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 Post subject: Precision 10-60 restoration
PostPosted: Sep Tue 07, 2021 12:09 am 
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Hi Folks

I am going to be starting the restoration of a precision 10-60 tube tester.

I have the 10-12s, and know them well...

The 10-60 has a number of variable resistors under the hood which are clearly for calibration of the various functions.

I did find one post on this here fro the 10-40: https://www.antiqueradios.com/forums/vi ... p?t=110265, which deals with the calibration, but not entirely.

I am having trouble using the search function on this forum with the term 10-40...

If anyone know of another thread which deals with the calibrating variable resistors on the 10-40 I would appreciate it...I have just started looking at it, but wanted to see what info was out there as I start.

Thanks!

Steve

Edited because I mistakenly entered 10-40 rather than what I have...the 10-60


Last edited by drworry on Sep Tue 07, 2021 4:26 pm, edited 1 time in total.

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 Post subject: Re: Precision 10-40 restoration
PostPosted: Sep Tue 07, 2021 12:41 am 
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Location: New Hampshire, USA
Not sure if this is helpful but I did find this: https://tubesound.com/precision-apparat ... be-tester/

According to the author, he’s not aware of a factory calibration procedure.


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 Post subject: Re: Precision 10-40 restoration
PostPosted: Sep Tue 07, 2021 12:50 am 
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Yes, Bob wrote that sometime ago, and it concentrates on the basic "electronamic" functions of the 10-12...not much help when it comes to the extra calibration resistors in the 10-60.

I actually wanted a 10-60 to see how successfully they incorporated gas and VR tube testing...

When I am done gathering info, I will go ahead and write it up for posterity....am hoping, though, that someone else has already done the equivalent of that!

Steve


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 Post subject: Re: Precision 10-60 restoration
PostPosted: Sep Mon 13, 2021 10:07 pm 
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Well, I am well into this project, and am finding it both instructive and frustrating...

First of all, I would like to know if any one else out there has one of these....It would be nice to be able to compare some findings.

For the purposes of this post, in other 10 series testers, the grid voltage is present and adjustable with the machine at rest (read meter not depressed) from 0-48.5 VAC.

On my 10-60, there is a 20 VAC potential on the grid relative to ground regardless of grid bias settings coming off of the 50V secondary. The grid bias adjustable settings do activate when the read meter button is pressed and are accurate (0-48.5 VAC).

From an examination of the schematic (as confusing as it is...) one can see from the circuit in red that one would expect to see a voltage potential when the meter is at "rest" (read meter button not pushed). The fact that I see 20 VAC rather than 50 VAC is likely do to some parallel resistances that are not really obvious.

I am hoping that someone who has either a 10-40 or a 10-60 to check this...the 10-40's schematic also looks like the 50 VAC should be there at rest. I am not really expecting someone to wade through this overly busy schematic to answer unless they have one...

There are other issues but I will raise them a bit later.

Thanks!

Steve


Attachments:
10-60 grid voltage.jpg
10-60 grid voltage.jpg [ 835.01 KiB | Viewed 442 times ]
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 Post subject: Re: Precision 10-60 restoration
PostPosted: Sep Tue 14, 2021 12:06 am 
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whoever drew that schematic didn't want anyone to truly understand the circuitry involved....

I've been mulling over the schematic of the Jackson 658 which is yet another mess...

-Steve

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 Post subject: Re: Precision 10-60 restoration
PostPosted: Sep Tue 14, 2021 12:23 am 
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The circuit is trivially simple. The transformer delivers the voltage and the meter measures the current.


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 Post subject: Re: Precision 10-60 restoration
PostPosted: Sep Tue 14, 2021 1:55 am 
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Actually, the circuit is simple...the drawing is not!

BTW, what is the purpose of the center tapped potentiometer? It turns out that there are some issues with the pot, and replacing it might be necessary...finding another center tapped pot would be challenging outside of a donor unit.

Steve


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 Post subject: Re: Precision 10-60 restoration
PostPosted: Sep Tue 14, 2021 11:56 am 
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You've got two threads now on this tester, one dealing only with the bad rheostat. That is going to make it very difficult to follow along, as you'll get answers in both places. Why?

You may be able to take a non centertapped rheo and actually add a center tap to it, easier than you can fix this one. Either try spot welding to the side of the winding (you've got a bad one to experiment with) or just a friction contact and you can drill/screw/glue to the cement frame of the pot perhaps ?

It looks like the purpose of the center tapped rheostat is to distribute the load on the transformer winding in the area that's being varied, or maybe to make the control adjustment less coarse, or both.

Whoever drew that schematic did a masteful job of making it as inscruitable and complex as possible. They decided (actually Hickok did this too) to keep each section of each switch in a "block" if you will, rather than distribute the contacts over the schematic to eliminate long wire runs. A perfect example of this is the on/off switch wiring.... they could have just put it between the plug and the transformer, but instead it's located within the switch bank, and therefore the wires have to go all the way across the drawing, and back. The technique of moving the contacts to where they'd eliminate long wiring traces, and calling it S1c or "part of S1" or something, I guess had not been developed yet.

This is why I re-drew the Hickok 539B to make it a little easier to follow a given circuit. But it took quite a few hours to do it.

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 Post subject: Re: Precision 10-60 restoration
PostPosted: Sep Tue 14, 2021 2:56 pm 
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Barry, I just wasn't thinking well....I thought that fixing a pot was a different enough topic to warrant it's own thread, and that the people who would have fixed pots wouldn't even look at a 10-60 thread.

Bad idea I guess.

I like your idea of drilling and adding a pressure fit tab...if I can think of a way to elongate the shaft.

My intention is to write a description of what I have learned about this tester...particularly around interpretation of the schematic and how to approach the unlabelled calibration pots...this unit has five..each one for a different function.

Calibration pots are as follows...any ideas more than welcome!

R32: all "electronamics" have one..it sets the meter to line set when your voltages are where you want them. No prob.

R19: all "electronamics" have one..it fine tunes the needle position when "read meter" is activated in quality test...I can not think of a reliable way to do this other than "qualitatively" test a known tube and set the needle as described in some of the literature...as you know, precision never described it..some have suggested setting the needle at a full scale current reading after computing FS mA from the schematic resistance values...been thinking about how to do that.

R27: adjusts the needle reading when testing a transistor for Icbo...I can certainly pick a transistor off the list, test it for Icbo in another unit, but he value I get won't be under the same conditions as applied by the 10-60..I will think more on this.

R7: fine tune for gas sensitivity...I like the idea already put out there on doing this with a calibrated 5µA leakage set up.

R23: haven't got ot his one yet...

SAteve


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 Post subject: Re: Precision 10-60 restoration
PostPosted: Sep Tue 14, 2021 3:03 pm 
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See if you can find a "shaft stretcher" on Ebay lol. No, not that kind .....

I suppose you can always have some precision shop weld an extension to it if there's no room for a coupler...

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 Post subject: Re: Precision 10-60 restoration
PostPosted: Sep Tue 14, 2021 3:58 pm 
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I just looked at my spare 50W 300Ω pot...no way to drill for a tab without drilling through the traces. I will go out to our local electronics recycling place to make sure they don't have one...

I am going to try a conductive pen that delivers conducting acrylic that I can get locally at circuit specialists to see if I can bridge the gaps...if that works, I may spend $50 for silver epoxy for a more permanent repair. The design of the original pot allows bridging on the sides of the coil that won't come into contact with the wiper...it might hold for a while.

Even disturbed, the pot allows bring the line to set for tube testing..(which is why I didn't notice it up front)..it is when testing VR tubes that one needs the full range of he adjustment from 70- 140V.

It is always something...

Steve


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 Post subject: Re: Precision 10-60 restoration
PostPosted: Sep Fri 24, 2021 5:43 am 
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So, both silver and nickel pens did not produce a long lasting fix to the discontinuous parts of the pot...predicted by y'all that it would fail!

I found a source for Ohmite 350Ω 50W pots that will fit (the chinese ones on Ebay will NOT fit..too big in diameter)...but will the vintage Ohmites will need to be extended...so I will fabricate some sort of extension to lengthen the shaft/screw collar.

I should get the battery powered spot welder tomorrow...fingers crossed...Bob, I could not get a simple capacitor arrangement to work...I will then try to repair the original using nickel to bridge th e open wiring.

So, a question....is the center tap really necessary? It would be far easier then to use one of the ohmites without also fabricating a center tap (though I did buy 2 so I could experiment with spot welding a center tap). If I had to guess, the addition of the center tap allowed better control, but does it help with heat/reducing needed pot wattage?

Steve


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 Post subject: Re: Precision 10-60 restoration
PostPosted: Sep Fri 24, 2021 11:37 am 
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Well, regarding your centertapped rheostat.... it appears that, assuming no "magic" stuff happening inside the transformer, this tester was designed for line voltage from 140V to 90something V AC line voltage. Since you're not likely to see anything other than 110-120 today, you could try just using an external rheostat (variac) and experiment with just using the 105V tap on the transformer as the input. I'm assuming your variac can range from below 100 to over 110 ...

If that works, you could certainly use the tester with an external variac instead of the rheostat for your LINE CAL adjustment when testing tubes. Make sure however the power transformer is not overheating. I cannot seem to fully reverse engineer the arrangement they've used here enough to decide exactly why they did it that way, unless as I mentioned earlier it's to allow for finer adjustment using a coarse rheostat, or a doubling up of the transformer primary winding in that area of the input windings.

Regardless, all you need is to get the LINE SET where it belongs, and at the same time, make sure you're not overheating the power transformer. I think a single, non center tapped rheostat can be used once you confirm this with an external variac. It would be a matter of picking one with the resistance that works with today's voltages, and also doesn't overheat itself.

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 Post subject: Re: Precision 10-60 restoration
PostPosted: Sep Fri 24, 2021 2:30 pm 
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To make it even more complicated, the rheostat is used far more than just for a line set when testing VR tubes...I actually think that the design encompassed these functions requiring the 140-70 range..

I have attached the overview of the VR function if one cares to decipher it..

Steve


Attachments:
B&W Operating Instructions for Precision 10-60.jpg
B&W Operating Instructions for Precision 10-60.jpg [ 517.81 KiB | Viewed 245 times ]
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 Post subject: Re: Precision 10-60 restoration
PostPosted: Sep Sun 26, 2021 3:01 pm 
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Well, sadly my trial of spot welding was not successful.

I used one of the battery powered devices used to spot weld thin nickel strips (0.1mm) to batteries.

Even at the lowest setting, and with the lightest touch, the current through the weld disintegrated the fine rheostat winding...

I attempted it on a couple of different rheostats all with the same result.

At this point, I will be trying silver epoxy next...

Even thought I found pots that I can make work, I am still stuck on how to affix a center tap now that the spot welding has failed me. Perhaps removing some of the conformal coating on the side and using silver epoxy to bind and then stabilizing the tap with a screw or lashing.

Sad...

Steve


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