EH Scott Radio Enthusiasts

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ELECTRICAL RESTORATION: All Wave 12 Deluxe

Hello all,

I have started another E.H. Scott restoration and wanted to share my progress for those interested.  Pictured is an early version AW12 Deluxe with AVC (Wunderlich tube) housed in an early version Tasman cabinet.  Upon receipt of the electronics, the set was wired for use with an 1930's era PM speaker.  The original E.H. Scott speaker was missing.  I purchased the set with a correct and working pair of E.H. Scott Rola twin speakers.  This Rola speaker setup has been seen and documented on early examples of the AW12 Deluxe chassis, which this set appears to be:

Serial number F-78

The All Wave 12 chassis/power supply and Rola speakers were added to this Tasman cabinet upon purchasing.  All is period correct thus showing an proper example as to what a complete set may have looked like brand new upon purchase via the Laboratory in the early 1930's.  Please feel free to comment as the restoration progresses.  I look forward to making this beautiful AW12 operational again!

Jon

 

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With the top side now complete, we move to the underside.  The coil wheel will first need to be removed to allow for total access of all parts in need of replacing.  The coil wheel is held onto it's shaft via two nut's.  One of two trimmer capacitors had to be carefully moved as it did not allow access to the securing nuts.  Once out of the way, both nuts were carefully loosed and removed.  Afterward, the wheel was then slowly removed. 

Very important to note:  The band switch needs to be set to the standard broadcast position prior to removing the wheel.  Once the wheel is removed, do not touch the band switch lever.  Such action will damage this mechanical system. 

On the top of the band switch gear housing (right below the coil wheel once removed) rest two sets of springs each complete with a guide bearing.  All three pieces are removable.  I cleaned all old grease on these parts and re-applied white grease in a very small quantity to each.  These parts move every time the band switch is engaged.

The tuner underside has been re-capped.  All resistors check within 20 percent tolerance and have been ok'd to stay within the circuit.  There are three bathtub capacitors along the rear wall of the underside.  Each are designated with a painted red dot.  This red dot was completed at the Laboratory upon assembly.  This dot signifies each capacitor having an inductor included within.  These capacitors/inductors are part of the I.F. circuit and will be left original for now due to their sensitive nature.  Low voltage travels across these parts, I will monitor each during first post restoration power-up.  Should issues arise during post restoration trials, each will be further examined.

Divider resistor checked within rated values as did all other components.

The entire underside has been re-assembled and ready for service.

Jon

Last issue on the tuner resides within the variable condenser unit.  There is a trimmer cap (shown below) which was either added to the circuit via E.H. Scott or likely added later.  The grounding terminal had come loose from grounding point (Chassis ground).  I have since re-soldered the tab to chassis.  I have not traced this component within the circuit yet.  Has anyone ever come across such modification in an AW-12 tuner?  It does not look original to me.

The tuner is now complete.  The power supply will be up next.

I first wanted to clean up the power supply chrome prior to doing anything else.  Shown below is the power chassis cleaned and polished.  One electrolytic can was replaced long ago, did not match the two originals still in place.  This mis-matched part has been replaced with an original period correct E.H. Scott electrolytic can (middle can).

A period correct power cord and brand new AC line switch w/period wire were replaced, seen in pictures below.

An underside view of the power supply shows several interesting items to note.  Two large brown ceramic resistors reside across the speaker connection, added to the circuit to allow use of a (non E.H. Scott) PM type loudspeaker.  Chassis wiring looks to have been replaced at some point in the past.  Wiring within the AW12 power supply is a rather interesting subject.  I have come across both cloth wire and rubber wire examples.  I am curious as to when the change over from cloth to rubber or vice versa took place at the E.H. Scott Laboratories.  Perhaps there never was a change over, results dependent on what was available at time of build.

Upon receipt of this AW12 power supply, the set was wired to use an PM type speaker.  The original E.H. Scott labeled speaker had been replaced long ago.

A closer look at the rubber wiring revealed areas of deterioration.  Bare wire is exposed within several areas of this power chassis.  All compromised wiring will need to be replaced before a first power up.

This AW12 will be taken back to what one could say "Laboratory spec's" upon the set being built new.  A twin pair of Rola speakers, used by E.H. Scott in early AW12 production will be utilized.  Research would indicate this early variation AW12 could had very well used the replacement speakers I will be using with this set.  I don't know the original setup as the original speaker/speakers used with this chassis are no longer present, no documentation present either.

As a result, the circuit was taken back to original spec's including the removal of the two ceramic resistors and several changes to the wiring.  Everything now matches up to the E.H. Scott twin speaker power supply schematic.  The AW12 could be had with either twin Rola speakers or a 12 inch Jensen dynamic speaker.  The power supply is wired to accept either.  Both power chassis schematics have a different layout, they are electrically identical though.

 

The power supply polarized and non polarized capacitors have been replaced.  All transformers tested good, fuse checked, switches cleaned, and all deteriorated wiring has been replaced.  Ready for a first power up! 

Upon first power-up, I discovered a short within the power transformer.  All wiring and replaced components were checked over again.  Primary and secondary's all tested ok.  The short was traced to the high voltage winding on the secondary.

As a result the power transformer will need to be replaced.  A replacement is on hand.  The original has been removed:

Measurement results of this power transformer below.  I am likely receiving a reading on the high voltage winding due to the shorted wire touching another wire on the same winding within.

 

Jon -  I have or have had examples of AW-12's with each speaker variation. And I have Scott literature consistent with the below time line. Speaker progression for the AW-12.

1) Spring 1932, the single dial AW-12 DELUXE was introduced using the same  11 inch Scott branded (by ROLA) speaker used on the 1931 Scott 2-DIAL 12  tube model. Same 5 pin speaker plug. Amp is like the 2-Dial amp with 5 hole speaker socket. An added chrome cover over the power transformer laminations.  Field coil is for the B+ circuit. Amp carries over the big ceramic voltage divider of the 2-DIAL model amp, too.

2  Summer 1932, "Laboratory Matched Twin Speakers" (ROLA) introduced along with an amp re-design. The twin speakers each have a separate field coil and share a single cable with 6 pin plug. The  big ceramic voltage divider is relocated  to the receiver. The larger speaker's field coil is for B+. The smaller speaker's field coil provides output tube negative bias. The set cable cable revised to using 5 wires and soon the 8 pin Jones plug is reduced from 8 pin to 6 pin version. The twin speaker are mounted on separate baffle boards secured to each other at about a 30 degree angle and then pushed into the cabinet against the cabinet speaker opening and secured.

3 December 1932, receiver re-design for AVC (automatic volume control) using the Wunderlich tube second detector plus other tube line up changes. Same amp and twin speakers carried over.

4 Spring 1933, Scott changed the speaker supplied from the twin set to the Scott branded 12 inch pedestal auditorium design (a special Jensen). The pedestal speaker is electrically identical to the twin speakers and has two field coils - one for B+ and the other for output tube negative bias. The twin set and  pedestal speakers are interchangeable, which I tried some years ago as a verification. 

Scott was a custom set builder and and made mid-production changes to improve performance of a model.

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