Need IF Expert(s)

[Joe Miller]

Hi everyone,

I've been trying to align my AW12 IF section but have found several "weird" characteristics that prevent me from getting the job done:

1.  First, here is the schematic http://www.nostalgiaair.org/PagesByModel/395/M0040395.pdf

2. All coils, resistors, connections are in spec--bypass caps have been replaced, tubes have been swapped out.

3. IF frequency is 470 kHz.  Only the IF coil on the output of the 3rd IF will peak--I can tune a peak, meaning tuning on either side of the peak causes the signal to drop off sharply.

4.  None of the other 3 IF coils will peak.  Signal is maximized by fully closing the tuning cap clockwise.  I assume there would be more gain if I could increase the capacitance further?

5.  The gains for each IF stage are strange--may be related to the inability to get a peak.  I sequentially moved the signal generator lead from the grid of the 1st detector to the grids of the 1st, 2nd and 3rd IF tubes expecting to hear the signal drop off in loudness as I removed each stage of amplification but found the following as I moved the signal lead from grid to grid (did not change the RF output of the signal generator):

• With Signal at grid of 1st detector--sound was low
• With Signal at grid of 1st IF--sound was about the same
• With Signal at grid of 2nd IF--sound was much louder (maybe 10 times)
• With Signal at grid of 3rd IF--sound was much lower (maybe 10% of what it was when signal was at grid of 1srt detector.

I think the IF gain of the 1st IF must be where the problem is but I'm unable to find the it.  Could this be part of the problem with not being able to peak the IFs.

I'm stumped on this one and would appreciate any suggestions any of you with strong IF experience could share.

Thanks,

Joe

 

[Thomas Day]

Joe:

I am not an "IF expert" but have about 40 years experience with electronics and doing alignments. The fact that the 50mmf trimmers have to be fully closed, at maximum value, would seem to indicate that something in the coil circuit has changed value. Since coils don't usually go bad except to open, or possibly have windings that short, then we need to look at everything else in the circuit. The only other components are the .0001 uf cap across the secondary and the cathode resistors and bypass caps. Reading thru the service manuals indicates that changes were made to several resistor values. It also makes reference to the adjustment procedure for the IF strip. Starting with the grid of the 2nd detector and working back to the 1st detector stage, gain should increase as you go.  

Since the first detector tube operates as a mixer for the RF and Oscillator signals, its gain will probably be minimal. The 'volume' control actually changes the screen grid voltage on all the tubes and should probably be set to maximum while doing the alignment, although this shouldn't have any effect on the frequency the IF is tuned to.

Does varying the IF frequency from 470Khz, say to 400 or 500 have any effect on the tuning? It seems Scott would vary the IF frequency to compensate for conditions in the customers region. This of course would mean that the Osc. coils would be varied to change the oscillator frequency to that required to produce the correct IF frequency. Measuring the Oscillator frequency would show this, since it is supposed to be 470 Khz above the incoming RF. 

Maybe not much help, but things I would look for.

Thom (In Chicago with an Allwave 23)

[Joe Miller]

All,

I replaced the 2nd IF coil and capacitor assembly from a parts chassis and re-aligned to restore the IF gains.  Still could only pick up one station though. 
This is a two dial set, one dial for the oscillator frequency and one dial for the station frequency.  I adjusted the oscilltor caps and voila, all the stations came in great.
This set is now a "boomer".  It sounds like a Scott is supposed to sound.
Lesson learned--on two dial sets don't forget to align the oscillator too.
Joe

[Thomas Day]

Glad you were able to get this working. 

[Angelo]

Joe good deal.The set should now play great  and sound good to.Glad you got it going.Angelo