[rfeenstra]

I saw that too, Ron.  I decided to leave both in the circuit.  Figured a little extra cathode bypass wouldn't hurt.

[Ron Ramirez]

Ok Rob. I think I'm going to try it with a single 0.1 uF cap in circuit. If it isn't to my liking it will be an easy matter to add a wire from the middle terminal of bakelite block (12) to the cathode of the RF amp tube.

I just remembered another question I had, and either you, Rob, or you, Ed L., can answer this -

Let's talk about compensating condensers (19) and (22). These are grounded on one end, and are constructed to be grounded on the end where the mounting bolt passes through the rivet. Did you replace these with ungrounded compensating condensers, or did you leave the stock compensators in place with one end grounded on each?

Remember, we must lift the grounded terminals of the two IF transformer secondaries or else AVC will not work.

[Ron Ramirez]

Since we're working from a modified version of the schematic from Service Bulletin 57-A, here is a corrected under-chassis drawing for 57-A, showing the correct connections for resistor (16).

[Image: https://philcoradio.com/images/phorum/70mod/70_019.jpg]

[rfeenstra]

Hmmm.  I left them to ground.  Missed that.  I wonder what would happen if I isolated the ground on those?  That sounds like an after church project!

[Ron Ramirez]

I think I remember what Ed L. did all those years ago to isolate those two trimmers -

Nylon hardware (screws, nuts, spacers) to hold those two trimmers in place, instead of the original steel hardware which not only holds the trimmers in place, but also grounds them.

Using nylon hardware is a quick and easy way to isolate the trimmers. Of course, wires must then be connected from the formerly grounded terminals of each trimmer to the respective terminals on each IF transformer.

Later today, if I am still feeling okay, I'll head to the nearby hardware store and (hopefully) get what I need to do this.

[jake wells]

Not to go off topic here Ron but are there any write ups for the model 50?

[rfeenstra]

That's what I was thinking, Ron.  I may use the metal hardware with nylon washers and put shrink over the screw threads, if that will fit through the hole.  I'll let you know how it turns out.

[Ron Ramirez]

Jake

Not that I am aware of. While an AVC conversion would be possible, I'm not sure that it would be worthwhile in a model 50.

[Ron Ramirez]

Progress:

Restuffed and reinstalled bakelite block (21).

Replaced resistor (16) and mica cap (15) with new parts.

Unsoldered the grounded secondary terminals of both IF transformers from their mounting brackets. I then, each IF transformer in turn, loosened the center nut and gently turned each transformer counterclockwise just a little bit, enough to ensure the formerly grounded terminals would not touch the mounting brackets.

Removed parts (23), (24), (25), (26), (27) and (28) as these parts are no longer needed in the modified circuitry.

Disconnected (33), 0.25 uF, for the time being. It will be restuffed and returned to the new circuit, connecting in the same manner as original but to two new, different value resistors.

I checked the resistance of resistor (11), which is supposed to be 51K. It measured 77K, so it will be replaced.

Oh, and this afternoon, I purchased nylon screws, nuts and spacers to isolate trimmer capacitors (19) and (22). Those will be installed later to replace the current steel screws, lockwashers and nuts, and brass spacers.

No photos for now. It's late and tomorrow starts another workweek.

[rfeenstra]

I isolated the grounds on the 2 compensating capacitors using nylon washers and a little heat shrink over the threads on the screw.  I re-aligned the radio and spent some more time tweaking the tuning cap bendable plates to try and maximize sensitivity and tracking across the band.  Tracking is now almost perfect all the way across the band.  Sensitivity is maximized on most of the band.  I loose a little on the low end but the only station I listen to down there at 590 is fairly strong.  650, WSM Nashville (playing blue grass tonight), comes in wonderfully (with some fading), as does zoomer radio 740. The AVC works great, reducing the effects of the fading. And, no more blaring stations!

Interesting comparison:  I have an AM frequency generator/audio modulator that puts out a very weak signal.  It can be picked up only a couple of feet away from the output conductor.  It's purpose is to drive an RF amp, which I don't have so i wrap the "antenna" around the antenna post of the radio.  it is capable of clean modulation from 20 to 15K Hz.  I have the 570 sitting right next to my model 96, feeding both radios with the signal.  The difference in audio quality is dramatic.  Granted, the 96 has a push pull output and a larger speaker, but it also produces a much cleaner top end, producing a more balanced sound.  The 570 seems muted in comparison.  ( I removed the added cap labeled "see text" in Ron's drawing.)  I was reading an article that was saying that the old TRF radios could produce a better sound than the typical, non HiFi superhet.  The superhet certainly has far better selectivity, but the TRF holds its own in the tone quality department.   At least the 96 does.  I may play with the 570 and broaden out the IF curves to see what it does to the sound at the sacrifice of selectivity.  It kind of sounds like fun!

Ron, keep plugging away as much as you can.  I'd like to know your results of this "experiment".

Rob

[Radioroslyn]

Rob, This is a little off topic but I have a couple of 370's also and noticed in the stock configuration that do to the placement in the cabinet of the spkr it's very near the power transformer and induces 60cy hum into the field coil. If you operate the set with the spkr out of the cabinet the hum is gone.
Have you noticed this?

Obtw Great series and will probably be doing to a 70B and a 370 after I gather up so bits and pieces. Both need the initial service and the B needs a front panel.
Tnx Guy's for doping all this stuff out.

[rfeenstra]

No, on the 570 the speaker is quite a way below the radio.  No hum.

[Ron Ramirez]

Terry

I know you were asking Rob, but...

It's been a while since I owned a 370 Lazyboy, but I do not recall hum being an issue in the one I owned.

[Ron Ramirez]

So in our last installment, as you recall...I accomplished everything listed in post #39 above.

Here are a couple photos.

[Image: https://philcoradio.com/images/phorum/70mod/70_020.jpg]

The under-chassis view after I had done everything listed in post #39. All components having to do with the old power detector circuit have been removed. Also, an old 500K potentiometer from a long-dead 96 or 112 has been installed in place of the former dual volume control.

You'll notice that the added 270 ohm resistor for the cathodes of most of the 35 tubes and the 27 tube (seen in upper right of the photo above) is a 5 watt unit. I know this is overkill, but I had several so I used one instead of buying a 1 watter just for this project.

[Image: https://philcoradio.com/images/phorum/70mod/70_021.jpg]

A close-up of the two IF transformers, showing how I unsoldered each of the lower secondary terminals from their respective mounting brackets, and bent them upward. You will also note that each IF transformer has been physically rotated counterclockwise just enough to keep the lower secondary terminals from touching the mounting brackets. As you recall, this was accomplished by loosening the center bolt where the transformer is mounted to the mounting bracket, carefully rotating the transformer itself, then retightening the bolt.

In my next post I'll talk a little about what I did to this 70 today (not much actually), and then I'll have another question for Ed.

[Ron Ramirez]

As I mentioned, I did not get a lot of work done on the 70 today. I did remove the tone control and bakelite block (31), along with the resistors connected to it. Don't worry, that bakelite block will be reinstalled once it has been gutted and restuffed with a new .01 uF capacitor. Ditto the tone control - it, too, will be gutted, restuffed, and reinstalled.

Goals for tomorrow - Restuff and reinstall those two parts mentioned above, and then start converting the old power detector tube to become a first audio amplifier. Also, get everything hooked up to the audio output tube.

Also to be accomplished if I have time - Install the extra resistors to the "B-C" resistor, and run wire leads from the "new" volume control to where they need to go in the circuitry. And lest I forget, restuff the AC bypass bakelite block with new safety caps, and attach a new cloth-covered power cord.

I intend to run the new AVC line last, after everything else is done.

Ed L., here's my question for you:

Referring to post #1, page 1, this thread or post #9, page 1, this thread, your schematic shows a 100 pF and a 220 pF capacitor on either side of a 100K resistor, coming off the bottom of the 2nd IF transformer (20). Yet the late 70 schematic, as well as most all other Philco schematics with this AVC circuit, show two 110 pF (100 pF) caps being used. Is there a particular reason why you chose 220 pF?

I think I am going to add a bakelite block with the two 100 pF caps inside (or one 100 pF and one 220 pF), to make a convenient tie point for the 100K resistor and the beginning of the AVC line. If there is room (and there should be easily), I'll add yet another bakelite block with a .01 uF capacitor inside to run between the center wiper of the volume control and the control grid of the first audio amplifier tube.