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! 

Views: 211

Comment by Kent King on April 1, 2017 at 7:46am

Steve - 

I don't know that anyone had tested the IFs on an SG9 before...only a handful of folks have bothered to remove the cover and restore as carefully as you are. I do know that Scott took pride in matching the Selectone IFs he sold for battery sets, so I agree that this was probably the way it was designed. It does seem odd however, and implies a very broad bandpass for an IF. I have some extra coils of this type in my parts boxes....someday I need to set up a scope and check the frequency on them. Rick Ammon has a great discussion of that process on his superhet web site.

- Kent

Comment by Steve Larrabee on April 1, 2017 at 9:23am

Hi Kent:  Just for fun, I'll attach a picture of the setup on the bench to have the IF assembly accessible for testing and repair.  I could use more colors for the jumpers to prevent Murphys. There are 9 wires out of the IF assy to the main bakelite chassis and 9 wires from the power supply - add the antenna and ground and that's 20.  Add the meter and the scope probe and the siggen cables and it's a great test for dementia when you turn it all on.  Everything checks good now, so will focus on reforming all solder connections and cleaning grounds.  Please, what is Rick's website link?  Thank you. best,

It just occurred to me - could it be that the "builder" back then mixed up the parts and put in 100kc

IFs from another set or earlier unit.  Ever heard of that?  My 1935 3 inch thick Ghirardi "Modern Radio Servicing" book discusses IFs well and does not mention (with reason) different freq IFs - bizarre.

Comment by Steve Larrabee on April 1, 2017 at 9:27am

Comment by Joseph W Strickland on April 1, 2017 at 10:35am

If the set used variable air trimmers to peak align the IF's, it may have had trimmers that allowed them to tune anywhere near the 130kHz frequency. Maybe it was just misaligned? Or perhaps had some fixed value padders remove or added?

Yes the set of jumpers looks challenging!

Joe

Comment by Steve Larrabee on April 3, 2017 at 7:06pm

I put the SGA back onto the bakelite plate chassis and wired it up after I reformed all the solder connections on the set.  It's working for the first time: with a lot of oscillations but good control on the front panel.  It is tricky tuning.  Problem with oscillations I think can be helped with shields for the tubes in the Shield Grid Assembly.  I made some with 3x5 cards wrapped with aluminum foil and grounded to each other and that cut the osc down quite a bit.  Does anyone have some shields for the old globe tubes they can spare/sell - there are 5 tubes in the assembly with 3 of them having grid caps on the top (22s).  Great to get the SG9 going........Thank you all for your help!!! best, Steve

Comment by Joseph W Strickland on April 3, 2017 at 7:30pm

It would be nice if someone could post a picture of one globe tube shield here so we know what to look for. In the pictures of some Shield Grid 9 receivers shown in the Collector's Guide it appears that the copper shielding you removed was to serve that purpose, so are you wanting round shields that you can use for temporary shielding while you work on the unit? Or alternatively you might fabricate something you can use while working on the set. Some aluminum sheet thin enough to work easily with tin-snips and a few nuts and screws to close the ends that have to meet to form a circular shield, attach a suitable length of wire at one of the screws and nuts with an internal star washer for good contact and then slip it over a tube, and ground the shield to RF/IF circuit ground. If you have some old aluminum pie tins lying about you might be able to use material from that to make shields.

Joe

Comment by Steve Larrabee on April 3, 2017 at 8:49pm

OK -- The Shield Grid 9 article (starting on page 96) in the Info Archive parts list contains 4 #680 tube shields and the article further on mentions each tube in the SGA is individually shielded.  As I mentioned, I tried some homemade shields and they helped - I can make those better if no one has the real shields or other shields that will fit + work.  Atwater Kent had some really heavy metal shields they used on their critical tubes - some grounded with a grid cap style connection and others with an open top for the tube grid cap when it had one.  Just hoping someone has some in the "box" they are not needing.  The lid for the SGA does shield the whole assy from the rest of the radio, but the individual tubes evidently need their own shields, too.  Thanks, best,

Comment by Joseph W Strickland on April 3, 2017 at 10:01pm

Steve;

OK, then it does indeed need additional shielding around the tubes even inside an already shielded enclosure. It is relatively easy in an IF strip to have oscillation develop if everything is not just right, particularly when the circuit is designed for best gain. Mr. Scott was going for lots of gain compared to other brands. One thought I had not long ago was to look and see if I could find a source of large ID copper or brass tubing to make a shield for an IF transformer. So far I have not found a reasonable source for the size I was looking for. I wonder if the OD of the globe type tubes was standardized or whether different manufacturers varied from a nominal size. Tolerances can sometimes result in trouble if the range is not known and a different source does not adhere to a specification well. There might be some sizes of aluminum tubing that would be large enough to use.

Achieving a good long lasting connection for a ground would be the issue with aluminum. In assembling aluminum tubing type antennas I have used Penetrox conductive paste. There are two types, one has ground up zinc particles in it and the other has ground up copper particles in it. Some manufactured amateur radio antenna kits include the copper paste to use with aluminum tubing connections where good lasting RF connections are exposed to weather. The copper product would serve well with aluminum using a short length of wire, star washer, nut and screw or a solder lug with an internal star washer end.

If you manage to locate some good OEM type shields that would be preferred from the standpoint of keeping it as original as possible.

Joe

Comment by Norman S Braithwaite on April 4, 2017 at 1:13pm

The original cast pot metal Scott tube shields are unobtainium but they are close copies of the Remler cast pot metal tube shields. They were even the same color! The Remler tube shields are hard to find but you are more likely to find the Remler version than the Scott. Norman

Comment by Joseph W Strickland on April 4, 2017 at 2:07pm

Norman;

Were either the Scott or Remier shields plated?

Joe

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