[Ron Ramirez]

Discussion

With this project at a standstill while I wait for delivery of some needed 0.1 uF capacitors, now is a good time to stop, back up, and talk about what we're doing and where we're going with this modification.

For clarity, let's compare an original, unmodified 70 schematic with the modified schematic.

Unmodified 70:

[Image: https://philcoradio.com/images/phorum/70mod/70.jpg]

Modified 70:

[Image: https://philcoradio.com/images/phorum/70mod/70_mod.jpg]

Let's go from left to right on each schematic and talk about what is being changed.

The first, glaring omission is the dual volume control (1). The requires a reworking of how the RF, IF, and oscillator tubes will receive their cathode bias. Ed has solved this problem by tying the RF and IF cathodes together with the oscillator tube cathode, and connecting all three to ground through an added 270 ohm, 1 watt resistor. Notice also how the mixer tube receives its cathode bias from this circuit also, through resistor (14), 5000 ohms. The former connection from the 27 (oscillator) cathode to point G is removed.

Antenna coil (2) must be modified to add another terminal, so we can feed AVC through the coil's secondary to control the amplification of the RF amplifier, which is changed from a 24 to a 35/51.

Dual bakelite capacitor (5) remains in circuit, performing the same jobs as in the original circuit.

The oscillator circuit is unchanged, save for removal of the connection from the oscillator tube cathode to point G as previously mentioned.

The IF amplifier tube is changed from a 24 to a 35/51. 35/51 tubes are remote cutoff tetrodes while 24s are sharp cutoff tetrodes. 35/51 tubes are more well suited to AVC action and will perform better with AVC than will 24 tubes.

The biggest changes occur at the former power detector tube, a 24. It, too, will be changed to a 35/51 and will become an audio amplifier (only). All of the parts which were used for detection - RF choke (27) and two mica capacitors (25) and (26) are removed. Bakelite block (28) is removed, which contains a .09 uF capacitor and a 250 ohm resistor. In addition, the connection from part (28) to point E is removed. We will also remove resistor (24) and capacitor (23). (23) is a 0.5 uF capacitor in a rectangular metal case. In place of these parts, we need to add a 100 pF capacitor, a 220 pF capacitor, and a 100K (0.1 meg) resistor as shown in the modified schematic. A new AVC line is also added, with 2.2 meg resistors and .05 uF capacitors as shown in the modified schematic.

For the new second detector, we make a concession to somewhat more recent technology with the addition of a 1N34A germanium diode between the top of the second IF transformer secondary and ground.

The bottom secondary terminal of the first IF transformer is factory soldered to the (grounded) mounting tab of this transformer. We will need to unsolder this terminal and then carefully bend it up and away from the mounting tab, to apply AVC to it. We will repeat this process for the bottom secondary terminal of the second IF transformer to be able to connect the new 100K resistor and 100 pF capacitor.

A new 500K potentiometer is added in place of the old dual volume control, with the wiper arm connecting to the grid of the 35/51 audio amplifier through a new .01 uF capacitor.

Points F and G now connect to the screen grid and cathode, respectively, of the new 35/51 audio amplifier. We will add an 850 ohm, 5 watt resistor between points F and G. A convenient place to do this is between the screen grid and cathode terminals of the audio amplifier tube...or between points F and G of the "B-C" resistor (37).

Resistor (29) is changed from 240K to 68K, and resistor (30) is changed from 99K to 47K. We'll change resistor (32) from 240K to 470K. And we will add a .01 uF capacitor from the plate of the 47 tube to ground.

In the power supply, we will add a 5600 ohm, 5 watt resistor between points E and F (across terminals 2 and 3 of B-C resistor (37) ); and a 47 ohm, 1 watt resistor between points G and ground (across terminals 4 and 5 of the B-C resistor (37) ).

Everything else remains the same in the power supply.

Only early production 70 sets have part (40), which was meant to "tune out" any residual AC hum. My 70 chassis does not have part (40) so this is one less part to worry about.

Finally, let's mention the capacitor shown in red in the modified schematic (now in its second revision). Once you have completed the modifications to your 70 chassis, if you find high frequency audio to be too shrill to your liking, add a small mica or ceramic capacitor of a value between 220 and 500 pF from the plate of the 35/51 audio amplifier (again, shown in red) to ground. This part is optional. If the radio sounds OK to you after modifications, you may omit this part.

Ed, can you confirm that all resistors are 1/2 watt with the exception of those labeled with higher wattage on the modified schematic, and the B-C resistor (37)?