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

[Norman S Braithwaite]

What happens if you feed the first detector grid a signal of 0.01 volts?

[Angelo]

Joe have seen mica caps go bad befor.Check them between each IF stage...I think your Osc is ok. Angelo

[Joe Miller]

Well I am back at it with this AW12.  It was playing on the one strong station at 1290 HZ and then quit.  I've checked all the voltages and all are in spec.  I swapped out the tubes with ones that tested good and finally got the channel back.  Turned it off and when I turned it on again a few hours later, nothing.

I'm going to try a signal tracer to see if I can figure this thing out.  Only thing left is the micas in the IF but I don't think this sounds like a mica problem?

When the 1290 HZ station comes in it is very loud and clear.  But WLS 890 is very weak and it is strong on all other sets I have restored.

This is a puzzler.

Joe

 

 

[Angelo]

May have said this but clean the band switch and check and make sure all contacts are good.May take several times of cleaning befor it realy is clean.Use alcohol to clean and switch back and forth many times let dry and clean again.Inspect each contact! Angelo

[Joe Miller]

Angelo,

I cleaned the band switch contacts and also the RF and Oscillator Coil pins.  Now when I tune in the strong station it breaks into a low frequency oscillation.  Something is definitely not right. After cleaning, the strong station tuned in at a slightly different spot on the dials.

Joe

[Norman S Braithwaite]

Joe:

Just a thought based on prior experience... Try loosening and retightening all machine screws through which electrical and shield connections are made.  The cathodic reaction of dissimilar metals between machine hardware and the chassis sometimes results in high resistance ground returns and diminished performance (including IF and RF feedback).  I don't give this a high probability of solving your problem but this possibility should be eliminated.

Norman

[David C. Poland]

Angelo -
i think you are describing the coil wheel and switches of the later models. Joe has the 2-Dial using plug in coils.

Joe - 'Tis a puzzle !!
- Is the oscillator really working?
- With a frequency generator, can you really pass an audio signal through each IF coil, and then down the whole IF amplifier section?
- That you only get the one station, and that it booms in makes me wonder if the reception of that station is independent of the tuning circuits by some quirk.
- Have you a frequency counter to check the oscillator circuit works, and the frequency generated is correct.
- Are you sure you are using the correct pair of coils for the broadcast band. Compare your coils with the owners manual coil by coil descriptions, lest some caps came off in the past and someone scrambled them trying to reattach them.

-Dave

[David C. Poland]

Do you have Ed Romney's FIXING UP NICE OLD RADIOS? He has a 4 page case study (pg 140-43) of the Scott 2-Dial, including a very detailed account of the design and function of each circuit. Then his challenges getting it playing correctly - and ultimatelywith "splendid" results on broadcast and short wave.
-Dave

[Joe Miller]

I've cleaned all the switches (incorrectly reported bandswitch above, it was actually the "gain switch")

I took all the shields off, cleaned them and reinstalled them and tightened the nuts snugly, but will check all the other grounding connections as Norm suggests.

Used signal generator and was able to pass IF frequency with an audio signal on it successfully.

I do have a frequency counter but not sure where to connect it to test the oscillator frequency. Can you tell me where to connect?  Also should the grid bias be negative if it is oscillating? 

As far as correct coils, all this set had with it were the broadcast band coils.  I think they are correct but no way to tell.  I could take a picture if anyone can confirm if they are the right ones. 

I have a set of coils from another set, but unfortunately they have different pin configurations and are worthless to me.  Would like to swap with someone who has the set I need.

I don't have the Romney book but will search for it to see if I can find one for sale.

Thanks everyone, 

I'm (we're) going to figure this thing out.

Joe

[Joe Miller]

Making some progress--I think I have an IF problem but it's working good enough to pass the stronger stations.  Here's what I found by poking my signal generator at various points in the IF chain:

1st detector grid--adjusted the tone for a gentle sound and left signal generator at this level for all other checks

1st IF Grid--tone was slightly higher (maybe a gain of 2 or 3) which means there was a reduction in gain across the 1st IF coil

2nd IF Grid--tone was much higher (maybe a gain of 10 or so) which means the gain across the 2nd IF coil was less than one

3rd IF Grid---tone was much lower (maybe a reduction in 20 or 30 from signal at 2nd IF Grid) which means the 3rd IF coil may be working okay?

I found the IF alignment caps all fully closed (tight) and there is no affect if I open them on IF signal strength.  Also, none of the IF coils are open.  Could this be bad micas after all?

The schematic calls for .0001 (microfarad I assume) for the micas, but the ones installed are brown and marked with one white and one orange color code. (that sounds like a 39 or 93).  I also have a AW12 parts chassis that has micas that are brown but marked with one red dot (2).  Before I try to replace the micas I need to figure out why I have different color codes in 2 AW12 chasses and what the right size is for this set.

Anyone have any ideas on what is going on here?

Thanks,

Joe

[Angelo]

Opps you right you have the coils forgot. .You should be able to find a copy of the book.Angelo