[Ed Locker]

Ron,
 I knew that either a 100pf or 220pf cap. would remove RF but to my ear the 220pf gave better fidelity response ( knocked down the highs ). The same with adding a 220pf to the plate of the audio amp. Your results may differ.

 Try a 100pf. It might sound OK to you. LMK

Ed

[Ron Ramirez]

Thanks Ed. I'm going to go with 100 pF.

Well, I didn't get as much accomplished on the 70 as I had hoped to, but what I did accomplish should make much of the remaining work go a bit easier.

I restuffed and reinstalled tone control (48) and bakelite block (31). As for the additional .01 uF capacitor which connects from the 47 plate direct to ground, I mounted it in the tone control housing. The grounded end is soldered to the metal frame of the tone control. The .015 uF section is switched as indicated on the modified scehmatic.

I came up with a bakelite block to hold the two 100 pF capacitors which are located in between the added 100K resistor, and installed it as well as the 100K resistor. I decided to remove the single bakelite block (21) and replaced it with a dual bakelite block. The added .047 uF capacitor inside that dual block is an AVC filter at the first IF transformer.

The wires from points F and G have been shortened and re-routed to what will be the 35/51 1st audio amplifier tube. I also added the additional power resistors which are connected to the "B-C" resistor (37). I only installed one at the B-C resistor as you will see in the photo below. The other additional resistor I added from the cathode of the 1st audio amplifier to ground.

IF secondary trimmer condensers (19) and (22) have now been isolated from ground, using the nylon hardware I bought last weekend for this purpose. Wires have been attached from the formerly grounded ends of each trimmer into the circuitry as indicated on the modified schematic.

An under-chassis photo showing this weekend's progress:

[Image: https://philcoradio.com/images/phorum/70mod/70_022.jpg]

and a schematic, the yellow highlights indicating what has been done:

[Image: https://philcoradio.com/images/phorum/70mod/70_023.jpg]

We're getting there...slowly but surely.

Until next time!

[Ron Ramirez]

Great progress was made on this project today.

The only work left to do is to add the 1N34A diode, restuff the twin AC line bypass bakelite block, add a power cord and a set of tubes - and this chassis will be ready to try out!

An under-chassis picture:

[Image: https://philcoradio.com/images/phorum/70mod/70_024.jpg]

and the schematic with yellow highlights indicating what has been completed:

[Image: https://philcoradio.com/images/phorum/70mod/70_025.jpg]

It's getting close...more to come tomorrow, so stay tuned.

[Ron Ramirez]

It doesn't work

I applied the finishing touches and tried it out.

No joy.

As you will hear in the video below, all it does is produce a noise that starts out as a rapid popping and slowly builds into a buzz.

Ed and Rob, I would like to see photos of the underside of your chassis. I'm wondering if my problem is a parts placement issue. I do have AVC capacitors sharing bakelite blocks with plate/screen bypass caps, and that was probably a mistake.

[Video: https://youtu.be/u31RcTUGY6s]

There's no feeling quite like feeling that you've just wasted the last three weekends...

[rfeenstra]

Interesting!  You know everything after the volume control works.  layout is nothing special on mine.  In fact, since I wanted the ability to turn it back into a normal 70, the AVC line is just kind of supported on the the end of the resistors soldered to the coils.  Not real neat and hig quality construction!  I was thinking of taking the chassis out this afternoon to test the frequency response of the audio portion.  I'll take a couple of pics for you.  As I told you yesterday, I left all of the old components in there so there is stuff just hanging unconnected.  I was just playing mine, and except for the roll off of the extreme highs, it sounds great.

You WILL solve this!

Rob

[Ed Locker]

Ron   

 Ryan is stopping over later today to take some pictures of the modded 70 chassis for you.

 Will be sending them your way soon.

Ed

[Ron Ramirez]

Thanks Rob and Ed. And Rob, it was great seeing you again yesterday.

Once I see the parts placement on your sets, I'll decide whether I need to move things around on mine or do further troubleshooting.

In 45 years of messing with old radios, I've never had one motorboat like this one.

[rfeenstra]

Here you go.  Notice that I suspend wire from the end of components!  This was a possibly temporary experiment, so I took the easy route in wiring, though I don't think I'll go back to an early 70 design.  I like the AVC!  In fact, I may now remove unused components and just save them with the radio for future owners.  I placed the resistor between the F and G terminals right on the tubular resistor.  The wires that used to go to the volume control now go to the screen and cathode of the 1st AF.

                   

[Ed Locker]

   
Here's a picture of my chassis. I have a couple more if you need them.

[Steve Davis]

Ron, don't get discouraged. With a major mod like this, you are bound to have a bug or two. The problem seems too be in the front end or the oscillator as the tuning has considerable effect. I would start pulling tubes one at a time to try to find the area of the problem. 

Steve

[Ron Ramirez]

In looking over the photos posted by Rob and Ed, one thing I see is that the AVC filter caps are placed away from screen/plate bycass caps - at least in Rob's conversion. Ed, I can't really tell where you placed your AVC filter caps (.047 uF)?

I don't know when I will return to this project. No time to work on radios weeknights, was gone all day yesterday, lots of other things to do today. We shall see. I might find a little time to mess with it today, and I might not.

And Steve - thank you. Pulling tubes sounds like an excellent starting point for troubleshooting, although I am still thinking that I am going to have to move my .047 uF AVC filter caps closer to their respective coils and away from bypass caps.

[Ed Locker]

Ron,

 On the top of the photo, there is a 2.2meg resistor connected to the hot side of the volume control to a 1 leg terminal strip. If you look closely, there is a thin black wire that connects from the terminal strip to one section of a dual bakelite block cap. The other section of the cap. has a 5.6K 1W connected to it. That's where the .047 cap. is.

 I hope that helps.

 Also, does your chassis put-put with the 27 oscillator tube removed?

Ed

[Ed Locker]

I think resistor 14 had changed value and I used a 5.6K 1W to replace it.

Again, it's been a while since I did this and I'm going by my fuzzy memory .

Ed

[Ron Ramirez]

Ed

No, the radio does not make the noise with the 27 removed.

Now we might be getting somewhere. I switched 27 tubes around and on the 4th one, it kept putt-putting but it began to receive the local Spanish station which broadcasts at 990 kc. Only it is receiving it at 1350 on the dial - with the tube shield off. As soon as I start to put the tube shield back in place, before it even touches the chassis, reception disappears.

Just for fun, I twisted the high frequency oscillator trimmer around a bit, with the tube shield off - and the frequency did not change.

Obviously there is an issue in the oscillator circuit.

I had not rewound the oscillator coil (or the antenna coil, or the RF coil) because it tested good. Perhaps I should pull it, remove the winding that typically goes bad, bake it, rewind it, and try again. I think I will also replace mica cap (9), 410 pF, because heating it up on both sides to remove/replace wires may have damaged it. That has happened to me before.

[Ed Locker]

Yes, replace that cap. 

Notice it was replaced on mine.

 I had that cap. go intermittent on a 71 once, I really pulled my hair out on that one. Since then, whenever that cap. gets disturbed while servicing the chassis, it gets replaced.

Ed