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 Post subject: Couple of Hickok 600a questions
PostPosted: Mar Wed 25, 2020 12:56 am 
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Location: Traverse City, Michigan
I just picked up a 600a and it works great but has a couple of issues.
It came with a data roll but no frame or the clear plastic window.
Someone also replaced the bias pot but the shaft is way too short.
Any suggestion on how to lengthen the shaft?


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 Post subject: Re: Couple of Hickok 600a questions
PostPosted: Mar Wed 25, 2020 1:11 am 
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I hate to break the bad news, but its not correct in every way and will not replace the Hickok pot that has a special taper. Dan Nelson may still have some of these pots or you can try Roger Kennedy You can likely find the roll chart mechanism along with a window on ebay.


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 Post subject: Re: Couple of Hickok 600a questions
PostPosted: Mar Wed 25, 2020 1:13 am 
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You could cut the shaft of another pot and file one end of it to fit in the slot and then drill a tiny hole perpendicular (90 degrees from the slot) then find a very small taper pin or strong wire (like one from the pointed end of a diaper pin or even a tiny drill bit to fit). Use a file to smooth it down flat and put a sleeve over the repair for added strength and to keep the pin in place.

That may be the easiest rather then remove the pot and either replace the entire shaft or the pot. Unless you have enough space under the chassis without having to move a number of wires or parts to get at it.

EDIT-Or do the same but with the sleeve drill a hole through both the sleeve and the shaft a just use a new shaft and slide it down into the sleeve and clamp it on with the setscrew in the chicken headed knob.

Or you could take another shaft and slide it into the sleeve and drill both shafts and either use pins or one pin then the top shaft with a tiny set screw like they use for glasses.

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Last edited by Radiosmoker on Mar Wed 25, 2020 1:35 am, edited 2 times in total.

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 Post subject: Re: Couple of Hickok 600a questions
PostPosted: Mar Wed 25, 2020 1:32 am 
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Location: Traverse City, Michigan
Radiosmoker wrote:
You could cut the shaft of another pot and file one end of it to fit in the slot and then drill a tiny hole perpendicular (90 degrees from the slot) then find a very small taper pin strong wire (like one from the pointed end of a diaper pin or even a tiny drill bit to fit). Use a file to smooth it down flat and put a sleeve over the repair for added strength and to keep the pin in place.


That's exactly what I thought I'd do. Thanks!


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 Post subject: Re: Couple of Hickok 600a questions
PostPosted: Mar Wed 25, 2020 1:43 am 
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Location: Pasadena CA USA
You can do this but your tester will not be accurate for the reason Bill states, that the taper is wrong.


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 Post subject: Re: Couple of Hickok 600a questions
PostPosted: Mar Wed 25, 2020 1:51 am 
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J. Hill wrote:
You can do this but your tester will not be accurate for the reason Bill states, that the taper is wrong.

Is there a way to measure the taper on the pot to compare it to the original?


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 Post subject: Re: Couple of Hickok 600a questions
PostPosted: Mar Wed 25, 2020 2:09 am 
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Yes there is but it requires either the specifications of the original, or a friend with a 600a with original. Easy way would be to measure the resistance of the pot in the friends tester as the knob is turned from zero to 100 and plot a graph. Do the same with your pot. This will be more difficult because you will have to fabricate a way to install the knob. But this would allow you to make a cheat sheet to correct for the taper. Therefore if the roll chart said to set it to 60 , your cheat sheet may say 60=66 so you set it to 66. Not the ideal situation, but a way to get the tester back testing tubes again.


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 Post subject: Re: Couple of Hickok 600a questions
PostPosted: Mar Wed 25, 2020 3:08 am 
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J. Hill wrote:
Yes there is but it requires either the specifications of the original, or a friend with a 600a with original. Easy way would be to measure the resistance of the pot in the friends tester as the knob is turned from zero to 100 and plot a graph. Do the same with your pot. This will be more difficult because you will have to fabricate a way to install the knob. But this would allow you to make a cheat sheet to correct for the taper. Therefore if the roll chart said to set it to 60 , your cheat sheet may say 60=66 so you set it to 66. Not the ideal situation, but a way to get the tester back testing tubes again.

I like it. Sounds like a solid plan if I can't locate an original pot.


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 Post subject: Re: Couple of Hickok 600a questions
PostPosted: Mar Wed 25, 2020 5:05 am 
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eBay occasionally has a used 3K bias pot for sale. They tend to be a little pricey thought: $30 - $50.

For some reason, a 1K pot from a 539 seems to be a little more common, but I am not sure one of those will work. Max bias when properly calibrated is around 42 volts which means 42 ma through the 60 ma bias lamp, so THAT will work,but will the cal hold?
<shrug> Who knows?


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 Post subject: Re: Couple of Hickok 600a questions
PostPosted: Mar Wed 25, 2020 10:56 am 
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You cannot just make any old pot work. Unless your new pot, and it's taper, match the original exactly, your roll chart values will be way off. It does not take much deviation from the original part to do this.

Making a cheat sheet would require that you have the original pot, in working order, so you could generate either a lookup chart, or a specially calibrated dial for the replacement pot. I had started figuring out how to rewind the original Hickok pot in another thread about a year ago, but got side tracked with about a million other things. It requires a certain wire with a certain resistance per foot, and a tapered form to replace the usually burned up Hickok original form inside the original pot. To make a new one from scratch would have to be a labor of love requiring infinite patience, and still you'd have to have the original to compare your new attempt to.

Best bet is just finding one on Ebay or here if someone has a parts donor or, if as mentioned, one of the long time tube tester repair gurus has one still in stock.


best of luck

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 Post subject: Re: Couple of Hickok 600a questions
PostPosted: Mar Wed 25, 2020 1:51 pm 
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Location: Centennial, Colorado 80122
Here is a Dial Setting to Voltage Conversion Chart for the TV-7D/U, which will give you an idea of the resistance taper. I think its the same for the 600A and others. If all else fails you could add pin jacks and with this chart and a good DMM to set the bias. Of course a small Digital Panel Meter could be added to the 600A, but that would require a 2 range switch and scaling resistors.


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 Post subject: Re: Couple of Hickok 600a questions
PostPosted: Mar Wed 25, 2020 3:22 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Pasadena CA USA
posted before coffee
Made error
Deleted same


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 Post subject: Re: Couple of Hickok 600a questions
PostPosted: Mar Wed 25, 2020 3:52 pm 
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I'm not sure exactly what the previous owner has in there. Goes from 0 to 48v with nothing in between.
EDIT: However, this was done without a tube in the tester. Not sure if this matters.


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 Post subject: Re: Couple of Hickok 600a questions
PostPosted: Mar Wed 25, 2020 5:59 pm 
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Location: Centennial, Colorado 80122
Here is some additional information of the Hickok tube testers and those that used their design. Its easier to list those that used a linear Bias pot as almost all used the special Hickok taper. This is the extent of my knowledge.

Hickok Models with linear Bias pot's.
539x With separate Bias Meter.
580 Meter had the Bias voltage scales with corresponding setup values.
6000B This tester was introduced in the late 60's and Hickok used a unique method that allowed a linear Bias pot to be
used along with the setup charts for the 6000 and 6000A models. If you can perhaps you can see on the
attached picture 2 non linear Bias scales were added that correspond to Bias dial on the other models. They
finally got cleaver near the end.

Hickok designed testers with linear Bias pots.
TB-2x Separate Bias Meters


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 Post subject: Re: Couple of Hickok 600a questions
PostPosted: Mar Wed 25, 2020 6:05 pm 
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Bill Eccher wrote:
Here is a Dial Setting to Voltage Conversion Chart for the TV-7D/U, which will give you an idea of the resistance taper. I think its the same for the 600A and others. If all else fails you could add pin jacks and with this chart and a good DMM to set the bias. Of course a small Digital Panel Meter could be added to the 600A, but that would require a 2 range switch and scaling resistors.


That is so helpful Bill. Thank you!


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 Post subject: Re: Couple of Hickok 600a questions
PostPosted: Mar Thu 26, 2020 7:46 pm 
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Well, I may have other issues. I measured the plate voltage on the octal socket and got 238v. Should be 150v per the Schoo calibration method. Ugh.


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 Post subject: Re: Couple of Hickok 600a questions
PostPosted: Mar Thu 26, 2020 7:51 pm 
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I made a resistance to degrees rotation chart for a 539B..... I’ll see if I can find it and post it up. Hopefully it’s the same pot, not sure.

EDIT: viewtopic.php?f=8&t=343637&p=2892504&hilit=hickok+539B+bias+pot#p2892504

If that doesn't take you right to it, the PDF is on page 4 of that thread

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 Post subject: Re: Couple of Hickok 600a questions
PostPosted: Mar Fri 27, 2020 11:04 pm 
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Location: Pasadena CA USA
cdoyal wrote:
J. Hill wrote:
Yes there is but it requires either the specifications of the original, or a friend with a 600a with original. Easy way would be to measure the resistance of the pot in the friends tester as the knob is turned from zero to 100 and plot a graph. Do the same with your pot. This will be more difficult because you will have to fabricate a way to install the knob. But this would allow you to make a cheat sheet to correct for the taper. Therefore if the roll chart said to set it to 60 , your cheat sheet may say 60=66 so you set it to 66. Not the ideal situation, but a way to get the tester back testing tubes again.

I like it. Sounds like a solid plan if I can't locate an original pot.


You don't need the original pot. If you had it you would not need to do this. You need someone with a grood pot to do it. I would do it but my 600A is 500 miles away now. i took it out of town and did not bring it back with me. I will get it in a couple of weeks and will be glad to make a chart for you then if you still need it.


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 Post subject: Re: Couple of Hickok 600a questions
PostPosted: Mar Sat 28, 2020 12:17 am 
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The inside of the 600a BIAS pot. I happen to have one in the parts bin.


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 Post subject: Re: Couple of Hickok 600a questions
PostPosted: Mar Sat 28, 2020 4:52 am 
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cdoyal wrote:
Well, I may have other issues. I measured the plate voltage on the octal socket and got 238v. Should be 150v per the Schoo calibration method. Ugh.


Did you shunt your meter with the proper resistor?

DM


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