig. 9B. Using the heater voltage, but with voltage doubling rectifier, to mute the radio in AM mode. D1/D2 doubler delivers about --17V to the AGC line, which completely mutes AM reception.
Fig. 9C. Using negative voltage from the "bias string", which many vintage receivers employ. This is particularly efficient if the radio has 6V6GT, 6AQ5 or 6F6G clone as an output tube. These tubes require relatively high bias voltage, --12...--16V, which is good for muting. Diode D1 is optional. As the bias voltage has some ripple, the diode D1 "skims" the negative lobes of the ripple, thus delivering the highest possible negative voltage to the AGC line.
The approaches depicted in Fig. 9B and 9C may be combined. For instance, if you connect the cathode of D2 in Fig. 9B not to the ground, but to "--Vbias" node in Fig. 9C, both the bias voltage and double-rectified voltage would combine, delivering up to --25...--35V to completely mute the receiver.
Fig. 10A. Muting by disconnecting cathode of the IF stage. If the stage has self bias, R1 and C1 would be already there. If in the originally the cathode was grounded, then C1 must be fitted close to the tube socket (to avoid possible parasitic high frequency oscillation due to the inductance of the wire to the switch). A bleeding resistor R2 is not necessary, but is recommended to avoid excessive cathode voltage if the tube develops internal leakage. A low value R1 is optional too -- just to prevent arcing of the switch when discharging C1.
Fig. 10B. Muting by shorting the screen grid voltage. Original screen grid dropping resistor is split in two (to the same total value): R2 and R3||R4. However, R3 and R4 value is selected so that to sink about 10mA to ground when shorted. Thus, in the AM mode the circuit would operate as per the original design. In FM mode, the IF stage would be shut down, but the current drawn through R3 and R4 would substitute for missing current draw of the shut down IF stage. (R3 and R4 are shown as two resistors, because a single 22K 5W rated resistor is not common.)
This arrangement with "compensational" current draw has certain advantage, as it helps maintain the same overall current draw and the same B+ supply voltage in both AM and FM modes. Without the current draw equalisation, in FM mode the B+ and the current through the output tube would increase, risking to exceed the plate power dissipation of the output tube and shorten its life. Such method is particularly recommended in the following cases:
- the radio has a fixed bias to the output tube from a "bias string" in the negative leg of the supply;
- plate power dissipation of the output tube is already close to its limiting value (usually the case with 6F6);
- the radio uses a speaker field coil as a choke in the B+ supply;
- screen grids of the IF stage and frequency changer (typically if triode-hexode) are joined together, and shorting disables both the IF and the mixer.
Electronic muting
What if a user would like to implement the AM muting is not keen on fitting any additional DPDT switches to the radio and wants to rely solely on the existing "RADIO / PICK-UP" switch? It is also possible, considering the auto-turn on feature of FA-5. Refer to Fig. 11.
Fig. 11. Muting (by screen grid supply shorting method) by a MOSFET switch. Once FA-5 module gets connected to the volume control potentiometer, FA-5 powers up, voltage about 8.5V appears on pin 11, which grives the MOSFET Q1, which in turn shorts the screen grid supply, similar to Fig. 10B. Any n-MOSFET rated at 400V or more will be suitable, particularly in a convenient insulated TO-220 package, e.g., Toshiba TK4A50D.
AM/FM switching by Tone Control knob
If the radio does not have a "RADIO / GRAM" switch, but has a tone kontrol knob (as majority of the radios with short wave band would have), it also can be used for AM/FM changeover -- with a "trick". A regular tone control potentiometer can be replaced by a switched one, and wired so that its most anti-clockwise position corresponds to the full audio bandwidth. A switch of this potentiometer will control an electromechanical relay, which in turn will be switching over from AM to FM in any appropriate configuration as discussed above. The rationale behind it is that in FM mode, when sound quality is good, there is no need to limit audio bandwidth by tone control. One example of implementation is shown in Fig. 12A.
Fig. 12A. AM/FM changeover by tone control switch and a relay powered from FA-5 module. Here muting is achieved by disconnection, not shorting the screen grid supply. Such method does not need power dropping resistors.
De-energised position of the relay should correspond to FM mode, energised -- to AM mode. The relay (5V coil is recommended) should be of a signal, "telecom" type with silver or gold plated contacts, designed for reliable operation at low currents and voltages. Panasonic relay TQ2-2M-5V is suitable, as an example. The relay, with an appropriate dropping resistor R9, is powered from pin 13 ("+8VS") of FA-5 module.
Thus, when the tone control knob is turned fully anti-clockwise past the "click", the relay would drop and the radio would switch to FM with full audio bandwidth. (There is no reason to cut off treble when listening to high quality sound from FM stations.) Turning the know clockwise past the "click" of the switch will energise the relay, switch to AM reception, and then, rotating the tone knob further, a listener will be able to "muffle" the treble, if needed, to make the sound "mellow" and/or reduce noise or "whistles".
Another, quite "elegant" method of operating the relay is illustrated by Fig. 12B.
Fig. 12B. AM/FM changeover by tone control with the relay in the power supply negative leg.
Here the relay inserted in series with a "bias string" of the radio and shunted by a switch coupled to the tone control. Opposite to the above Fig. 12A, the relay is energised in FM mode. Shunting resistor R1 helps to keep the relay coil voltage within its specification, as usually the current draw of the radio exceeds the relay operation current. In this case, muting is done by applying the bias voltage, compounded with the relay coil voltage, to the AGC line (similar to Fig. 9C). But muting is more profound, as the bias voltage and relay voltage are added up. Further, in FM mode, the voltage drop of the relay coil somewhat counteracts B+ rising due to the IF shutdown. Thus the B+ equalisation, similar to Fig. 10B and Fig. 11, is achieved, but without high wattage resistors. If required for the complete equalisation, an extra dropping resistor R2, selectable on test, can be added. It would further increase the available AGC muting voltage.
Coupling to FM antenna
A telescopic FM aerial probably is the best, but would look really out of place on a vintage radio. The simplest acceptable solution is to place a piece of wire and attach it to the inner top surface of the timber, bakelite or plastic cabinet of the radio or thread through the holes of the back board. It is quite sufficient for clear reception of the local FM stations, as the module is quite sensitive with its RF preamplifier. If such surrogate antenna is separate, connect it to pin 3 ("H") of FA-5. If the same surrogate antenna is intended for crude AM reception as well, connect it to pin 2 ("A2"), as per Fig. 5B. Note that a long wire antenna, which is good for AM, may not be the best for FM, as it may be prone to multi-path reception. Thus, if a decent AM antenna is expected to be used with this radio, a separate FM antenna would be preferred. It is recommended to fit an RCA audio socket the the back of the radio. Then it will be possible to plug any surrogate antenna into it with a standard RCA audio jack, as well as a coaxial feeder from an outdoor antenna.
Finally, in case of a balanced 150...300 Ohm feeder (ribbon "noodle" cable, nowadays not so common) a three-turn coupling coil can be wound around the existing 6-turns inductor L1.
Note that tuning of the front end LC tank to the middle FM band frequency 100MHz can be done by stretching/compressing the turns of the coil L1 or by the optional trimer capacitor C1 (if fitted). In practice, fine tuning is not really necessary, but for the absolute best sensitivity, the coil can be either compressed (to increase inductance) or spread out (to decrease inductance), or the trimer capacitor (if fitted) adjusted. To fine tune, find a weak station, reception of which you would like to optimise, and, by adjusting the coil, try to achieve reception with minimum static noise.
Mounting of the FM tuner module
Because of a relatively small size of the module, it is always possible to mount it somewhere not far from the tuning gang variable capacitor (or the frequency changer tube) of an AM radio. The best position is probably under the chassis, close to the bend between its horizontal and vertical surfaces. But practically, it is usually more convenient to fit it on the top of the chassis. The module should not be placed too close to the power transformer. Some examples of the installation and the "gimmick" are shown in the below pictures.
Fig. 15A. "Gimmick" coupling to the oscillator section of the tuning variable capacitor (Australian His Master's Voice radio, model 42-71).
Fig. 15B. FA-4 FM tuner module fitted into an Australian Philips model 125 vintage radio. Gimmick is the white/green striped wire. Antenna post is shared. A decoulping (though practically useless) inductor can be seen on the left, in a clear plastic tube.
Fig. 15C. FA-4 module and AM/FM slide switch fitted to Scharnberg-Strauss 4-valve radio, model 41. A separate FM antenna terminal is provided with a surrogate antenna (piece of wire) attached.
Fig. 15D. Oscillator signal pick-up gimmick in the Scharnberg-Strauss radio.
Fig. 15E. FM tuner module FA-5 in His Master's Voice model 65 ("Super-5") radio, mounted on the ferrite rod antenna frame. AM/FM band switch and FM antenna post are fitted at the back of the chassis.
Fig. 15F. FA-3 module in a Kriesler (model 11-81) radio. Inductive tap is used for coupling. A screw terminal is fitted for FM antenna.
Fig. 15G. FA-5 module fitted to Kriesler (model 11-7) radio. Inductive tap and a short piece of a coaxial cable RG-174 with RCA plug are used.
Fig. 15H. FA-5 in HMV (model 61-51) radio (with the inductive tap for coupling). A tuning indicator LED is surface mounted style, shining into the edge of the dial glass and held by hot-melt glue, which is safe on the LED. Chemically aggressive glues, particularly "Superglue" shall never be used, as its vapour can cause clouding of the LED lens.
Fig. 15I. FA-5 in AWA Radiola (model 573MA) radio. It is one of the few radios where there is more room under the chassis for the module. RF signal is fed to FA-5 via a coaxial cable (black) from a RCA socket at the back of the chassis.
Fig. 15J. FA-5 fitted to Philips BX495A radio -- installed on the inner side of a metal shielding plate.
Fig. 15K. FA-5 fitted to Philips model 224 radio -- more convenient to fit onto a bracket supporting the face metal panel of the chassis.
AM/FM switch is installed onto the aluminium heat shield at the back.
Practical schematic diagrams
Below are schematic diagrams (click on them to view full size) of conversion of some Australian AM radios to FM reception.
Fig. 16A. Connection of FA-5 to a HMV "Super-5" (model 65) radio. As the first audio stage is integrated with AM detector, it is preferable to mute AM chain in FM mode. It is achieved by shorting screen grid voltage of the IF stage.
Fig. 16B. Connection of FA-3 to Kriesler model 11-81 radio. FM antenna is coupled by a tap to the coil. Enabling of the FA-3 is done simply by supplying ~6.3V in the FM mode. As the first audio stage is free from AM detector, muting of the AM chain in FM mode is not necessary, however, just in case, a partial muting is achieved by 1N4004 diode, which sends about --8V to the AGC, significantly reducing AM sensitivity. This diode is optional and not necessary.
Fig. 16C. Connection of FA-5 to Kriesler model 11-7 radio. To avoid cross-talk, 1N5711 and 1N4148 diodes are used for AM and AGC detector respectively. A tone control 3-way switch is now used for selecting AM, FM and gramaphone pickup modes.
Fig. 16D. Connection of FA-5 to HMV model 61-51 radio. IF stage is muted in FM mode by opening the 6BA6 cathode circuit, but a 330K bleeding resistor helps prevent overvoltage in (an unlikely) case of leakage in the 6BA6 tube. A resistibe divirer 330K/120K at the AM detector equalises volume of AM and FM reception and does not allow AGC circuit "floating" in FM mode.
Fig. 16E. Connection of FA-5 to Philips model 125 radio. "GRAMO" position of the band switch is be used to activate FM mode. In this position, fortunately, the local oscillator continues to run as in MW band and further, the AM chain is muted by shorting the IF stage screen grid voltage to ground.
Fig. 16F. Connection of FA-5 module to AWA model 573MA radio. R32 provides some load to the AM detector in FM mode and prevents AGC line from floating. R31 attenuates audio signal in AM mode to equalise volume of AM and FM. "Ground leak" resistor R30 is optional an fitted just in case to prevent excessive voltage on IF 6BA6 tube cathode in case the tube develops internal leakage in AM mode, when the IF stage is disabled.
Fig. 16G. Connection of FA-5 module to Philips model 224 radio. In the FM mode, the AM IF stage is disabled by lifting the cathode of V2 (6BH5) of the ground. R1 and R2 are optional. R1 makes C1 discharge softer when the mode switch is being thrown to AM mode. R2 is a bleeder in case an old V2 develops an internal leakage.
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