[jake wells]

I would try it un shielded the heater wires in my rca are unshielded.

[Phlogiston]

Well, that would be a good spot to inject noise into a very sensitive receiver. And they probably did it for a reason - given, that reason might have been "we have these cables in stock". But I would replace it just because it was built that way.

[Ron Ramirez]

OK Russ, thanks. Of course I don't have any suitable shielded wire. So I have ordered some, after checking that 22 gauge wire would safely handle the current of three tubes plus four LED dial lamps (it will). The wire I ordered, from Mouser, has PTFE insulation so it is more resistant to soldering iron heat. While I was at it, I also ordered enough mica capacitors to replace all 473 micas in the Magic Brain.

(No, it doesn't use quite that many mica caps...but it seems like it)

I suppose as long as I am doing this right, I should perform the 6SG7 mod as well? This mod replaces the 6K7 RF amp tube with a 6SG7. The 6SG7 will ensure that band "D" (23 to 60 mc) will work across the entire band, not just the low end as with most 6K7 tubes. It requires a new octal socket with all eight pins - the 6K7 socket is missing the contact for pin 6.

But then again, what can be received between 23 and 60 mc utilizing amplitude modulation these days? Hmmm. Upon second thought, it seems like a lot of work for little or no return. I don't think I will do that.

I'm not going to work on the Magic Brain tonight. Maybe I will tomorrow night.

[Phlogiston]

I thought that the problem with those was the osc. tube. The last one I played with was good to about 27mhz and only if you tuned it from low to high (pulling the osc along with you). If you tuned from 60mhz down the osc would come up further down the band.

[Ron Ramirez]

It was John Goller who advocated the 6SG7 mod on ARF.

https://www.antiqueradios.com/forums/vie...6&t=212711

https://antiqueradios.com/forums/viewtopic.php?t=166151

I suppose if I really want to do this, that I can do so later on. Right now I just want to have the set playing by Christmas if at all possible - that way, this can be my Christmas 2019 electronic restoration.

[Ron Ramirez]

One VERY expensive order of mica capacitors later...I began digging into the Magic Brain in earnest.

   

I've replaced about half of the micas at this point.

And no, I did not bother to restuff the four .01 uF paper capacitors. The underside of the Magic Brain is so jam-packed...plus when this is finished, it will be totally enclosed and no one will see under here unless they remove the bottom cover.

I also removed the interconnect cable which terminates in an octal plug in preparation to replace the individual wires with new ones:

   

I don't feel very comfortable running the heater supply through the 22 gauge shielded wire that came with my Mouser order. One side of the heater string runs through the center conductor...the other through the shielding.

I do have some 20 gauge shielded wire coming from a different supplier, but it won't be here until Saturday at the earliest. But still...both sides of the heater supply running through a shielded wire. Hmmm...

Oh, by the way, folks: Even though I bought a couple 6SG7 tubes, I've made a firm decision not to do the mod. I don't like the idea of running a really long control grid lead from topside down under the chassis. That, coupled with little to nothing to listen to on the very high shortwave band (23 to 60 mc)...I will stick with the factory 6K7 setup.

Well, that could be a good use of the 22 gauge shielded wire, I suppose...but no.

[rfeenstra]

"nothing to listen to on the very high shortwave band (23 to 60 mc)"
Here's a little FYI as I just learned a bit about this frequency band.

There is some interesting info on the "Apex" band if you google it. These high frequencies were being experimented with for HiFi. The FCC allowed a much wider bandwidth for better audio frequency response and less static. The McMurdo Silver Masterpiece V and VI went up to 70 mhz to take advantage of this. Other radios began to include these frequencies as well. At one time there were upwards of 50 stations broadcasting in this area. Transmission distance was thought to be line of sight like today's FM and so transmitting antennas were placed on high points, hence the name APEX band, and most stations were in high population areas. It only lasted a few years as FM supplanted the HiFi am stations. Most of the APEX band stations converted to FM fairly quickly. As Ron said, there's not much there now!

http://www.theradiohistorian.org/Apex/Apex1.htm

[EdHolland]

That is interesting Russ - something I didn't know. I have always wondered why some notable sets had such high frequency coverage, above the conventional range for short wave. Thanks for the link

Ron - you are doing a beautiful job as always - I don't envy you all those cap changes and the rubber wire menace, but the end result will be well worth it.

Ed

[Ron Ramirez]

Thanks Ed.

Progress:

   

   

There are a few mica capacitors which are soldered to the trimmer capacitors, such as this:

   

I decided not to replace these, since those metal rings you see around the trimmers are made of solder, and I didn't want to mess them up.

Topside progress:

   

Still to do: One more mica cap underneath, one more mica cap and one more resistor on the top, plus one more grid cap lead to replace on top. Also polish and reinstall the shields for the open coils you see above. And them make and install a replacement wiring harness and the Magic Brain should then be complete.

If all goes well, I hope to have the Magic Brain back in the main chassis sometime tomorrow - and also hope to apply power to it for the first time tomorrow as well.

[Ron Ramirez]

Progress continues, slowly but surely.

   

I'm basically finished with the Magic Brain save for rebuilding the interconnect cable and reattaching the dial scales. I really don't know why I bothered polishing the two coil covers which I had to remove to replace rubber-covered wires, since these coil covers will not be seen with the Magic Brain cover back in place when the unit is reattached to the main chassis. Just habit, I guess.

   

Here, I have removed the covering from the interconnect cable. This will help me to determine how long each individual wire was, so I can cut new ones to the proper length.

[Ron Ramirez]

New interconnect cable.

   

   

The Magic Brain is now ready for reinstallation into the main chassis.

[Ron Ramirez]

Back together again.

   

   

I have a problem: I'm having a very hard time getting the mechanical linkages of the bandswitch assembly to work properly. Because of this, I have yet to try the radio out. Hopefully tomorrow I will get this issue resolved.

Somehow I managed to make the interconnect cable slightly longer than the original.

   

I guess that's better than having it too short.

More tomorrow!

Edit: I forgot to mention that the 20 gauge shielded wire arrived today. This is what I used as part of the new interconnect cable.

[KCMike]

Looking good Ron. There's a lot of work in your restoration.

[Ron Ramirez]

Thanks Mike, but apparently not enough work...

Good news and bad news.

The good news: The radio is up and running and performing very well on AM. It seems to be working on LW as well, but there isn't anything to listen to on the LW band in my area.

Also, I lubricated the mechanical band switch linkages and it is changing bands much better now.

The bad news: It is not receiving any shortwave. At all.

I certainly hope that I didn't mess up replacing any of the mica capacitors...I dread removing the Magic Brain again but I may have to. I'll try injecting signals first, and go from there.

Forgot to mention: As I was giving the radio a trial run, it suddenly stopped playing (AM). I noticed the 5Y3GT tube filament had gone dark (someone had stuck a 5Y3GT in place of the original metal 5Z4). I replaced it with another 5Y3GT and the set came back to life.

[EdHolland]

That is a lot of work Ron. I hope troubleshooting the SW reception does not prove too difficult.