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]

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! 

[Steve Larrabee]

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.

[Kent King]

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"?