Allwave 23 - IF Diode Adjustment

[J.N. Shetler]

Here's a question for you guys that have done numerous AW23 sets (Norman, Kent, Bill). I am finishing up a 23 (new caps & resistors), it is performing well, tons of gain regardless of bandwidth. Moving between selective and high fidelity works great. No issues (it was sweep aligned). One concern that I noted was the IF diode adjustment. I am totally bottomed out to the minimum amount of capacitance on the trimmer. It was a very gradual gain to the end of travel. No sharp peak. I am wondering if this is normal, or whether I need to decrease the fixed silver mica cap inside the IF diode can. What have you guys found in your own sets?

Justin

[Norman S Braithwaite]

Justin:

The IF diode should peak just like the IF transformer adjustments but less sensitive.  Try turning the adjustment all of the way in and back it out to see if you can find a peak with the slug on the opposite side of the coil.  Also did you peak align the set to 465 kc (or if marked in grease pencil on the inside of the back apron, that frequency) before sweep aligning?  If not, it may be possible that the peak is out of the proper range for the IF diode circuit.

Norman

[J.N. Shetler]

Hi Norman,

I appreciate your reply. The set had no markings on the apron and was peaked with a generator & freq counter to 465kc. The IF cans peaked sharply and then it was sweep aligned with a scope. I have a nice peak at the selective position and good wide even bandpass at the fidelity setting. I just went through the sequence again, and the IF diode still has the highest output on the meter with the trimmer completely closed. As I open the trimmer, the output slowly falls until it bottoms out. It is at it's highest point with minimum capacity on the trimmer. It's almost as if the padder across the trimmer is too large. I confirmed that it is a good silver mica with a value of 50pf.  

The other thing of note, I can't seem to find any specifications for the RF-AVC (RF diode) trimmer settings. I have the Scott instructions for aligning the receiver and see no mention of it. I would assume it would involve high-level signal injection on the broadcast band and adjusting the trimmer to change the grid bias on the 6D6 RF tube. Anyone know the factory procedure for this.

The set is a real performer "as is" right now, I just want to get the most out of it that I can before installing it in the cabinet.

Justin

[Norman S Braithwaite]

Justin:

I had forgotten that the IF diode adjustment was a trimmer capacitor rather than a slug.  I do not know why the IF diode is not peaking but it should.  Options include adding a small amount of capacitance or replacing the entire diode assembly with another (I have several available).

Going from recollection the Scott service literature does not identify the alignment procedure for the RF diode.  I align this diode by introducing a very strong signal to the antenna and adjusting for minimum signal output (volume).

Norman

[J.N. Shetler]

Hi Norman,

Upon close inspection, the coils in the IF diode checked out ok, i think the trimmer was overtightened at one point and some of the fine threads were damaged limiting it's range. I added 10pf to the padder and it peaked. I must have been right on the edge before.

Odd that Scott didn't address the RF diode in the instructions. With a very strong signal, I aligned for minimal gain using bias on the RF tube. The circuit tunes broadly, but there is a definite "correct" null. Interesting to inject signal and watch the circuit in action. There is a definite delay to account for signal fading with atmospheric conditions. But on a strong "local", the gain of the 6D6 is cut considerably.

The only other thing I noticed which may be inherent to the design, the behavior of the IF gain across the bandwidth setting varies more than I'd expect. I know the bias setting on the 39/44s is varied with the bandwidth control; seems the range is a little greater than need be. This may be an artifact from sweep alignment. The peak alignment led to a very sharp peak. After sweeping in hi-fi position, the last couple IFs were detuned a little to even out the aprons on the scope and provide a nice flat topped response. I guess the IF gain variation may be a consequence of that.

These sets are real performers, the sensitivity must be down to a couple microvolts. It bests everything I've worked on from that period except a few "boatanchor" receivers.

Justin

[Norman S Braithwaite]

Justin:

Readjusting the last couple IF transformers while sweep aligning after peak aligning is the way to go.  In the "high fidelity" position the IF bandwidth is nominally 32-khz wide to accommodate the short lived experimental AM high fidelity stations.

Norman