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 Post subject: Bradleyometer
PostPosted: Sep Fri 05, 2014 1:46 am 
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Ever see one of these?
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This is out of my GE S-22 (Cousin to the RCA R-7 Superette).

At first, it looked like a wirewound, but that's actually a stack of carbon wafers interleaved with metal disks. It's supposed to be 500K, but it has drifted up to a little over 1 M. (I'm going to use as is---tone control)

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 Post subject: Re: Bradleyometer
PostPosted: Sep Fri 05, 2014 2:33 am 
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I worked on a Stromberg 145 that had one of those for a VC and was tapped.


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 Post subject: Re: Bradleyometer
PostPosted: Sep Fri 05, 2014 1:39 pm 
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Interesting! And no, I have never seen one before. But I must admit that only recently (since joining this forum) have I ever looked inside of a pre-war radio. I was mostly concerned with 50s and above.

Thanks for the picture!

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 Post subject: Re: Bradleyometer
PostPosted: Nov Wed 19, 2014 1:39 pm 
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My 1936 Arvin 61M tombstone radio uses that same control for it's tone control.

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 Post subject: Re: Bradleyometer
PostPosted: Nov Wed 19, 2014 4:11 pm 
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Hard to judge the scale, how big and heavy is it?


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 Post subject: Re: Bradleyometer
PostPosted: Nov Wed 19, 2014 4:24 pm 
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You will find these in many 30s Crosley sets as a tone control... when you see them, buy them!!!

Jim

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 Post subject: Re: Bradleyometer
PostPosted: Nov Wed 19, 2014 5:43 pm 
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adicarlo wrote:
Hard to judge the scale, how big and heavy is it?

buried in the radio ATM----I think its about 1" X 1.5"---fits in the same space that a "normal" pot would.

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 Post subject: Re: Bradleyometer
PostPosted: Nov Wed 19, 2014 9:28 pm 
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The French set I just worked on uses two of similar design for the filament controls. I think I have a few of the US made ones as well, but the French ones are not inside of a case.

viewtopic.php?f=1&t=263205

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 Post subject: Re: Bradleyometer
PostPosted: May Wed 11, 2016 12:25 am 
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As the 1933 Zeniths use these Bradleyometers, I have been working on repairing them.

The problem is they typically have increased in resistance by as much as 3 times. Two I have which are supposed to be 500K ohms are now 1.5 to 1.6 MEG ohms.

Allen Bradley made these by interleaving small carbon wafers with metal disks. Essentially making a "Stepped potentiometer". My plan is to replace these carbon disk stacks with a small circuit board with surface mount resistors.
A friend helped me design a small PC board and send the "Gerber" files to a manufacturer to have them made. Each board will be about an inch long and a half inch wide. Being so small, I had to make them a bit oversized to fit within the PC board manufacturers requirements. Plus, there is a minimum order, which means I now have 240 of these little boards! :shock:

They have 28 steps between min and max volume and I made provisions for a loudness tap, which the Zenith pot uses. (actually 6 positions to choose from)

My next step is to figure out the resistance contour or taper of the Zenith pots. Hopefully the taper is ok even though the resistance has increased. Then solder on the proper resistors and assemble the pot.

The contacts are gold plated to help with aging and noisy controls, though how much the plating will last with a wiper arm going across is yet to be seen...

I will probably make these little boards available as I do not forsee using all 240 of them! :lol:

I will post more as my project progresses.

I hope to find some more of these pots at Kutztown.

-Steve


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board.jpg
board.jpg [ 110.72 KiB | Viewed 3677 times ]

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 Post subject: Re: Bradleyometer
PostPosted: May Wed 11, 2016 3:41 pm 
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You already know that the Strat uses these for both volume and tone. The volume control has taps for loudness control which make it rather complicated. Other than one of the carbon disks being damaged , mine are fine. The loss of the one disk causes a slightly elongated spot where the volume does not change but is hardly noticeable. Loss of more of the disks would be bad.

It would be nice to have a spare set of restive elements incase a failure ever did happen. The loudness contour is one of the unique characteristics of the Strat.

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 Post subject: Re: Bradleyometer
PostPosted: May Wed 11, 2016 4:26 pm 
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Russ, yes, the 1000Z uses type AAS for the Volume control and type AA for the tone. Both are the dual-pot design.

Very much custom made for the 1000Z.

Zenith used them almost exclusively on the 1933 console chassis with the 4 window escutcheon, but by the 1934 models, they were no longer used.
It is interesting that they were chosen for the 1000Z which is a 1935 model.

The contour is important as is the loudness tap. I know my little circuit board has less "steps" (28) than a full Bradleyometer which has around 50, but it shouldn't be too big of an issue. At least I think.

I will soon find out, right? :D

I am studying the audio taper contour of the 63-231 volume control as well as where the loudness tap is. I provided 6 convenient tap pads on my board for locating it as close as I can to where it should be.

The two samples I have of the audio taper isn't what I expected. Though they do agree with each other.
I would have expected a nice logarithmic curve, but it is not.

Another Bradleyometer which I have (not from a Zenith) has a VERY nice audio taper curve. All 3 have increased in their values by roughly 3 times...

More to come.

-Steve

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 Post subject: Re: Bradleyometer
PostPosted: May Wed 11, 2016 9:47 pm 
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You know, Steve, the next guy who opens your Bradleyometer is going to have a heart attack. :)


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 Post subject: Re: Bradleyometer
PostPosted: May Sun 22, 2016 1:38 am 
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Dave, yes I know. Finding a PC board inside will be a unusual surprise indeed!

Now, I'm working on the selection of resistor values to put on my board. All 27 of them. I wanted to duplicate the audio taper, so I carefully read the resistance as I rotated the control. Its not easy as there is backlash in these things as well as any error of my rotation. So, I graphed the resistance going up, then coming back down.

The taper looks messed up on the two samples i took. They both have risen in value about 3X the original 500K. Even so, I would think the taper would have stayed intact.
Both samples are very similar.

As an experiment, I took a 3rd sample of a non-Zenith used bradleyometer which ALSO has increased in value about 3X the marked resistance rating.

It has a beautiful audio taper curve!

So, I suppose I need to take more samples from the Zenith radios I have. I have a feeling they will be the same.
At this rate, I may as well make it a linear taper...

-Steve


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bradley-curve1.jpg
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 Post subject: Re: Bradleyometer
PostPosted: May Wed 25, 2016 4:58 pm 
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Well, as there are no thoughts, I asked a guy here at work, who came up with this plot in Excel.

I suppose its a reasonable compromise.

-Steve


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bradley-curve.png
bradley-curve.png [ 20.7 KiB | Viewed 3401 times ]

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 Post subject: Re: Bradleyometer
PostPosted: May Wed 25, 2016 5:15 pm 
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Making a PC board and all that is an admirable effort, but the problem of high resistance drift in carbon resistance elements is common and frequently due to absorption of humidity. This is reversible. Designers accounted for and relied on the heat produced under a radio chassis in daily operation to keep the resistances in tolerance. When the equipment is not used daily, humidity gets in and the resistance goes up. Same thing is true of fixed carbon and carbon comp resistors.

Very often, a week or so of continuous operation with the set in its cabinet, or a few days in a drying oven, will bring the resistance down quite a bit, usually to the point where a resistor or a carbon control can still be used. I always try to drive the moisture out of old stuff first before taking more drastic measures.

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 Post subject: Re: Bradleyometer
PostPosted: May Wed 25, 2016 6:22 pm 
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Chris, Now you tell me! :wink:

Yeah, I will have to try heating them up and see how close they get. It would be nice not to disturb the originals.

however, I have a couple I will need to fabricate because I don't have the originals. So, not all is lost in my efforts... I hope?

Thanks for the advice!

-Steve

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 Post subject: Re: Bradleyometer
PostPosted: May Sat 28, 2016 2:12 am 
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Great job Steve! I'm looking forward to seeing a picture with all of the resistors soldered in place.

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 Post subject: Re: Bradleyometer
PostPosted: May Mon 30, 2016 3:16 am 
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Not a waste of effort at all. If a carbon resistor has changed value because of temperature or voltage stress, or it's been physically damaged, no length of time in a drying oven is ever going to bring it back. What you are doing is the kind of thing I love to see, because once you put the covers back on those Bradleyometers, they will look and work like originals. I'm sure I am not the only one who says, "oh no, what happened here?" when looking under a 1930s radio chassis and seeing a brand new replacement pot in bright yellow "Japaneseum*".

(*Japanesium, the only non-ferrous metal that rusts).

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 Post subject: Re: Bradleyometer
PostPosted: Jun Wed 01, 2016 1:58 pm 
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Thank you both John and Chris!

Here is the board with the resistors in place.
I've yet to give those other two a good drying in an oven. But as I need more of them, I'm still going to continue fabricating at least a few.

-Steve


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bradley-board-resistors.jpg
bradley-board-resistors.jpg [ 80 KiB | Viewed 3211 times ]

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 Post subject: Re: Bradleyometer
PostPosted: Jun Sat 04, 2016 7:10 pm 
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Yeah, still working on this...

I would like to lubricate the wiper arm holder, which is made of Bakelite. It pivots on a metal pin and has teeth which engage the gear on the knob shaft.

I'm thinking a silicone based grease as it is best for plastic, but I am unsure about Bakelite.

Any thoughts?

My plan for the wiper arm on the contacts is to use GC Electronics "Tunerlub" Cat. No. 26-01 which I believe is a petrolatum (Vaseline?) based lubricant for switch contacts.

-Steve

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