1938 Detrola RF Coil

[Daniel Bingamon]

I have a bad coil on a 1938 Detrola Model 227 Table Radio
The schematic below in the upper left the broadcast and short consists of four coils on one form.



The lower photo shows the 0.05 uF capacitor attached to the coil this seem to identify the antenna lug.
The very next position to the left has a resistor and capacitor in parallel and this seem to identify the pick up coil for the shortwave.    I've number the coils 1, 2, 3 & 4
Now, I can tell the this shortwave pickup coil (2) which eventually makes it to the grid of the 6A7.   And this is the  closest winding to those terminals.   Now, near the end of this coil there is another shorter coil on top of it for a short distance.  Is that one that is on top of it coil 1 or 4.
Are the other coils further back 3 & 4?

[rsjsouza]

The lower photo shows the 0.05 uF capacitor attached to the coil this seem to identify the antenna lug.

It should be the 0.001 tied to the binding post that also ties to the wire in red, no?

Now, I can tell the this shortwave pickup coil (2) which eventually makes it to the grid of the 6A7.

Indeed. It will make its way through the band selection rotary switch.

And this is the  closest winding to those terminals.   Now, near the end of this coil there is another shorter coil on top of it for a short distance.  Is that one that is on top of it coil 1 or 4.
Are the other coils further back 3 & 4?

It is, since the coils 1 through 4 are air coupled.   

One notice: remove your e-mail address from the image - radiomuseum tends to insert those artifacts in the files they distribute.

[ingalopez]

Coils 1&2 are clearly the short wave band, and the two beefier ones, 3 &4 the ones for the broadcast band.
How do you know there are bad? no continuity? Doesn´t look toasted, may be a careful inspection may reveal a broken wire.
Good luck

[vk6zgo]

Interestingly, that schematic looks like the radio is designed to be a DC only radio, not the much more common (outside Oz) postwar AC/DC type, as you have no rectifier, & the speaker field coil is connected to the incoming Mains supply.
You will have disappointing results if you plug it into 120v ac.

I take it all back----the envelope-less "squiggles"  at the bottom right are the rectifier.

[Dave Wise]

The rectifier is type 25Z5, with two sections wired in parallel.  Omitting the circles around the electron tube symbols sure makes them hard to see.

[Daniel Bingamon]

The schematic replaced with same from another source.

Yes, the red wire is the antenna wire and it goes to the 0.001 cap.

There is a part of the coil that appears burnt and there are broken wires there in the middle.  Hard to see in the photo.

This is my attempted redraw of it in Tinycad:

[Daniel Bingamon]

Also, regarding the 500pF cap and 100K resistor in parallel C7 and R4 near L2 on the new drawing.  What does that do?  How does it contribute to LC?

[andy3055]

To begin with, replace those Dublier and Sprague wax capacitors! Those will be leaking.
A good source is Mr. Carlson's Lab YouTube channel.  He restores a lot of these old radios and you can pickup a lot of ideas.

[Daniel Bingamon]

These are original photos.  The caps are already changed.
When bought it there was linkage missing on the variable capacitor, the IF was extremely out of tune.
I'm beginning to wonder that the tuning circuit was modified by someone beforehand in a failed attempt.
The front pushbuttons were also missing.

[Dave Wise]

Errata: R8 cold end goes to 6Q7 cathode.  6Q7 grid is not connected to cathode.  Tone control capacitor is on output transformer primary not secondary.

L1/L2 is the SW transformer.  (Single-layer windings.) L3/L4 is the MW transformer.  (Universal windings.)  The unused windings are shorted out.  C7 couples SW signal to the 6A7 signal grid.  R4 gives DC return.  Together, they help avoid overloading near strong signals by biasing the signal grid negative.  (Study "grid-leak detector".)

[Daniel Bingamon]

On the errata, yeah you're right.  Thanks
Does C7 contribute to the Frequency determination or does R4 prevent that?

I'll look up the Grid-Leak Detector as you suggested, I have seen that in "Impoverished Radio Experimenter" series of books - I have all six.  Good series.
It just seems a little strange for the R4/C7 set to be between the Variable Capacitor and the Coil rather than closer to the grid.

[Daniel Bingamon]

What does "Universal Windings" mean?

[Dave Wise]

C7 does not affect frequency, it's just a coupling cap.  For some reason, Detrola wanted C4/R7 only on the SW band, so they have to come before the switch.

"Universal Winding"... Let Me Google That For You... https://www.antiqueradios.com/forums/viewtopic.php?f=19&t=303945     https://antiqueradios.com/forums/viewtopic.php?f=19&t=290185    https://ieeexplore.ieee.org/document/1696757

In your photograph, they are the windings at the top of the screen.  Machine-wound in a zigzag pattern that (a) reduces parasitic capacitance, and (b) builds up layers without needing a bobbin.  The SW windings are less inductance and therefore get by with a single layer.

[Daniel Bingamon]

Many thanks.  Now that I know that I can measure each coil and check the inductance and with the variable capacitor and trimmer values I can determine if the wire breaks are in a place that makes the right inductance or not then I might be able to repair the coil.   If I can't repair it then I might try to use portions of it to produce a replacement.  I would want to avoid have to replicate these "Universal Windings".   The coils I normally make are limited to plastic sewing bobbin.

Some people would remove the SW band and just try to get the AM working - I don't want to do that, this is a challenge and a way to learn new old things.

[Daniel Bingamon]

I now have 8 wires connected to this coil and I have some measurements.
Going from front (simple coils) to back (Universal windings), this is what it's showing as:

40uH
13uH
243uH
1953uH

I'm thinking that this 1953uH value I thought was a bit high for the AM band but when you put the 50pF cap in series in the circuit it changes the inductance.
The 40/13 set would make sense being the SW windings. 

[rsjsouza]

At what ftequency did you measure the inductances? This can show some variation.

I am not entirely sure of the order of coils in your list, but the 243ųH is compatible with a tuning 47-470pF variable capacitor typically used to cover the resonance frequencies of 530 to 1600kHz.

But it seems the coil is not broken. I have heard of coils changing inductance due to humidity accumulating on the silk insulation, but this is reversible (at least in part) by re-baking the coil and applying a new coat of wax to keep the moisture out.

[Daniel Bingamon]

The coil was broken.  I repaired the breaks.  I measured the inductances with a Proster BM4070 Digital LCR Tester.
The tuning capacitor I measured about 17-350pF BC and 13-181pF on LO.
Just trying to determine if the coil is working right or baked and shorted.

[Daniel Bingamon]

I connected it to the variable capacitor, the other winding to a short antenna lead and ground and the ground and VC to an old Radio Shack signal tracer like a crystal set and it's tuning in a 3 mile away 50KW station right where it supposed to be.   I'd say the AM winding are now working after fixing the broken wires.

[Dave Wise]

Definitely making progress!

[Daniel Bingamon]

It just bugs me that the primary winding on the AM coil on the antenna side is 1953uH.  That just seems awful high.  As a transformer that would then go to the secondary which measures 243uH and my spreadsheet calculated 228.3 so that's in the ball park.  But that inductance ratios 8:1