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TRF RADIOS

TRF Radios In The 1920s#

Mr Ernest Eversure, tuning a Grebe Synchrophase vacuum tube radio receiver in 1925, made by the A. H. Grebe Co

S. Gordon Taylor / Public domain

A tuned radio frequency receiver (or TRF receiver) is a type of radio receiver that is composed of one or more tuned radio frequency (RF) amplifier stages followed by a detector (demodulator) circuit to extract the audio signal and usually an audio frequency amplifier.

Despite the fact that Edwin Armstrong had already invented the regenerative receiver, which was a far more elegant design, it appears that this type of receiver was popular in the 1920s.

Early examples could be extremely tedious to operate since when tuning in a station, as each stage had to be individually adjusted to the station's frequency. However, later models had ganged tuning, the tuning mechanisms of all stages being mechanically linked together, and operated by one control knob.

By the mid 1930s, it was replaced by the superheterodyne receiver patented by Edwin Armstrong..

A More Simplified Tuned Radio Frequency Receiver#

A more simplified Tuned Radio Frequency receiver has its frequency selection circuitry at the front end of the receiver. The following amplifier stages are NOT tuned such as was the case with the Grebe 1925 system.

Many receivers have some form of band selection circuitry at the front end. However, here instead of selecting a band of frequencies, we select a narrow section, which is the frequency we actually desire.

A crystal set is a simple form of TRF receiver using the crystal, after the Tuned circuit, as a sensitive diode to produce audio from an Amplitude Modulated signal.

The following TRF receivers are really just amplified crystal sets.

The Mk 484 And Its Equivalents#

The MK484, TA7642 and other equivalents are normally packaged within a transistor style TO-92 case, so they look like transistors. However they contain 10 transistors.

Inductiveload / Public domain

Internal Circuitry#

To-92 Package internal circuitry (equivalent circuit) internals

M0OOZ - YooFab / CC BY-SA

A MK484 style radio#

Super attentive and highly driven persons with some knowledge of radio, will instantly recognise that this type of receiver is really similar to a crystal set, but with the signal amplified. The amplification first occurs here at radio frequencies, after which the audio signal is then amplified.

MK484

M0OOZ - YooFab / CC BY-SA

This is a simple radio with a very poor audio section and it is not optimised for voltage.

These chips are just a sequence of RF amplifiers as can be seen above in the diagram under the heading The Internal Circuitry Of The Above To-92 Package. However, although the internal circuit does have an kind of automatic gain control (AGC), the voltages presented to pin 2 and 3 need to be correct: by this I mean that the voltage affects the reception quality which in-turn is dependant upon the received signal strength. Thus it is that the AGC is only partly automatic and needs some manual tweaking of voltages for the circuit to be within the scope of signal strength that this circuit requires to function correctly.

Lacks some awesomeness#

The design lacks certain degrees of awesomeness but does have redeeming qualities. Additionally it is educational and it works!

Details of this offering#

  1. There is a horrible circuit around the volume control, showing two 470n caps before and after the pot.
  2. This is stupid and although it does not convert the volume control into a kind of volume/ tone control, only one of the caps are needed here.
  3. A reasonable thing, is the lowering of the 9V supply to a more reasonable ~2V using diodes to drop the voltage.
  4. Even this part of the circuit is not very precise, as we don't want restrict ourselves to a 9V supply.
  5. However, upon closer introspection, a variation in voltage from 9V to 13V produces only a few millivolts difference after the 3 diode voltage dropper.
  6. So it is acceptable to almost all modern omnipotent deities, all luminaries, savants, makers, enthusiasts, tinkerers and other persons of interest. Ergo: it may be be used with caution.

Curing The Voltage Problem#

We can cure the voltage problem with something like the following circuit.

Instead of a fixed voltage dropper, we now have a variable voltage setup, which has a number of operational advantages apart from our voltage supply.

A circuit that deals with the voltage problem

M0OOZ - YooFab / CC BY-SA

Setup procedure#

  • A) Set at 1.00 - 1.05V with no signal (for specific IC's)

  • B) Adjust at -0.6V for strong signal and -0.9V for weak signal.

  • ZN414Z, voltage supply 1.1-1.6 V, idle current 0.3-0.5 mA

  • MK484, voltage supply 1.1-1.8 V, idle current 0.3 mA typical

  • TA7642, voltage supply 1.2-1.6 V, idle current 0.2 mA

  • Batteries:
    • Alkaline battery, 1.6 V
    • NiMH battery, 1.4 V, 1.25 V typical
    • LIPO battery, 3.7 V typical

The trick to setting up the chip supply voltage, VA at point “A”, is to quickly evaluate how a particular chip (and circuit) behaves:

  1. Set the AGC voltage at point “B” to 0v using Pot-B. That’s like having “no signal”.

  2. Adjust the supply voltage at point “A” to a PRECISE value - like 1.05v - using Pot “A”.

  3. Now increase the AGC voltage to about 0.8 - 0.9v or so by adjusting Pot “B” - and start tuning around the band from 530 kHz to 1700 kHz.

  4. Take note of the apparent bandwidth and sensitivity as you tune across the band. They change as you go from 530 kHz to 1700 kHz.

  5. Now, reset the voltage at point “B” to 0 again, and adjust the supply voltage at point “A” to a new value using Pot-A. And repeat the above procedure - noting the apparent selectivity and sensitivity. It does not take long.

Once you tune-in a signal, you can increase or decrease the volume by adjusting Potentiometer “B”.

Note the 100uF capacitor is in the wrong place. It should be after the 10K potentiometer. Also this circuit has no regulation of voltage and is designed for specific 1.2 - 1.6 volt batteries.

We want a 9 - 14 Volt supply for allowing the use of a headphone amplifier or even a amplifier to power a small speaker.

Use A Mix Of These Circuits#

To get what we want here we should use a mixture of the best parts of these circuits.

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