EH Scott Radio Enthusiasts

The Fine Things are Always Hand Made

Looking for ideas - I haven't dug into this too far yet.

I recapped an AW23 (late) and I'm now working on the alignment. Voltage checks were in spec. IF alignment went right by the service data and trimmed out nicely. By the time I got back to the 1st IF, I had significantly reduced the generator output to keep the meter from going over - gain was excellent. On to AM band - 1400 trimmer is super close, slight tweak and then dial down to 600. At about 750, the two neon VR tubes go dark. Turn it back up above 900, they relight. A mA meter on the B+ shows a pretty good jump in set current when the bulbs go dark. 

Looking for ideas on where to start when I go back to the workbench after dinner...

Kent 

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Comment by Norman S Braithwaite on October 7, 2020 at 11:09am

Another item to check are the connections to all three stators on the tuning capacitor.  The connections were made by machine screws through a phenolic insulator.  The phenolic insulator shrinks allowing connection to become loose and corrosion to enter the connection.  The solution is simple.  Just loosen and retighten the machine screws holding the solder lugs to the stator sections.  But this requires removal and replacement of the tuning capacitor.

Norman

Comment by Kent King on October 6, 2020 at 8:17pm

Will do. I didn't get more time out there tonight. I did one thing - pulled the osc tube and the VR came back. So it is probably a parasitic oscillation. More when I have time to test.

Comment by Norman S Braithwaite on October 6, 2020 at 8:08pm

While you have the tuner cover off, clean the wipers.  Sometimes the grease hardened causing the wiper to lift and possibly cause parasitic oscillation.

Comment by Kent King on October 6, 2020 at 6:18pm

Per Norman, I'll double check the grounds. I had the tube shield completely off during cleaning, I'll make sure the grounds are all tight in that area. And I have several 76 tubes to try, so these things give me a se of tests for this evening. Thanks for all the ideas.

 

Comment by Kent King on October 6, 2020 at 6:15pm

I'll pull the tuner cover to check the plates, that may be the best thought. I went back and looked at the pics I took as I was disassembling, there is a chance some of the plates may be touching near full closure. I think that will be the first stop.

Comment by Norman S Braithwaite on October 6, 2020 at 6:14pm

And check the ground connections near the oscillator tube socket.  A very small resistance can cause problems.  You may need to create a solder bridge from the lug to the chassis bypassing the ground connection by rivet.

Norman

Comment by Norman S Braithwaite on October 6, 2020 at 6:11pm

That sounds like a parasitic oscillation problem in the oscillator circuit.  The neon bulbs only regulate voltage to the oscillator plate.  I presume the inner bottom plate was installed.  Retest with another type 76 tube in the oscillator position.  Also try loosening and re-tightening the nuts holding the oscillator tube shield to the chassis.

Norman

Comment by mike hadley on October 6, 2020 at 6:06pm

As the local oscillator is running the assumption that the 76 is working ok, is the 50pf grid capacitor ok?

and rhe vanes of the tuning cap aren't shorting

as only the local oscillator is connected to the vr tubes.

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