[Ron Ramirez]

I have a pretty clean Philco 70 chassis, to which I plan to add Ed Locker's AVC modification.

This has been previously discussed starting here.

The purpose of this mod is to not only add automatic volume control (AVC) to a radio which was designed without AVC, but also to eliminate the distortion which is inevitable in radios which use the "plate detector" circuit.

With this mod, the plate detector will become a tetrode audio amplifier, and a germanium diode will be added to perform the second detector function.

I've taken the liberty of redrawing Ed's schematic below (NOTE: Two errors were found in the schematic previously posted here, and have now been corrected below):

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

Hint: Right-click the image above and click View Image to see a full resolution version of this schematic. Then right-click that image and click Save Image As to save it to your computer.

I'm looking forward to performing this mod. Both Ed Locker and Rob Feenstra report excellent results from doing this mod to their radios.

[Ron Ramirez]

Here is the candidate for modification:

[Image: https://philcoradio.com/images/phorum/70mod/70_001.jpg]

[Image: https://philcoradio.com/images/phorum/70mod/70_002.jpg]

This is a very nice, unmolested, unusually clean chassis. It was purchased from an ARF member at Kutztown two or three years ago .

Notice that it has Sprague electrolytic cans instead of copper Mershons. It is apparent that Philco used Sprague electrolytics during some production runs of 70, 90 and late 112 sets, although the majority of Philcos made during this era have Mershons.

Under the chassis:

[Image: https://philcoradio.com/images/phorum/70mod/70_004.jpg]

I'll be removing the old dual volume control and replacing it with a 500K potentiometer. That added-on paper electrolytic will also have to go.

[Ron Ramirez]

Fun(?) with Restuffing Electrolytics

A few years ago, I attempted to open up a Mershon electrolytic following directions I had read online. Only, the top cap would not come off. That project ended in disaster as the top of the Mershon ended up crushed.

Until now, I have not made another attempt to to open up a Mershon or early Sprague.

But reading of how some Phorum members had successfully restuffed these, I decided to try it again.

I ran into some issues with opening up the first electrolytic can that I removed from the chassis. You can read about those woes here. Since these electrolytics are part of this 70, I'm presenting a condensed version in this thread.

Attempting to wiggle the bakelite base from the electrolytic can resulted in a breakage:

[Image: https://philcoradio.com/images/phorum/70mod/70_005.jpg]

Then, trying to get the rest of the bakelite base removed resulted in more breakage as seen on the left in the photo below.

[Image: https://philcoradio.com/images/phorum/70mod/70_006.jpg]

I set the electrolytic with the broken base aside while I concentrated on the second electrolytic. I discovered that, by carefully cutting the old rubber seal away from the perimeter of the bakelite base, that the base could be removed fairly easily as you can see at right in the photo above.

[Ron Ramirez]

I restuffed the second electrolytic and put it back together. However, my attempts at rolling the lip back over the base did not turn out very well. It looks very sloppy. And in addition, the rolling resulted in a gap between the bottom of the can and the negative contact above the insulator on the 70 chassis. So I ended up soldering not one, but two copper rings made from 14 gauge electrical wire to make up the difference.

[Image: https://philcoradio.com/images/phorum/70mod/70_015.jpg]

I then turned my attention back to the first electrolytic.

I managed to glue the broken Bakelite base back together...more or less.

Then I proceeded to restuff the can with a new 8.2 uF, 630V Solen film capacitor.

[Image: https://philcoradio.com/images/phorum/70mod/70_012.jpg]

[Image: https://philcoradio.com/images/phorum/70mod/70_013.jpg]

This time, I followed Bob's (klondike98) suggestion to glue the base into the can with epoxy.

The base kept trying to spring out, so I have put one of those cheap Harbor Freight clamps to use:

[Image: https://philcoradio.com/images/phorum/70mod/70_014.jpg]

When the glue sets, I'll remove the clamp. I will not reinstall this electrolytic until the glue is completely dry.

Oh, by the way, I soldered a wire to the inside of the copper can, and soldered the other end of that wire to the top end of the Solen cap. The bottom end of the Solen is soldered to the 1/4-20 screw which passes through the bakelite base and forms the positive terminal.

The Sprague cans shine up nicely with Mother's Mag & Aluminum Polish.

[Ron Ramirez]

Turning my attention back to under the chassis...

We have to modify the antenna coil in order to add AVC to this set, removing one wire lead of the coil's secondary winding from the ground terminal, adding a terminal to the coil form, and soldering this lead to the new terminal.

Please note that one lead of the antenna coil primary is connected on top of one lead of the antenna coil's secondary winding. This means that you must carefully undersolder the ground terminal, (very carefully) unwind the primary lead from the ground terminal, and then (also very carefully) unwind the secondary lead. And once that is done, wrap the primary lead back around the ground terminal and solder it back in place, leaving the secondary wire dangling for the moment.

[Image: https://philcoradio.com/images/phorum/70mod/70_007.jpg]

I found a solder terminal in my pile of junk. You can also sacrifice a terminal strip by using its ground lug as a terminal.

[Image: https://philcoradio.com/images/phorum/70mod/70_008.jpg]

Remove about 1/3 of a turn of the secondary winding, poke the lead through the hole in the form above the new terminal, and solder the lead to your new terminal.

[Image: https://philcoradio.com/images/phorum/70mod/70_009.jpg]

Now the primary and secondary windings are isolated from one another, and AVC may be applied through the secondary of the coil.

[Ron Ramirez]

The Tuning Condenser

In my years of working on Philco sets, I have found that the screws which hold the stators in place on Model 20, 70 and 90 tuning condensers are almost always loose.

So as part of a restoration, I remove the tuning condenser and tighten these screws.

[Image: https://philcoradio.com/images/phorum/70mod/70_010.jpg]
The tuning condenser, removed and ready to be worked on.

[Image: https://philcoradio.com/images/phorum/70mod/70_011.jpg]
Tightening one of the screws.

On model 20 and 70 tuning condensers, there are six screws on each side of the tuning condenser. All must be tightened for proper operation.

Once this is done, I like to replace all of the wires with new ones. Afterwards, using new rubber mounts, I reattach the tuning condenser to the chassis.

[Ron Ramirez]

So I was going to start replacing capacitors inside bakelite blocks...but I soon discovered that I am completely out of 0.1 uF caps!

Therefore, I have placed an order with Sal's Capacitor Corner.

This project is now on hold until those arrive.

[Ed Locker]

Man! If it can, it will! Just when the story was getting good!

Good job on the redraw of my messy schematic. Thanks.

Ed

[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)?

[Ed Locker]

From what I remember (it's been 15 years) I used a 1 watter for the common cathodes resistor (270 ohms) and a 1 watter in parallel with the 70 ohm resistor from point "G" to ground on part 37 (47 ohms). This is all from memory. I could be wrong. 

 If needed, I will pull the chassis on one to verify. LMK

Ed

[rfeenstra]

Question Ron: Late model 70's and Ed's schematic tie the center tap of the 2 filament windings (4 and 7) to ground rather than 7 and 12 being connected together as in the early 70's.  I don't know if there is any noticeable difference in sound between the 2 ways of doing it.  What are your thoughts?  On my mod, I connected 4 and 7 to ground as per Ed's schematic.

I also added the cap on the plate of the 35.  It does make a very small difference (I actually increased it to 500 pf).

[Ron Ramirez]

Oops! Another oversight on my part regarding the filament winding center taps. Third revision of the schematic is forthcoming. Thanks for catching it, Rob. I'll defer to Ed's work on this.

And Ed, thanks for the clarity on those two resistors - I'll also add that info to the third revision of the schematic.

[Ed Locker]

You know, I didn't rewire the filaments......I don't understand why the schematics are different.

I just removed the 27 tube from one of my revised 70's and measured the resistance from the filament to ground.....my fluke read 243 ohms.

Ed

[Ron Ramirez]

Ok Ed. That means your B filament line's center tap is not grounded, but is connected to B- as per original.

I don't see where it would make a difference whether the B filament line is grounded, or connected to B-.

Schematic revision #4 on the way...

[Ron Ramirez]

Ok. Fourth revised 70 mod schematic posted in posts #1 and #9 above. We can now assume all resistors are 1/2 watt except as noted in the modified schematic, and except for B-C resistor (37).

Since Ed's 70 has the filament center taps connected as per original factory wiring instead of what he shows in his drawing, I have left the filament taps alone. I may, or may not, experiment with this once I get my 70 going to see if changing the filament center taps makes any audible difference.