Shield Grid 9 Renewing...........

[Steve Larrabee]

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! 

[Steve Larrabee]

Joe:  Tnx but they have all been replaced with new .1 orange caps - visible in pix of the IF chassis above.  I think I can determine the freq of the IFs, and I don't see why I can't bypass those two filament resistors to give me 5 volts on the filaments of the four designed for 5 volts tubes they affect.  I have dismounted the SGA again; making it much easier to trace through it.  I also built a four pin extension plug and socket six inches tall so I can measure the tubes buried in the SGA better = very helpful.  Thank you for the help.............

[Joseph W Strickland]

Steve;

Maybe I am missing something, but it looks like in the picture the 2 X .1uF capacitors are still in circuit. Could this be contributing to the problems you are dealing with?

Joe

[Steve Larrabee]

[Joseph W Strickland]

Steve;

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

Joe

[Steve Larrabee]

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

[Steve Larrabee]

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

[Steve Larrabee]

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

[David C. Poland]

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.

[Steve Larrabee]

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

[Kent King]

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.