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 Post subject: Rebuilding the 1920's Homebrew Set
PostPosted: Oct Tue 06, 2020 3:55 pm 
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I started a brief 'hello' and intro thread here:

viewtopic.php?f=1&t=384927

Wanted to use this thread to detail some of the progress on rebuilding / restoring a homebrew set I recently acquired. Seems like the general feedback is guiding me to suspect it's a mid/late 1920's or possibly early 30's build. Though, of course with anything homemade there is no manufacturer insignia, date, model, serial, etc. So seems like that is the best we can pin down. With most any antique I come across, I generally enjoy keeping as much original as possible, but above all, it must function!

Briefly poking around this set, it seems this is going to need A LOT of work, cleaning and rebuilding. Looking at the pics, it's a little beyond the 'nice antique patina' stage and more into the 'ick, gross, wash your hands! mold and mildew' stage. My idea is to document, clean, repair, restore and rebuild - keeping as much of the old as possible, but also add a bit of my own 'homebrew' to spruce it up a bit and send it on down the line for the next 100 years. Hopefully this will be a bit like putting a nice frame around a classic work of art.

So if you are reading this around 2120, I will be long gone and turned to dust, but hope you are enjoying some nice music and the warm glow of the tubes.

So - on to a few 'as found pictures':


The front:
Image

Original guts:
Image

Little better view with the vacuum tubes removed and set aside for safe keeping. Guessing by the kludge of wires at the transformer in the lower left, this would have originally had two 'hedghog' transformers? Suspect one may have died at some point and been replaced by the rectangle brick in there now and the current hedghog is also dead. So looks like some rewinding in my future?

Image

Point-to-point wires under the chassis. If I'm not mistaken it looks like the basis for the 'breadboard' might have been a plank of mahogany. Ideally, I'd like to leave this as untouched as possible. Seems the wires are in at least 'usable' shape:
Image

Electricity isn't the only thing that might 'bite' on this set! Though this duo might have given up the ghost and been quietly desiccating for the past half century.

Image



Cleaning up some parts / comparison

Mica caps. These appear to be a little low on the capacitance but as one solid block of phenolic, not much way to crack them open and get at the guts. I suppose adding a capacitor to the backside should get the values to the required levels. Guessing 00025MFD would be 0.25nF in modern nomenclature.

Image

Couple of tube sockets after a swim in the ultrasonic bath. It's almost as if they layer of grime has protected everything underneath!

Image

Testing:

Looks like the bathtub caps are shot... they are labeled "5 MF" - which I take to be 5 uF with current designations. Luckily, it seems like it should be easy to melt out the tar, embed some modern caps and scoop the tar back in. So we can keep the old look but get the capacitance back where it needs to be.

Image


Surprisingly, the worst looking cap is the better of the two, though still falls a bit short of 5 uF.

Image

On/Off switch - apparently the grease had turned to glue and in combination with the crudded up terminal (the technical term) - it was not making contact. Luckily, with bending a few tabs, it comes apart for cleaning, too.

Image


More to come...


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 Post subject: Re: Rebuilding the 1920's Homebrew Set
PostPosted: Oct Tue 06, 2020 4:42 pm 
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This should be a fun radio to get working!

As for capacitor values, they can have a wide variation in tolerance where they are used in bypass/decoupling as well as coupling.
The main issue is with current leakage. Usually mica caps hold up well with age, so I wouldn't worry about them.

The bathtub caps are good candidates to melt out and install new capacitors inside. They are usually paper which have absorbed moisture and are leaky.
This will affect their value considerably on a cap meter such as what you are using.

I would more believe these are supposed to be 0.5 microfarad in value rather than 5.

Looks like a fun project! :D

-Steve

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 Post subject: Re: Rebuilding the 1920's Homebrew Set
PostPosted: Oct Tue 06, 2020 7:12 pm 
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Cool set. Those are straight line frequency tuning capacitors, I think.


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 Post subject: Re: Rebuilding the 1920's Homebrew Set
PostPosted: Oct Tue 06, 2020 10:03 pm 
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.00025uF is equal to 250pF which equals 0.25nF

There should be nothing wrong with the mica capacitor. How low is it? I have worked on 20's sets for over 50 years and cannot ever recall seeing one of those moulded capacitors bad.

And you would never find a bathtub cap of 5uF of that small size in the twenties!! More like 0.1 to 0.5uF. More than enough for decoupling.

The old 20's capacitors stand well over age. Don't assume because they are old that they are bad!


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 Post subject: Re: Rebuilding the 1920's Homebrew Set
PostPosted: Oct Wed 07, 2020 4:26 am 
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Thanks for all the input! You all might be right on those caps. After more cleaning I popped the cleanest one under the microscope and sure enough, there might be just a hint of a "." in front of the "5". Definitely no leading "0". So 0.5uF

Image

Image



I'll go back and check the other caps, too. Possibly with a different meter to double check. Based on condition, I was just expecting them to be bad, so when the numbers came up pretty far off the value, I didn't go much further. But if they're good, or even serviceable, that's one more thing we can keep OEM!


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 Post subject: Re: Rebuilding the 1920's Homebrew Set
PostPosted: Oct Wed 07, 2020 3:42 pm 
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That is a .5 mf... or old speak "half a mike"... Leading "zeros" are not common practice in the 20's...

A 20's era 5.0 mf paper cap. would be roughly 10 times larger or more from insulating fillers and internally comprised of 0.1 to 0.5 cap packets all wired in parallel.

Modern conventions of labeling components do not apply to 20's, 30's components, same rules for drafting schematics, in the 20's it was more common to find a schematic drawn with diagrammatic component illustrations. Works fine for the unskilled radio fan but a bear to systematically trouble shoot...

In time find also that a digital muti-meter will tell lies by making erroneous measurement of resistance of an inductor and making similar errors attempting to make a leakage check of a leaking capacitors as well as attempting a capacitance measurement of a leaking capacitor.

A common service bridge instrument will measure all but the very leakiest capacitor...

A once common, analog VOM will also do well and be FAR less miss leading as to a component value....

Move carefully when cleaning as it is possible to ruin components. Much of 20's radios are not entirely made from Bakelite, but of hard rubber sometimes called Vulcanite, also of a shellac and coal based thermoforming compound that has no official name other than "mud". Certain coil forms are of cellulose acetate. Both mud and the hard rubber will flow and melt at soldering temperatures, cellulose coil forms can burst into flame, it has happened to me simply soldering a loose wire...

Your radio is a TRF with no apparent method to control feedback from triode RF stages, other than the orthogonal placement of the RF coils. Generally that would mean the "B" voltage for the RF stage will likely be no more that 67-1/2 volts. This is a "knock-out" type of set with point to point wiring, this should not be re-routed as the "instructions" used to build may have wiring laid out to reduce unwanted oscillations.

Fun radio when working correctly.

Advice for any 20's radio, re-solder every connection ,clean every electro-mechanical joint.

If the insulation has failed on the wiring underneath replace the wiring. Old rubber insulation isn't and will leak at points weer it crosses over... chas

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 Post subject: Re: Rebuilding the 1920's Homebrew Set
PostPosted: Oct Thu 08, 2020 11:00 pm 
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Would have been someone's home made pride and joy many years ago. Good to see the old set being brought back to life for another run.


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 Post subject: Re: Rebuilding the 1920's Homebrew Set
PostPosted: Oct Fri 09, 2020 4:38 am 
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Can't thank everyone enough for all the valuable info and feedback! Had a couple of quick questions:

Any tips on removing those Kurz-Kasch dials? Three small nuts on the back - I removed those and the top of the bezel seems loose. Bottom seems hung up on the small dial and wheel which turns the big wheel. I don't see any set screw in the knob and they don't seem to slide off with any reasonable pulling (though I think slide-off knobs were later, anyway?0

Also, I made a schematic, but it seems different from two comparable radios I was looking at... Freshman Masterpiece and AK model 20. Additional notes below. If you have any thoughts, it would be great to have the input


Anyway, here are some more more updates and pics

So I spent much of the past couple of days numbering, checking, testing and playing connect the dots! If anyone wants to keep track, those vacuum tube sockets are a bit 'interesting. For the 201 tube, as pictured, the holes toward the bottom of the image are filament, the upper left is grid and the upper right is plate. The tube socket rotates those 1/4 turn clockwise, to the left two terminals are filament, upper right is grid and lower right is plate.

Red = general connections, Green = ground network, Blue = original markings
Image

Image

Carefully desoldering everything and keeping all the wires ordered... about 40 wires, all cut to fit one specific spot, and all the color of dirt/dust!

Image


Big fight of the evening was one screw in the center of the formica face place which stubbornly refused to budge in the wood. Even my sharpest/nicest flat blade screwdriver with all the force I could muster still stripped out the head before it would budge. Finally had to get a dremmel with a diamond tip and carefully grind a new slot so I could get some real twisting power. In the end, it finally gave up!

Image

Finally a bare wood board. Hopefully will have some time this weekend to scrape off the layer of gunk and see about refinishing. I'd like to get the top clean/presentable, but plan to leave the bottom original as testament to time. I just need the hot tip on those dials, now!

Image



With all the numbers and wire tracing, I was able to put together this schematic. Slightly different than what I was considering as 'similar'... a Freshman Masterpiece or an AK Model 20.

[Edit: Much better schematic, revised with community input, is attached in a later post]

Image


On the AK, variable pots for filament control seem to be on the A- side. My set seems to have the filament control on the A+ side. Not a big deal, I suspect... two ways to skin the polarity cat?

Also on the AK, one filament control pot seems to control the first two RF input tubes and another pot seems to control the detector + two audio amp outputs. On my set, one pot seems to control the 2 input tubes and 2 output tubes as one group, then another pot controls only the detector tube... again just a choice of where to control and how?

Lastly, the hacked-in audio transformer to replace what I suspect was one dead hegehog...The secondary side of both transformers was tied together, but a third piece of wire had also been tied in, but was hanging loose when I got the set. I suspect this should be grounded to A-?

If you see something that looks out of place, give me a shout! Outside the A+/- and B- terminals, nothing was labeled... and I'm a chemist by profession!


Last edited by KCs_in_KS on Oct Sat 10, 2020 4:41 pm, edited 1 time in total.

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 Post subject: Re: Rebuilding the 1920's Homebrew Set
PostPosted: Oct Fri 09, 2020 5:40 am 
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That schematic is the work of a professional!

I have looked it over and it appears to be correct. Having the two .5 mf caps paralleled would be fine...

The "flying lead" is a connection for a "C" battery the probably floated around in the cabinet. The "C" will have no current drawn so it would last at least 5 years. Some set designs were without a "C" thus this apparently was initially that may be why no terminal was brought to the terminal board for a "C", thus, added later. When the owner "discovered" the benefits, longer tube and "B" battery life.

I cannot recall how the KK vernier is removed. Wait until someone can tell you how, Bakelite can break easily...

FWIK the panel is hard rubber, the oxidized green is a tell tale, scratch and sniff the sulfur. The the high sulfur content of the panel corrodes brass and steel hardware.

Beware cleaning the plate circuit rheostat. The construction is a roller and metal band bearing against the resistance strip, supposed to be "noiseless". The carbon film is attacked by almost all solvents... Try a dry cotton swab...

The "A" battery voltage is 6.3 to 6.6 the voltage from a lead/acid battery. The rheostats provide a voltage drop for the filament a bias from the drop as well as a volume control. If too low an "A" voltage is used the surface of the thoriated filament will be depleted and no "new" thorium will come to the surface. The tubes will become "paralyzed" of the course of time... Proper filament temperature is essential to have slow migration of thorium to the surface....

IMHO, use a pair of vintage transformers, they do not have to be Hedgehog, Thordarsons, Peerless, Dongan, Federal, ACME, Freshman, General, Kellogg, Karas and many more made audio transformers, some of which have stood the test time and are very serviceable. These will have sufficient turns and winding resistance to provide the correct load for the tubes and ample gain for the audio...

The detector grid condenser is typically for BC band 250pf, call it in nf or mf or mmf. That, is the working value. The purpose of the separate filament rheostat for the detector is for the radio fan to tinker with the detector to make it more sensitive to weak signals at the expense of volume, which is recovered by advancing the RF and or the AF rheostats. Access to the grid leak was important too for the same reason, that is why it was in a replaceable cartridge and also the leak often failed by going very high in resistance. The first "B" battery in the "B" string had a number of B+ taps from 12 to 22-1/5 volts. That also was for sensitizing the detector....chas

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 Post subject: Re: Rebuilding the 1920's Homebrew Set
PostPosted: Oct Fri 09, 2020 11:48 am 
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The Kurz Kasch dial is relatively simple.

To remove the front part, you will need to gently tilt the tuning knob up to compress the little leaf spring under it. (basically levering it down in back)
While doing this it should come off if you've removed the nuts on the back.

The dial itself is held on the variable capacitor shaft with a collet and nut.

Yes, all made of Bakelite, so be gentle. The only metal parts are the spring, collet nut and little studs. It relies on friction and spring pressure for the drive.

-Steve

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 Post subject: Re: Rebuilding the 1920's Homebrew Set
PostPosted: Oct Fri 09, 2020 2:20 pm 
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Congratulations on the restoration progress! That is a fairly common circuit for that era; two RF stages, a grid leak detector, and two AF stages. Put an ad on the Classified, and someone should be able to come up with a pair of good audio transformers.

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 Post subject: Re: Rebuilding the 1920's Homebrew Set
PostPosted: Oct Fri 09, 2020 3:15 pm 
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Here are a couple sources of new transformers.

https://www.arbeiii.com/Transformers/
https://www.tubesandmore.com/products/t ... tage-10-ma

-Steve

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 Post subject: Re: Rebuilding the 1920's Homebrew Set
PostPosted: Oct Fri 09, 2020 3:47 pm 
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Hello guys,
He did outstanding job on the restoration so far .
Can't wait to see the finished radio .
Sincerely Rich


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 Post subject: Re: Rebuilding the 1920's Homebrew Set
PostPosted: Oct Fri 09, 2020 4:25 pm 
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Incredible job ! But I have a question on the schematic.
The 2nd RF grid does not have a DC return path as shown ? On all my AK20 and Freshman
radios, they always have a return path thru a resistor or transformer.
On this schematic the DC return path is blocked by the two paralled .5 uF caps.
EDIT::: OOPS, now I see the connecting dot, so ignore my question.

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 Post subject: Re: Rebuilding the 1920's Homebrew Set
PostPosted: Oct Fri 09, 2020 6:04 pm 
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As more and more "common" three dialers are rescued and restored. I am seeing that the circuits are not so common. The "Neutrodyne" (patent) is the only one without variation. TRFs appear to have a variety of devices or circuits to prevent oscillation of the stages as well as control of the overall gain. This radio is no exception... chas

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 Post subject: Re: Rebuilding the 1920's Homebrew Set
PostPosted: Oct Sat 10, 2020 2:26 am 
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OK - another round of thanks for all the great info in order!

...and another couple questions:

The radio has three variable pots, what would these typically be called? The main one controlling B+ to the RF coils, which I guess would be "volume"? Another pot controlling voltage to the detector tube filament and a third pot controlling voltage to the other four tube filaments. Believe I've seen one dial called 'color'?

Any tips from the pros on how to restore the hard rubber face plate? Looking under the dials, it seems it was originally sort of a semi-gloss black. Can I just scrub/sand/polish with progressively finer grits to restore the shine?


On to more good stuff...

Chas - Funny you should mention the addition of a C battery - I some extra pieces came with the radio... one of which happened to be a C battery! Though I didn't exactly see any terminal for it! So that explains the loose wire which would be C-, then the C+ should also share the A- post? Darned battery seems to be dead, though... so likely past that 5-year time frame! Have to do some restoration there, too!

Image

Paul - I revised the schematic to show the new C battery, added some component values, and corrected my error in showing ".25pF" cap on the grid, where it should be ".25nF", Changed the A battery to 6V, and reorganized to possibly make it a bit easier to see the caps/grounding scheme. This should be an equivalent circuit to what I posted earlier, though I tapped it out on a laptop over lunch, so if you see any goofs, just let me know.

Image


Steve - That pressure / bending tip did the trick for the dial - success there! Since the top two posts seemed to slip out fairly easily, I went ahead and slid them out, then a bit of pressure 'up' on the bottom dial to press the spring down, like you said, and the bottom popped out. ...and a sigh of relief!!! Now to do the other two.

Image


The main 'volume' pot is all disassembled, too. Thanks for the tip on the carbon trace... no solvents there! Though the shaft is in a solvent soak right now. The copper shaft was essentially seized in the brass bushing. Down side is that it seems the copper is swedged to the whole assembly, so no easy way to take apart. Hoping the ultrasonics and solvent can work down between the shaft and get things loosened up.

Also - Thanks also for the hot tip onthe audio transformers... Have some components on order and yet another plan brewing!

Thanks again for all the input and help guiding a newbie!


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 Post subject: Re: Rebuilding the 1920's Homebrew Set
PostPosted: Oct Sat 10, 2020 4:00 am 
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There is un-vulcanized free sulfur in the hard rubber panel. If sanded and polished ti will develop sulfur flowers is some 10 to 20 years and look green, even if creatively sealed. I tried on a Maclite one tube radio with a hard rubber panel, looked great for several years now it is as green as ever, having been exposed to the UV from the florescence lighting. The green may be below the surface.

What some 20's builders did to dispense with the sheen of the hard rubber panel was to sand it with a medium grit. Using a block and backing strip of wood to keep the block parallel. The strip was incremented across the panel to keep all the scratch lines running in the same direction. When the desired non-reflective surface was achieved, the panel was rubbed with boiled linseed oil and allowed to cure, then repeat treatment with the linseed.

The local corrosion seen is in part a reaction to the free sulfur.

Three "C" cells usually comprise the "C" battery of 4-1/2 volts. The "C" battery is not required if the B+ is 67-1/2 volts. If 90 is used then the 4-1/2 is appropriate. There are better and easier to "stuff" battery cases and better scanned battery artwork out there. If you had the same brand in "B" battery then restoration of the set of batteries would be practical... A homemade set of "B" batteries will typically last about three years, if carefully assembled without shorting and qualifying the cells with a volt/load test.. Sunbeam cells are about the cheapest. about $15 for 45 volt battery...

There is a company that supplies a black phenolic sheet in 3/16" and 1/4" Norva plastics. The material and thickness goes in/out of stock. It is supplied in high sheen and with a pebble finish. http://norvaplastics.com/

RF gain, Detector sensitivity, AF gain. Many radio manufacturers would give the controls creative names like tone, volume, brilliance. Changing the filament intensity will not only change the volume but change the tone to some degree. Once, the listener decides on a station, changing the controls to get the desired volume without distortion and the most pleasing tone... There are a couple of stations that still broadcast quality music, that is where the TRF excess because of its broad sensitivity, as long as there are no adjacent interfering stations...... chas

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 Post subject: Re: Rebuilding the 1920's Homebrew Set
PostPosted: Oct Sat 10, 2020 6:09 pm 
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Some observations...

On the Kurz-Kasch dial, you have disassembled it but it is still not removed. It is a friction fit on the condenser shaft, a four-segment gripper-type of construction. The whole thing should pull off. Here is a picture of the back side friction piece.
Attachment:
2020-10-10_12-56-28.jpg
2020-10-10_12-56-28.jpg [ 23.24 KiB | Viewed 627 times ]


Your loose wire to the transformer secondaries could go to A- or to a C battery, your choice.

And to appease the nit-pickers, those are rheostats, not potentiometers.

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 Post subject: Re: Rebuilding the 1920's Homebrew Set
PostPosted: Oct Sun 11, 2020 12:47 am 
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Yes, that is a collet type of fastener.

-Steve

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 Post subject: Re: Rebuilding the 1920's Homebrew Set
PostPosted: Oct Sun 11, 2020 1:00 am 
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Great K-K vernier dials, love the 0-100 and 100-0. Pick the nits going up or coming down :lol:

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