Allwave 15 questions

[John Poulton]

I'm new to this group, so please make allowances.. :) 

A friend and colleague inherited a beautiful Scott Allwave 15 and has asked me to help him get it going again.   This is the first E H Scott radio I've had the privilege to work on, and I've been collecting documents and reading this discussion group to get going.

At this point I have only the tuner and ps/amp chassis, and I'll need to get my friend to bring along at least the speaker before powering the radio up.  But in the meantime I've been assessing what's there.

The radio is in what seems to me to be amazing condition.  The chrome finish is nearly perfect, and the tuner looks to have been unmolested.  The power supply/amp.. not so much.

Starting the inspection with the power supply, I immediately found some weirdness.  The photo below shows the underside with an initially mysterious component:

It's difficult to see how this thing is connected into the circuit, but after some head scratching, I finally figure out that this is an 8H center tapped inductor, and that the circuit is this:

So.. have any of you seen this variation on the AW15?  If so, what's in the "22" box?  Is it just the standard interstage transformer, with primary disconnected?  Is there a source for the AW15 schematics between the "Early" and "Late" versions that might include this variation?

[Thomas Day]

It looks like the driver transformer part 22 went bad, and someone bypassed it with the tapped inductor, the 14k resistors and caps to provide b+ to the driver tube and signal to the output tubes. If you remove the indicated part, you may find clipped leads. 
Thom. 

[John Poulton]

Thanks for the reply, Thomas!  I'd wondered if that was the case, and I really should take transformer "22" out and see if the primary leads are still tucked inside... But, the center-tapped inductor is mounted in a little steel box that looks much like the case for the 2uF capacitor, and it's riveted to the chassis with chrome-plated rivets.  So, it *looks* original!  I'm wondering if this was one of the many variations in the All-Wave 15's design, a bit of an experiment with "resistance coupling"..? 

Forgot to mention that this receiver is S/N L-402.

[John Poulton]

Thomas, following up on your suggestion, I took "22" out and examined it closely..   It only has 3 wires emerging from its potted innards, and no sign that any other wires have been trimmed off.  

The thing really looks as though it was fabricated this way on purpose at Scott, and I think all the evidence points to this radio just having a different audio driver circuit than either "early" or "late".  Go figure.  FWIW, I measured the inductance and resistance of the one winding:  473Hy @120Hz and 538 ohms, end-to-end.  It's a seriously weird circuit, but it does seem that the Scott engineers had a thing for center-tapped audio chokes..see the circuit for the "hi-fi" version of the audio power amp, with 4 2A3's.

I drew up a schematic for the final amp/ps circuit, after component replacement:

[Steven Mckelvey]

yes - I have the same addition and like you I think its a factory addition as other components are moved to accommodate it and the mounting to the chassis looks factory to me as well.  Keep me posted on progress.  If you look on my page, you will see my posting and photos.  What is your serial number?

[David C. Poland]

There are several diagrams for the Scott AW-15, showing considerable evolution during 1934.

The Riders diagrams marked early and late are both later diagrams, using the 55 tube for 2nd detector.

Explore the Info Archive at the site top, then open set folders. then open Allwave 15 folder.

Then open Wunderlich 15 to find the two early diagrams of April and May 1934 for the earlier models using the Wunderlich tube for the 2nd detector. If your receiver has the 2 red antenna posts, then it is certain yours was built for the Wunderlich tube, although a number of examples were later converted to use 55 tube after the Wunderlich tube became unavailable. There was evidently a brief time mid 1934 when the last I F transformer had the center tapped secondary and the 55 tube diodes were connected in a push-pull manner like the Wunderlich sets. 

My experience is there is little difference in the power supply during model run for the Scott AW-15.

Although not shown on the amp diagram, the earphone jack is on every AW-15 amp I have seen - 2 positions, fully inserted the earphone jack silenced the set speaker.