Restoring prewar FM tuner

[Troy Taylor]

Hello-

 
A couple of weeks ago at the annual Seattle antique radio swap meet I purchased a prewar FM Scott tuner.  It has been determined that this is the same one that was sold at the Estes auction in January 2023.  How it got from Ohio to Seattle and ended up on a table at a local swap meet here is anyone's guess.  But it was very fortunate for me that it did.

This set is all original and has never been restored.  All but one of the tubes are the original Sylvania green-leaf type installed at the factory.  It is complete, except that it is missing the builder's plates from the rear of the chassis.  So I do not have any serial number information to contribute to the database.

I have started re-capping the chassis, taking the time to individually re-stuff each one of the waxed cardboard tubes in order to preserve the original appearance.  The original resistors are all 10% tolerance, and a few have drifted so far out that I had to replace them, but I am not replacing any that are within 15% to maintain the original appearance as closely as possible.  Does this seem reasonable, or should I replace all that are more than 10% out?

Using the schematic from Riders looks like there are a few little differences from what's drawn and how it was really made.  There are two .25uF caps in the set and one of them is tied directly to the positive side of one of the 30uF filters.  This is not shown on the schematic, but it is clearly original, so I plan to leave it that way.

As the reception frequency is the prewar FM broadcast band, I have been making arrangements to purchase one of the frequency converters that tunes the present day FM broadcast band.  The goal is to be able to actually use this tuner after restoration.  It is presently sitting on top of my AW23 in a Warrington cabinet, and looks at home there.  Hopefully it will sound fantastic through the phono input on that radio!

Also- does anyone know if it is possible to get reproduction builder's plates made somewhere?  I seriously doubt there are any junk chassis out there to purchase originals from, so I'd like to explore the possibility of getting reproductions made.  It seems likely to me that someone has already had this kind of work done on all manner of vintage items missing this type of plate.  But I don't even know what this sort of plate with recessed lettering is even called.

That being said, I don't have a water-cooled checkbook, so cost is a factor.  Any suggestions on how this might be done, who is able to do it, and what information is needed would be most welcome.  At the very least, I assume it will require some high resolution photos of what's missing, scaled at exactly 100%.

Any information/suggestions are most welcome.  As this is an uncommon item, I want to do all the work properly.

Thank you-

Troy

[mike hadley]

Hi Troy,
What's needed is either a decent photograph or a scan of an original, then the rest is fairly easy to do,
photo etch on brass Is probably the way to go, then plated with nickel and the background blackened.
Mike

[Robert Meader]

If this tuner is like the FM tuner section of the philharmonic and phantom receivers it will have a drift problem.  The cure is to place the 10pf capacitor connected to the oscillator coil with a 10pf N750 capacitor.

[David C. Poland]

Yes indeed - my 1941 Scott Laureate with AM, FM & SW bands, the FM drifts for upwards of 30 minutes. This model was designed to include the pre-war FM band, and only one SW band. It has the 15 inch coax speaker version of the optional hi-fi speaker system.

I have Bill Lier's digital converter of modern band to the pre-war band, the version with the modern FM frequency display screen. With it, you tune the converter to the desired frequency, say 90.5 MC, and fine tune reception with the Scott FM band set at approximately at 45 MC. When finally well warmed up, sounds pretty good.

Whereas the Philharmonic and Phantom were existing AM models to which the FM band was substituted for the ultra high AM band (used mainly for receiving the audio signal for experimental pre-war TV broadcasts in a few cities). The pre-war FM tuner is pretty much the same FM circuit plus a built in power supply.

So evidently the buyer had a choice of 1) TV audio on the standard model or 2) FM broadcasts on the FM Philharmonic or Phantom Deluxe ... but 3) not both features in 1940 &1941 Scott radios. Incorporating the audio  as part of the FM broadcast signal was a post war development for later TV's.

[David C. Poland]

Robert- can you elaborate on your modification to reduce drift.

[Robert Meader]

I got in front of me the schematic of the Model FM Tuner.  Looking at the mixer (6sa7) the 10 mmfd is in parallel to the oscillator col and 25 mmfd trimmer.  If you look to the left of the 10 mmfd capacitor you will see a 50 ohm resistor connected between "X" and ground. The "X" is 6.3 volt filament.  Physically the 50 ohm resistor is taped to the 10 mmfd capacitor.  The FM Philharmonic worked by simply epoxing the resistor to the capacitor.  This worked on the philharmonic but not on the Phantom.  To correct the problem with the phantom and maybe Laureate you will need to change to 10 mmfd  capacitor to a temperature compensating capacitor selected by trial and error.  Note if you use the  Scott FM converter you will need a capacitor with a different temperature coefficient as the FM converter drifts as well.

[Troy Taylor]

Hello-

Thank you all for the information about plates and the 10pF cap solution.  I have a new question about caps.  Yesterday I received the new 30uF electrolytic filter caps I had ordered.  Although I have been restuffing the wax caps, I am not intending to drill out rivets and cut into the electrolytic cans and attempt to restuff them.  So my plan is to install these new caps under the chassis and keep the original cans intact, but disconnected from the circuit.  I have done this on all my other radio restorations in the past.

The new caps are rated at 500V and are large enough that they cannot be put in exactly the same locations as the original cans.  I will be able to stay pretty close though.  Will moving the location of these caps slightly be likely to cause any problems to the circuit?  Sometimes the physical layout and dress of leads makes a significant difference in operation.  Everything else is exactly the same as original after restuffing, but these three caps will require a change.

Can anyone tell if a slight change in the electrolytic positions will cause problems, or if this will be OK to do?  My intent in installing them this way is to make the new caps completely removable in future, should anyone wish to restuff the original cans.  This change would be totally reversible and not involve any alterations to the chassis.

Thank you and best regards-

Troy

[Alex R. Whitaker]

Gary Marvin, a well known Seattle collector/restorer, bought the set online.  He had it shipped, along with some other radios, to Seattle.  I did let him know how rare it was, when he posted the set on a Facebook antique radio group.  He told me that he bought it to sell.  

[palegreenthumb]

Wow,  I wish I had seen that!  I get very little time to look around since I'm always a volunteer.  Nice find. 

[Troy Taylor]

Hello-

After a long delay, I'm finally looking at this tuner again.  When I tried it before I got absolutely NO output at all.  Taking a closer look at everything I realized that perhaps some damage to the shielded audio-signal out cable could have potentially severed the signal path.  So I measured the continuity and found less than one ohm resistance along the wire. 

But I also found 320K ohms resistance between the audio line and the external shield.  In my mind there should be infinite resistance between the signal and the shield, so I'm wondering if this 320K path is effectively shorting enough of the audio to ground to make it inaudible.  But, before I decide to dig into this wiring looking for a partial short, I want to ask: is this actually a problem, or is there an internal connection somewhere that makes this OK?

Should I leave this alone, or look for a short somewhere?

Thank you and best regards-

Troy

Edmonds, WA

[Troy Taylor]

I thoroughly examined the full length of the audio cable and found that all the damage was limited to the very last foot.  So I went ahead and shortened the cable by one foot to remove all the damaged areas, and re-soldered the original connectors in exactly the same relative positions as they were located from the factory. 

The damaged piece of cable measures infinite resistance between audio and shield all by itself, so it is now clear that the damage did not create any electrical path between them.  Cosmetically ugly, but electrically still OK.

The 'new' cable (all original, but one foot shorter) now measures 290K ohms between audio and ground.  So I am doubting that this is actually a problem or an explanation for my lack of audio signal.

Thank you and best regards-

Troy
Edmonds, WA

[Scott Seickel]

Clip your ohmmeter onto the audio out leads and then turn the volume control.  You will see the resistance between the audio out hot lead and the shield vary as the volume control is turned.  Volume control is acting as a variable voltage divider.  

[Brad Winder]

We have a local company here called Photometals, and I've had them recreate a couple chassis tags for me. Very hard to tell the original from the recreation, unless you have them side by side.