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 Steve Larrabee on March 23, 2017 at 5:38pm

Comment by Joseph W Strickland on March 25, 2017 at 8:44am

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

Comment by Steve Larrabee on March 25, 2017 at 9:05am

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.............

Comment by Steve Larrabee on March 25, 2017 at 9:58am

In doing some more research on this set, I find that the four #680 tube shields are missing from the SGA.  So there may be some interaction between the IF tubes when I get it working.  Note the Short Wave antenna terminal is directly fed to the RF tube plate where 90 vdc plays.  I would suggest a small value capacitor in that line to avoid angry birds.

Comment by Joseph W Strickland on March 25, 2017 at 11:19am

Steve;

Sorry, it just looked like the caps were still in circuit.

Those filament resistors look like they may be providing an RF choke value too. Perhaps someone installed resistors instead of chokes? I tend to agree that having the filaments operating that much below their actual rated voltage would not be very good for operation. It is possible that a somewhat reduced voltage might extend life, but by how much?

Joe

Comment by Joseph W Strickland on March 25, 2017 at 11:25am

In looking at the schematic again I see that the RF stage gain is apparently controlled by changing the voltage delivered to the filament/cathode. It may be normal for these tubes to be operated somewhat less than the full 5V rating in order to help establish the desired overall bias point of the tubes. These direct heated cathode circuits are a different animal than the indirect heated cathodes of later generation tubes.

Joe

Comment by Joseph W Strickland on March 25, 2017 at 5:22pm

Steve;

I agree about the SW antenna input to the RF amplifier circuit. I think a .001uF capacitor would work fine for anything on the SW band frequencies the radio could receive. The +90VDC present in that circuit would probably not be too bad to encounter with a hand, but it might make you pull your hand back quickly and damage something in the radio.

This radio has such interesting construction. Individual contacts are used for each tube pin to be inserted into rather than using tube sockets that we are all so familiar with. I imagine they had to use a drilling template to get the holes in the bakelite positioned correctly in order to have them line up with the tube pins - no small task!

Joe

Comment by Steve Larrabee on March 25, 2017 at 6:40pm

Aye-Joe.  The RF tube rheostat serves a volume control for the input and also as a control for oscillation that may occur.  The other 3 five volt tubes with the two resistors in the filament line are factory nichrome wire-wound flat standard units and included in the schematic as designed.  I am running the set with less than five volts (4.75v on the ARBEIII battery eliminator), as you say, reduced filament voltage I think we agree preserves the tubes.  In the discussion of the Power Pack, they are using 6 volts for A+, so that may be why those resistors are there.  There is no control of the filament voltage of those three tubes. The second rheostat on the right controls the fil voltage to the three 22 IF tubes - a must - since they are rated for 3.1 volts or so.   I will increase the input fil voltage to 6vdc and see what that does.  On the SW antenna, I found it interesting they put 90v  on the antenna and also that they bypass the RF tube.  I like my RF amp when on the SW bands.  Scott recommends a 70 foot outdoor antenna, so SW performance with that was apparently acceptable.  I can shut off the filament voltage completely with the RF tube filament rheostat full CCW.  Next step for me is to try 6vdc A+ and to figure out the design IF.  Thanks for the chat. best,

Comment by Joseph W Strickland on March 25, 2017 at 8:05pm

Steve;

Yes reduced filament voltage does help extend tube life. A number of more modern hi-fi and stereo equipment did that with DC applied to 12AX7 or 12AU7 filaments at something less than the full 12.6V and the tubes just never seem to go bad. I have a nearly 60 year old Harman-Kardon A300 amplifier with 12AX7's operated from voltage developed by the cathode circuit of push-pull output tubes and they are still operating fine after all these years.

I have seen some other sets that use RF amplification on some bands and not on others. I have a Hallicrafters SX-25 which does something similar, except that it has two RF amplifier stages and one stage is cut out on the broadcast band, but not on short wave - the opposite of the Shield Grid 9 unit.

Good luck with the restoration!

Joe

Comment by Steve Larrabee on March 31, 2017 at 8:58pm

Slow progress:  I have determined the IF frequencies for the SG-9 are #1 130khz, #2 100khz, #3 130khz, #4 100 khz by scope and tuning siggen down slowly from 500 khz.  Research indicates in 1935 that 130 kc was a common early superhet IF freq , but there weren't many "commons' in 1928 - and 100kc was the lowest used.  Interesting that the four IFs alternate between the 2 freqs in a pattern which tells me they were designed to do that.  There is a padder for each IF that has no effect on the freq, and the capacitor on the coils is fixed and the same at .0001mfd each.  I have never encountered two different IF freqs in a chain of a bdcst receiver.  Since they are not adjustable they must be very broad. The set is very clean and well cared for so I am thinking to leave it alone and move on with the troubleshooting.  Has anyone any experience with this issue?  Any ideas or suggestions welcome!!   Different subjects:  upping the filament input voltage to 6 vdc from 5 vdc helped with no evil consequences. The schematic calls for 135 vdc to the speaker or headphones but the set was wired for 90vdc.  I had a good audio stage with 90vdc but I rewired it per schematic and now have more audio with headphones than I think I want - even half-deaf.  Will likely revert to the 90vdc.  Continuing the trblshooting on the RF side next.  Thanks for the help.

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