EH Scott Radio Enthusiasts

The Fine Things are Always Hand Made

Hi Kent:  I am working on a Shield Grid 9 now that is in very good shape. I have dismounted the SGA (shield grid assembly) because of low filament voltage on the three 22s - max I can get is 2 volts with 5 volts on the entry bus.  Taking that SGA off and opening it was much more of a chore than necessary because the designer apparently didn't think it would ever be required.  The paperwork discusses the short wave ability of the SG-9 and the need to change the coil in the 630 Antenna Coupler copper box (+ the .0008 added capacitor).  Only one of the 4 copper housings has 2 bolts mounting it to the bakelite.  Do the covers of the other three copper housings pull off? or how do they come off?  Is desoldering the lugs from under the chassis to free the assembly required?  The bakelite is so fragile that I don't want to apply any damaging pressure.  Any ideas/help greatly appreciated! 

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Comment by Kent King on March 13, 2017 at 10:27pm

Steve - The unit that is bolted down is the 640, which is an audio output transformer. Unless it is open, you should not remove and upset that unit any more than needed to clean it. The 3 other plug in coils (610,620, 630) are a much bigger problem. There is no way in except to unsolder the copper cans, which usually results in discoloration and burning of the lacquer over the copper. Those 3 coils should not be soldered in, the should just plug in to the chassis. To operate on SW, Scott offered different coils, the 621, 631 and the 622, 632 pairs. The problem is this: these are almost impossible to find. I have a set that I've had for years, but have hardly ever seen more, usually just one or two at a time, never sets. Can you send a pic of the connections to the 3 "plugin coils"? 

Comment by Steve Larrabee on March 14, 2017 at 11:16am

Kent - Thanks for the info.  At first I thought this SG-9 was a kit built by an amateur but the more I got into it, it looks like a factory job with some questionable soldering here and there.  I went through it carefully checking coils, resistors as I could, connections, liquid de-oxit with a tiny brush on the tube socket pins, tested tubes, etc and fired it up.  I have a good RF front end, a good oscillator (by the way, Radio Attic's write-up says the 340 is the oscillator - it is the 1st detector/mixer - the real oscillator is the 301A to the right of the chassis), and a good audio section. My problem is in the SGA, somewhere.  I have 20 pictures of the dismounting operation (felt like an appendectomy) but I'll attach a couple of the coils/transformers soldered into their positions on the bottom of the chassis.  The inside of the SGA is a marvel with its coils and caps from 1928 - the set paid little attention to the serviceman having to work on it years ahead.  So far, I have no issue with the copper cans and have no need to dismount them.  I was just trying to understand how they were mounted and how to work on them if necessary.  Your SG-9 paperwork is a big help, as is the reconstituted "Manual".  Interesting timing in that the latest MAARC Radio Age has a terrific article by Ed Lyon discussing the oscillation prevalent in the 3.5v filament "22" tubes that were only used in 1928 - quickly industry replaced by the 24 and 24A.  I'll keep you posted of my slow progress. Thanks for the help.

Comment by Steve Larrabee on March 15, 2017 at 7:53pm

Inside the Shield Grid 9 copper box, the Choke very fine wire (36  gauge?) leads were green with corrosion of copper.  They had been worked on some time in the past because new wire was wrapped and soldered onto the original leads in the little spaghetti tubes.  Rewound the whole choke thinking that brittle 88 year old wire is vulnerable to fail again.  Now my voltages are good all around and 22 filaments are back up to 3.1 volts max.  The black caps in the SGA are dual .1 mfd and none tested good, so I wired in new ones.  No signal getting through to audio stage.  Will try siggen and scope tomorrow..........good challenging project! 

Comment by Kent King on March 15, 2017 at 8:06pm

Steve - Very nice work...opening the SGA unit is a pain, far too easy to break fine wires. So....Scott did not build the SG9 sets at the factory. He recruited "professional set builders". I have several brochures and ads seeking such individuals. These guys were supposed to demonstrate capability and quality work. I'm not sure how much Scott checked up on them, but for the most part, the SG9 sets that are found are about as you described: nicely built with an occasional bad joint or even a non-fatal wiring error (most of those probably got caught at run time. Personally, I have always been impressed with the skill level most of the hobbyists had back then. There are some very nicely built kit sets, many of us today would swear it had to be "factory". Scott was avoiding RCA, which was why he did not sell his own completed radios, he just provided the various coils and other parts at a discount to his builders.

Comment by Steve Larrabee on March 15, 2017 at 8:31pm

Hi:  That's fascinating info, Kent.......fits right into this set nicely.  Clever that he had kits, too, and this SG-9 was available as a kit with well explained directions: numbered and colored wires and fairly simple to build.  The SGA unit is really unusual in its design and assembly; it contradicts the rest of his sets so well designed, I think.  This is an early set - interesting that he wanted to avoid RCA's clutches that strangled so many competitors.  I'll let you know of reportable progress....best, steve

Comment by David C. Poland on March 17, 2017 at 7:38pm

Yes - RCA was the big gorilla in the 1920's. Held the superheterodyne patents and would sue manufacturers of superheterodynes radios - except a few who got access before RCA acquired the patents. RCA finally forced to license  superheterodyne manufacture about 1930.

Comment by Steve Larrabee on March 23, 2017 at 12:27pm

I've been working on the SG9 all this time, and have a couple of questions.  (1) The schematic shows a 2 ohm resistor in the neg filament line feeding the oscillator and the mixer/converter: my filament voltage on those two tubes is 3.0 volts with 5.0v on the input to the set.  There is another resistor in the neg filament line to the detector and audio tube of 2.4 ohms: voltage on those filaments are also 3.0 volts.  (.pdf file schematic attached)  All stages appear to be working but I get no output into 2K headphones when set fed with a modulated sig gen 1000 khz RF signal.  Shouldn't those four tubes filament voltages be 5.0 volts?  Why are those resistors in there at all?  Is 3v filament voltage harming performance?  Note that this is separate from the intentional fil voltage reduction for the 22s required.  (2)  Are the three IF stages tuned at all?  What frequency are they designed for and how are they peaked to that frequency?  Thanks for the help!! best, steve

Comment by Steve Larrabee on March 23, 2017 at 12:29pm

schematic .pdf file didn't upload -- how is a .pdf document file attached?

Comment by Steve Larrabee on March 23, 2017 at 12:29pm

schematic .pdf file didn't upload -- how is a .pdf document file attached?

Comment by Joseph W Strickland on March 23, 2017 at 5:12pm

Steve;

For this part of the site a *.jpg or *.png file type comes through OK. Can you provide one of those types?

Joe

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