Need help...Scott Allwave 12 manual and schematic

[Mark Kaplan]

Hi all,

I'm very fortunate to have acquired a Scott Allwave 12 in decent shape.  I believe it is  a "transitional " model.  It has one dial, two speakers,  three 58 tubes and a Wunderlich tube for the detector.

I did find one schematic online but it doesn't seem to match my set.  In particular, the large power resistor (voltage divider?) mounted to the side of the chassis doesn't match .  There are five taps on this large power resistor.   Tap closest to front of chassis is ground.

The other sections read: 1080, 600, 1180 and 4300 ohms respectively.

I really really need a schematic which shows the values of the parts (resistors and capacitors) and possibly a manual.   An electronic scan would be super and I can print it out.

I've posted a few photos of the set.

Someone has removed all the wiring from the original electrolytic caps and there are a few new caps tacked in  various places and two or three other replacement capacitors but pretty much things look original.

I would appreciate any help or advice that can be provided.

[David C. Poland]

From info in my file for past 12 tube Deluxe sets:  

Max receiver voltage is 250 volts at high side of the power resister.

The 2 top side filter caps - both were 8 MFD - so replace with modern 10 MFD  450 volt filter caps. 

       -one at the 250 volt high side lug of the power resistor to ground

       -and one at the next power resistor 100 volt tap lug to ground.

The power resistor - I have a diagram showing  voltages but not the sections resistance.

       lug voltages of    250V   100V    50V    25V (unused)  and Ground.

   

[Mark Kaplan]

Dave,

Thanks for the help, especially with the chassis electrolytics.  I'll start recapping tomorrow.  I plan to disconnect a couple of the small bathtub capacitors to check them at voltage.  Most of them seem to go to ground and they seem to be 0.1uF at 200V.  It will be interesting to see how leaky they are.  I probably will replace them but keep the original parts in place.

I'm not sure why it has taken me so long to get back to this set but a house move and life seems to  have intervened in the last few years.

Regards,

Mark

[David C. Poland]

Mark -   Not sure the resister color codes were very standardized in the very early 1930's.

*My info indicates:

The 2nd AF tube pin 3 grid to ground resistor is 1/2 meg   (500,000 ohms) 

2nd electrolytic - 8 MFD  (use a 10 MFD)  from 100 volt lug of power resistor to ground. 

              (First 8 MFD electrolytic is from high end of power resistor to ground.)

Don't have power resistor section values, but from the high end, voltages at the lugs should be:

                   250V     100V      50V      25V (unused)       and ground

The IF circuits have a 1/2 meg resister between the AVC line and the IF Tran secondary grid circuits.

The last IF transformer secondary - the center tap has a 250M resistor to ground and a 1 meg to the AVC line back to the IF tube grid circuits.

Hope this helps.

[Mark Kaplan]

This is all helpful Dave.  I'm surprised by the value of the 2nd AF tube pin 3 grid to ground resistor is 1/2 meg   (500,000 ohms) .   My resistor sure looks like a 2500 ohm dogbone and it reads 7800 ohms (pin 3 grid to ground) which is nowhere near 500,000  ohms.   I guess I can try both extreme values and see how things perform?

Mark

[Mark Kaplan]

Another Capacitor question:   There are three "bathtub" type capacitors on the rear wall of the chassis.

These caps have TWO terminals each and the right hand side of each is labeled .01uf  200V.   There is a resistance of about 40 ohms between the two terminals of each of these capacitors suggesting there is a coil inside the bathtub.  The left hand terminals of all three capacitors are joined together and go to the sensitivity control.   What are these capacitors  (labeled part 7204) and how do you test/replace the 0.1uf cap in each one since it seems to be linked to the coil inside each can??   The right terminal of the capacitors go to the cathode/suppressor grid of each of the three 58  IF tubes.  (see attached photo to make my explanation clearer).  Thanks for any help/suggestions you can provide.

Mark

David C. Poland said:

Mark -   Not sure the resister color codes were very standardized in the very early 1930's.

*My info indicates:

The 2nd AF tube pin 3 grid to ground resistor is 1/2 meg   (500,000 ohms) 

2nd electrolytic - 8 MFD  (use a 10 MFD)  from 100 volt lug of power resistor to ground. 

              (First 8 MFD electrolytic is from high end of power resistor to ground.)

Don't have power resistor section values, but from the high end, voltages at the lugs should be:

                   250V     100V      50V      25V (unused)       and ground

The IF circuits have a 1/2 meg resister between the AVC line and the IF Tran secondary grid circuits.

The last IF transformer secondary - the center tap has a 250M resistor to ground and a 1 meg to the AVC line back to the IF tube grid circuits.

Hope this helps.

[mike hadley]

Mark;
The bathtubs you mention are similar to those used in the allwave fifteen, during the recent refurb of my fifteen I removed the bathtubs and restuffed, I have attached a photo of the internals, coil to the right and cap to the left, the red dot on the can denotes the coil cap junction.
Mike

[Mark Kaplan]

Mike,

How did you remove the "tar" to get at the wire for the coil/cap junction.  I assume you left the coil in place and just replaced the capacitor in the can?

Thanks,

Mark

[mike hadley]

Mark;
It takes a few minutes work with a hot iron (800f tip) and some soderwick to get the back off, be careful poking around as the wires from the inductor are quite fine, the pitch is easy to remove as the bathtubs are lined with wax paper before the original contents are put in,
Desolder the red terminal and carefully separate the cap and inductor wires, if you are careful you could even snip the old cap wire close to the terminal in the can,
the new cap is soldered between the lug and the bathtub box, there is enough room to get a polypropylene axial cap in there.
If you look carefully there is a small notch on the back cover where the original cap was earthed
Mike

[Mark Kaplan]

Mike,

Thanks,  I'll get started on those tomorrow.

Regards,

Mark

[mike hadley]

Mark,
A quick tip, when you desolder the lug nearest the red dot, melt the solder with your iron tip, poke the tip onto the eyelet, then flick the molten solder into some old newspaper, being careful not to burn yourself in the process, this works every time and leaves the eyelet clean for the new cap lead and solder,
Mike

Btw the caps in my bathtub filters were all leaky.

[Mark Kaplan]

Well, I've got the set recapped except for the small square brown capacitors which I assume are Mica and likely OK.  Initial tests have revealed a problem with the volume.  When I first turn the set on, as it warms up, I have good volume for maybe 5-10 seconds.  Then the volume decreases rapidly and stabilizes at a relatively low level (broadcast band).  It's almost like the AVC is doing its job too well and keeping the volume low and steady.   Selectivity and sensitivity on AM band seems OK.   I hear almost nothing on the shortwave bands.   The multi position sensitivity control seems to work and I have it set at maximum sensitivity on AM band.

I tried adjusting the four IF coils using the screw adjustments on top of the coils but cannot get any improvement in volume.

I'd appreciate any thoughts or suggestions on where to start looking for a problem.   Oh, yes, I tested all the tubes on a TV-7 and they are good.   I have both a Wunderlich tube and a modified 55 tube I can interchange for the 2nd detector but it makes no difference.

I will start checking some voltages tomorrow after I figure out how to stabilize the large chassis on its side so I can get some measurements.

Thanks,

Mark

[Mark Kaplan]

Strange Voltage drop problem and solution:  Concurrent with the volume drop that I observed in about the first 30 seconds after turning the radio on, I also noticed a voltage drop, measured at the top of the voltage divider resistor, from an initial value of 255V down to about 175V.   Clearly something was dragging down the voltage as the tubes warmed up.  I rechecked all my capacitors and all seemed OK.   Next I removed  seven tubes from the chassis, leaving only the two 56 audio tubes.  I noticed a similar voltage drop when I turned on the set and an audio signal injected at the volume control likewise dropped in volume during the first 30 sec or so.

Next I removed the two 56 audio tubes (1st and second audio) from the chassis and amazingly , to me, the voltage held at about 255V.   I replaced those 56 tubes and now the set seems to play with LOTS of volume on AM and nice loud hissing on short wave.  I still need to align everything so this is just an initial test but I am thrilled and mystified.  I rechecked the two removed 56 tubes and they still look good on my TV-7 with no shorts.  Not sure why one or both of these tubes would cause the voltage drop but there you  have it.

Still more work to do tomorrow but for today I am a happy camper.

Would appreciate any theories as to what might be going on here.

regards,

Mark  (definitely NOT an electronics expert)