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Tech Section: Neutralization of Philco Neutrodyne-Plus Receivers

Applies to Models: 511 Series, 86, 87

The term "Neutrodyne" refers to a particular type of Tuned Radio Frequency (TRF) radio receiver, in which the inter-electrode capacitance of the triode RF tubes are neutralized with neutralizing condensers connected between the grid and plate of each RF amplifier tube.

The circuit, developed by Hazeltine Laboratories and used by Philco in its "Neutrodyne-Plus" models (which includes the 511 series and Models 86 and 87), takes particular care in its alignment because of the neutralizing condensers, which are adjustable.

To properly align a Neutrodyne receiver, not only must the set be aligned for best performance, but also the set must be neutralized.

Use a Neutralizing Adapter

Usually in vintage service literature, the service man is directed to use a dummy tube in place of the RF amplifier tube in the particular stage being neutralized. The dummy tube is defined as a good tube which has had one of its filament pins cut off.

However, there is a better way to neutralize a set, and that is by using the tube that will actually be in the circuit. In this manner, the inter-electrode capacitance of the tube being used is neutralized, which allows the set to operate at its peak efficiency. Yet, the tube must not be operating (filament off) while it is being neutralized, else the neutralization will be ineffective.

Some people advocate using a section of heat shrink tubing to cover one filament pin of the tube being used, neutralize the stage the tube is in, remove the tube, remove the tubing, reinstall the tube, repeat for the next stage.

But here is another method - use a special adapter for neutralizing a Neutrodyne-Plus Philco receiver.

In order to construct this adapter, you need a four-prong tube plug (a base of an old, broken tube will be fine) with a four-prong socket mounted in the top of the tube base.

Schematic of a Neutralizing Adapter

Figure 1. Schematic of a neutralizing adapter for Type 26 tubes.

A wire should be run from each of the tube socket contacts to the corresponding pin on the tube base - except that you leave one filament pin disconnected. Once you build the adapter (shown in schematic form in Figure 1 above), you will be able to use the actual tube being used in the circuit to neutralize each particular stage.

Notice there is no wire between one of the filament contacts of the socket and one of the filament pins of the plug. This gives the same effect as cutting off one of the filament pins.

Using the actual tube that will be used in the circuit of the stage being neutralized is desirable for two reasons:

  1. There is a finite supply of Type 26 tubes, therefore, we do not recommend cutting the pins off good tubes; and
  2. The neutralization procedure will be more precise if you are neutralizing the stage with the actual tube that will be in the circuit.

Alignment & Neutralization Procedure for Models 86, 87 & 511 Series

Remember - Align the RF stages of the set first, then perform the neutralization!

After you neutralize the set, touch-up the RF alignment again for best results.

The procedure must be performed as follows:

Set your signal generator for a weak 1300 kc signal. Connect the high end of the signal generator to the radio's antenna (ANT) post through a 200 pF capacitor. Connect the low side of the generator output directly to the set's ground (GND) post.

Tune the radio to 130 on the dial (carefully tune in the 1300 kc signal on 511 sets, as they have a 0-100 scale). Turn the set's volume control to maximum. Use an output indicator across the speaker output terminals of the 511 or across the voice coil of the 86 or 87's speaker.

Align the 3rd RF trimmer (number 3 in Figure 2 below), the 2nd RF (number 2) and finally, the 1st RF trimmer (number 1) for maximum output.

Now it's time to neutralize the radio. If you leave the good tube in the radio's socket and attempt to neutralize the stage, you will achieve very poor results; in fact, you will be doing more harm than good.

Turn the radio off.

Location of alignment and neutralizing trimmers, Models 511, 86 and 87

Figure 2. Location of alignment and neutralizing trimmers, Models 511, 86 and 87.

Following Figure 2 above, pull the 3rd RF tube (a Type 26) out of the set and plug the tube into your new neutralization adapter. Now plug tube and adapter into the radio.

Please note: The six trimmer condensers are accessed from the bottom of the chassis in the Model 511 series, and from the top of the chassis in Models 86 and 87. For the 511 series, there are holes in the metal bottom shield to provide access to the trimmers without having to remove the bottom shield.

Set your signal generator for a strong 1300 kc signal, and turn the radio back on.

Adjust the 3rd neutralization condenser (number 6 in Figure 2) for minimum signal.

Turn the set off. Remove the 3rd RF tube and the adapter. Remove the tube from the adapter, reinserting the tube (only) back into its socket. Now remove the 2nd RF tube, plug this one into the adapter, and install the adapter (with 2nd RF tube attached) into the 2nd RF tube socket.

Repeat the above procedure for the 2nd stage, adjusting trimmer number 5, and finally the 1st RF stage, adjusting trimmer number 4.

Once neutralization is complete, check the set for proper operation. If the neutralizing condensers had to be turned (adjusted) more than a small amount, perform the RF alignment again (you do not need to neutralize it a second time).

If you experience oscillation at any point on the dial after carefully aligning and neutralizing the radio:

Philco purposely misaligned its 3rd RF trimmer condenser to compensate for oscillation in some Neutrodyne-Plus receivers. Simply turn the 3rd RF trimmer (number 3 in Figure 2 above) clockwise about 1/4 turn. This misaligns the set somewhat, but cures the oscillation. And, yes, this was a factory "repair".