Need Help with AW12 Dual Dial

[Joe Miller]

Hi all,

Just pieced together an AW12 from two chassis.  All recapped and audio transformer replaced (had an open secondary).  Powered it up and was able to get a local station loud and clear, but....

I don't know how to tune this thing.  First both the dial cords are broken off.  I have the metal clips that attach to the dial wheel, but don't have a dial stringing diagram so if anyone can help or has info please let me know.  I think I will have to take the tuner assembly off to restring it.

I was able to get the local station by tuning each of the dials separately (I think one of the dials is on wrong, but not sure).  What does each dial do?  What is the procedure for tuning in a station?

Would like to hear from someone with experience with the dual dial AW12.  

Thanks,

Joe Miller

[Joe Miller]

Well, I'm running out of ideas:

I pulled all the covers, tube shields (even unbolted the bottoms and removed them), cleaned everything up.  Replaced all the grid cap wiring to the 24 tubes, inspected all the coils (all look fine), put it all back together.

All resistors are in spec, caps are all replaced, checked with variac at 100 Volts and up and no change.

Maybe I am expecting too much from this set.  I get the local station loud and clear, WLS comes in ok but just can't get the other stations my other sets can get in the same location, using the same antenna.

I'm going to power it up and tap all the components with it playing on WLS to see if I can find something amiss.

[Angelo]

Joe make sure the radio uses a 24A  tube a 24 tube may not work in it.Check the tube sockets very good! They can be cracked dirty or have a short on them.Try several det tubes if you have them.Check the tuning  cap to. Iwonder if you checked the non paper caps ? Check antenna input post ,have seen them be shorted befor.Angelo

[Joe Miller]

Hi Angelo,

Just checked the tubes and all are 24As

Just noticed that the oscillator tube (27) wouldn't light up so I tapped it and it did.  Don't know yet if it is the tube or the socket.  This isn't my problem but one that needs to be taken care of.  I'll clean all the sockets to make sure as you suggested.  And I'll make sure the second detector is a good, strong 27.

I did check the tuning cap and the padder cap, looks okay.  All caps were replaced except for the ones inside the cans and in parallel with the IF coils.  They looked like they were new. 

I don't think I have a shorted antenna post or else I wouldn't receive the strong local station or WLS

Will keep you posted.

Thanks

Joe

[Norman S Braithwaite]

Joe, when you say "they look like they were new" are you referring to the supposition that these capacitors were replaced with capacitors of recent manufacture or the fact that they look particularly clean?  If not replaced with recent manufacture film type capacitors, if leaky these capacitors will greatly affect receiver performance.

Norman

[Joe Miller]

Norman,

They were just particularly clean looking--they had not been replaced.  I couldn't test them since they are in parallel with the IF coils.

They look like mica caps and I assumed micas would be okay.  

 

Do you think I should replace them too?   If so, I'll have to find some "film types" to replace them with.

I assumed that since I was able to get the IF frequency to pass during alignment that they were okay, maybe a bad assumption.

Thanks,

Joe

[Norman S Braithwaite]

Ahh, mica caps.  I thought you were referring to the paper capacitors in the IF cans.  The mica capacitors are seldom problematic.  With the signal generator feeding the first detector (converter) and the signal generator output fully attenuated, are you able to get decent output at the speaker?  If so the IF and audio circuits are probably OK and the problem is likely to be in the antenna, RF, or oscillator stages.

Norman

[Joe Miller]

There were no paper caps in the IF cans, only mica types. Did this set originally have paper caps in the IF cans?  Maybe someone has been there before me?

I was able to get the IF signal through the IF chain but not sure how attenuated the signal generator's output was when I did that.  Seems like I had to raise the signal on the lower frequencies but I'm not sure.  I'll redo the alignment check.  Also the adjustments are fully clockwise to get the max sound out---I thought that was a bit strange.  I thought I should see a peak where either side of it the sound volume would go down.

Maybe I will get out my oscilloscope to see if I can see where the signal is being attenuated on the lower frequencies?

 

[Norman S Braithwaite]

Joe:

I can't remember which models have wax paper capacitors in the IF cans and which don't!  Most do.  Anyway, each IF adjustment should have a reasonably distinct peak.  You should focus on that first.  The fact that some IF adjustments are maximum at the end of travel indicates that the IF circuits are not peaked.  To what IF frequency did you align the receiver?  Usually this model had the IF tuned to 470kc.  If you cannot peak all IF circuits at this frequency due to components changing value over time, you may need to try a little higher or lower.

Norman

[Joe Miller]

I'll take another stab at the alignment.

Come to think about it, I need to check the adjustment to make sure I know how to move the caps.  It has a nut and a screw for adjustment.  I have a spare chassis so I can play with it to see if I can figure it out.

I looked in my spare chassis and it  also has micas rather than paper caps inside the IF cans

[David C. Poland]

Loosen the nut, then adjust the screw. Then re-tighten the nut (to lock the it). Watch the metered out put to be sure tightening the nut does not alter the adjustment.

To turn the locking nut, I made a non metallic tool from an old bakelite/plastic test probe. The inside diameter was about right and the outside diameter fit the hole in the cover. I used a file to shape the inside diameter into a hex shape to fit the nut. You may have to use a wrench to get the nut loose and work it so it turns freely. Then it should turn easily enough to snug it after you have peaked eack IF.
-Dave

[Joe Miller]

Thanks Dave,

I think that's why I wasn't able to peak the IFs.  I think the nut bottomed out before I was able to adjust the cap to a more closed position.

I have a spare chassis and did some playing around with it.  It appears the screw will prevent full travel, so I was going to remove the screw and adjust the nut to peak the IF and then put the screw back in and gently torque it against the nut. 

This seems like a crazy way to set up an adjustment--not sure why it was done this way.  Maybe to keep owners from monkeying with it and messing up the IF alignment.

Joe

[Joe Miller]

Now this is puzzling, been busy all week and just now got back on it.

I hooked up the signal gen to the grid of the 1st detector via .1 microfarad cap.  Tuned to 470 kc and got a nice loud tone.  Hooked up the scope and measured at the various stages:

@ 1st detector grid  .92 volts (all p-p)

@ 1st IF grid  .18 volts

@ 2nd IF grid .040 volts

@ 3rd IF grid .010 volts

Now I realize this makes no sense as this suggests the gain is much less than one for each stage.  How could I pick up any station if that were the case?  Any ideas what I am doing wrong?

Could the oscillator be messing up the IFs and preventing me from getting much station senstivity?

Would appreciate any ideas?

Thanks,

Joe