You can seriously injure yourself or get yourself killed. This section is not intended to be a complete guide to safety in tube equipment, just to hit the high points as refresher for those of you who have some experience. The best way to learn the requirements and practices for safety in tube equipment is to find someone who will teach you one on one. Do not, ever, ever, leave the equipment plugged in and start work on it unless you specifically intend to make some live-voltage measurement. Leaving it plugged in guarantees that you will have hazardous voltages inside the chassis where you are about to work.
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To convert from one to the other, divide , by the known figure this conversion works both ways. The result should be rounded up or down to the closest whole number. Thus , divided by KHz gives Metres. Conversely, , divided by Metres gives KHz. This confirms that Virgin is received at the Metre point on the dial the former position of the Light Programme on MW. As noted in the table, the wavelengths used by current UK broadcasts are the same as those allocated after the war of course the stations are different now so the stations still line up with indicated stations on the dial.
This sort of information can be useful for checking that the tuning is reasonably accurate. It is worth calculating and noting down the wavelengths of the stronger stations in your area for this purpose. Some of these stations may have additional relay transmitters on different frequencies in some areas. This was changed when the wavebands were revised in the s to bring it into line with the 9kHz spacing between allocated wavebands on MW and LW.
Please note that kHz is a modern term, meaning kilo-Hertz. Period literature and service sheets may use the term kcs, meaning kilocycles per second.
Both values are identical - 1 Hertz is 1 cycle per second. Kilo-Hertz means one thousand Hertz. This low-cost set has MW and LW bands only. The various sections of the waveband switch are marked m MW or l LW and are closed when the set is switched to the band indicated.
L1 and L2 are the coils on the ferrite rod aerial. S1 and S2 select the appropriate one for the waveband selected. The aerial is tuned by C4, with C3 being a trimming component. The received signal passes to the control grid of V1b. V1a is the local oscillator, which is tuned by L5, L6 and C10 to C C14 is ganged with C4, so the oscillator frequency varies as the tuning is adjusted. S3 and S4 are the wavechange switch. The oscillator frequency is in this case KHz above the tuned frequency.
V1b is the mixer stage the type of combined valve used for V1 is often referred to as a "mixer-oscillator" or a "frequency changer". This mixes the oscillator signal with the received signal and produces four output signals on the anode. These are the two original input signals, the oscillator frequency plus the received frequency, and the oscillator frequency minus the received frequency.
Modulation on the received signal is also present on these last two outputs. Since the oscillator is always KHz above the received signal frequency, the oscillator minus received signal will be at KHz, regardless of the tuning setting. This is the IF Intermediate Frequency. The IF varies between different sets, but KHz is the most common value.
The service sheet for the set will give the frequency used, but you only need to know it if you intend to realign the set. Chas E. The change to kcs the RMA recommended figure was virtually complete by about In the USA IFs from about kcs to kcs were used in early superhets, with kcs taking over from just before the war. Local Oscillator Testing If noise is heard which alters in note and volume as the set is tuned across the band, with the increase coming towards the low frequency high wavelength end of MW and the high frequency low wavelength end of LW, this indicates that the local oscillator is probably not working.
Measure the voltage on the anode of the oscillator section. If the oscillator is running the voltage will be between about 50 and V. If it is not oscillating, faulty the voltage will be much lower, probably around 10V. To confirm that the local oscillator is at fault, tune the receiver to a position on the dial where you would expect to receive a strong station such as Radio 4 on LW or a local station on MW.
Connect a signal generator to the control grid of the mixer-oscillator valve, and tune the generator across a band of frequencies around KHz to KHz higher than the station frequency for example KHz to KHz for Radio 4. This simulates the action of the local oscillator, and if a station is heard it proves that the local oscillator is indeed faulty. IF Amplification To obtain good selectivity and sensitivity, several stages of IF tuning and amplification are required. Coupling between the IF amplification stages is by IF transformers.
These are tuned on the primary and secondary windings, giving two stages of tuning. One stage of IF amplification and four stages of tuning is adequate for good reception in most cases, and is the arrangement used in most sets.
Additional stages can be added, and will result in improved performance and sound quality, particularly when trying to pick out a weaker station which is close to a strong signal. The anode of the IF amplifier valve will be fairly high, generally only a few volts lower than the HT rail feeding that part of the circuit. Applying the meter should give a crackle from the speaker if the stages following this valve are in order.
The voltage on the screen grid varies widely in different designs, but anything below about 60V is cause for suspicion. The control grid should be between 1V and 3V negative relative to the cathode with no signal present. This may be derived by various means but the most common is a cathode resistor. The voltages on the mixer section of the mixer-oscillator should be similar to those mentioned above.
If a crackle is heard when measuring the mixer anode voltage, the IF amplifier is probably OK. Advantages of Superhets The advantage of superhet circuits over earlier TRF arrangements is that the signal passes through several stages of very sharp tuning, which gives superb selectivity and sensitivity. Since the IF is at a fixed frequency, the IF tuning is fixed. This is much more straightforward and reliable than attempting to keep four or more variable tuning stages in line with each other across the waveband.
This is carried out by a diode, which strips off the bottom half of the IF signal b. The result is then passed through a low pass filter to remove the IF, leaving the audio intact c. This passes to the volume control, and then on to the amplifier which has been described previously.
In the Philips circuit repeated here , the detector diode is contained in V2. The anode of the diode is shown to the right of the shared cathode, which is at the bottom centre. The IF is filtered out by C19, leaving the audio signal present across the volume control not shown, to the right of R There will generally be additional IF filtering components in the audio amplifier stages.
The second diode in V2 is unused in this set, so the relevant pin is connected to the cathode. In better quality sets it would be used as a separate AGC diode as detailed below. Automatic Gain Control AGC The purpose of the automatic gain control sometimes called automatic volume control, or AVC is to ensure that all stations, regardless of the signal strength, give a similar output level.
Without AGC, the stronger local transmissions would come through much louder than programmes that are more distant. Fading at night would also be more pronounced. In the Philips set shown above, the AGC voltage is developed by the same diode in V2 as is used for detection. Since the cathode is at 0V potential, the positive peaks of the IF will be held at this level due to the diode action.
The negative peaks will be at some level below 0V, depending on the signal strength. This is smoothed to an average DC level by R9 and C2. Therefore as the signal level increases, the negative DC potential on the junction of R9 and C2 increases.
The gain of V1a and V2 can be varied by altering the biasing voltage on the control grid. This sort of valve is often referred to as vari-mu, and is sometimes denoted by a diagonal arrow through the symbol this indication is not universal though, and is not shown on this circuit.
Therefore, as the signal strength tends to increase, the negative AGC voltage increases. This increases the control grid biasing voltage on V1a and V2, reducing the gain, which compensates largely for the increases signal. The AGC voltage is at a very high impedance. This will give excessive gain for the signal level received, resulting in possible distortion and instability.
It is normally worth replacing all the AGC decoupling capacitors if the set is prove to noise and whistles, or the performance does not seem quite right. This uses separate diodes for the detection and AGC. Detection is carried out by the diode whose anode is shown to the left of the shared cathode of V3, and operates in the same manner as described above. R7 is the volume control and C13 is an IF filter. The AGC voltage is developed across R11 by the other diode in V3 with the anode shown to the right of the shared cathode.
The IF is picked off from a tapping on L7, and coupled to the diode by C The use of separate diodes for detection and AGC gives better performance, particularly when receiving weaker stations, than using a single diode for both. This arrangement was used on many AM sets for this reason. However, the performance does not suffer as much as might be expected, possibly because the valves used are optimised for this method of operation. Also most people would use these sets mainly for listening on VHF, so the designers were probably not that concerned about getting the best possible reception on the other bands.
Few listeners thought it worthwhile to erect decent FM aerials simply to duplicate what they could already receive satisfactorily. Note that in those days the 10kcs station separation permitted far better AF modulation on AM. Two contributors have described it properly for me! Nigel Hughes says: AGC delay was provided specifically to prevent every signal from being weakened, no matter how small. However, it is also true that some sets had QAVC Q for Quiet to provide noise suppression when tuning between stations.
This was effectively what modern radio communicators call a "Squelch" circuit, which muted the audio stages until the AVC level rose to a certain threshold. Miller added: The purpose of delayed AVC is to prevent the AVC from coming into action until the incoming signal is strong enough to provide sufficient output from the set to provide good listening volume.
So-called simple AVC without delay comes into effect with any level of incoming signal and thus can reduce an already weak signal to an unusable level. The delay voltage may be obtained in a double-diode-triode or -pentode by returning the AVC diode anode to chassis, thus effectively biasing it negative by the amount of voltage existing on the cathode of the valve, or by the application of a negative voltage onto the AVC diode anode as mentioned above.
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