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 Post subject: Are Miniature 10-turn Pots Still Manufactured in 500K?
PostPosted: Feb Sun 03, 2019 5:36 am 
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Hi, All-

I'm having a hard time finding a multi turn pot in 500K for the power
supply I just bought (Heath SP-2717A) I want to replace the single-
turn B+ Adjust pot with a multi-turn for better voltage setability, even
though the performance is really hobby grade on this supply.

I saw some Bourns types on the internet in 500K but the seller
had them only new in cartons of 10 pcs. BTW this has to be a mini-
size pot, 1in dia- it can't be a big old Beckman Helipot type because
that wouldn't fit my front panel. I have a couple dozen of these
miniatures in 3t/5t/10t but all are in the range 1K/2K/5K/10K.
I suspect Bourns may have stopped making them above 100K.

I was hoping that a fellow forum member might have one or more
of these min. 500K multi-turn pots and be willing to part with one.
No response to my WTB over in the Classifieds so far... New or
used would be fine, 3t/5t/10t also fine- but it needs to be an actual
multi-turn type and not just a vernier type...

David

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 Post subject: Re: Are Miniature 10-turn Pots Still Manufactured in 500K?
PostPosted: Feb Sun 03, 2019 7:31 am 
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Hi David,

I checked Mouser and Digi-Key.

They have the standard full-size (1-13/16" dia) version, but no 1".
And they go for almost $100.

Have you tried Surplus Sales of Nebraska?
http://www.surplussales.com

- Leigh

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 Post subject: Re: Are Miniature 10-turn Pots Still Manufactured in 500K?
PostPosted: Feb Sun 03, 2019 12:30 pm 
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Good suggestion, Leigh- but they seem to only go through 125K for
a single control. Not too bad at 10-15 dollars, but not giving them away,
either.

On Ebay I know where to get 500K ten-turn units, and 10 of them
would cost me 'only' about 85 dollars. If they would sell singles at
10 dollars each, I would snap one up. Maybe two...

Surplus Sales has a mini dual 300K. I suppose one could hook
them up as 'series opposing' rheostats- one increases, one
decreases with CW rotation, and you use their junction as the 'arm',
but you end up with a 600K 10-turn pot. At 95 dollars, I'll pass...

BTW there is a hint that Bourns once made the 500K version-
a German Ebay vendor had a 500K listed, but there was a drop-down
menu where you had to choose a 'wert'. Well, the highest 'wert'
was 100K but there was a 500K still in the choices but 'grayed
out'.

Thanks, David

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 Post subject: Re: Are Miniature 10-turn Pots Still Manufactured in 500K?
PostPosted: Feb Sun 03, 2019 12:54 pm 
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Beckman-Helipot made some multiturns in the smaller diameter, as did Bourns and Spectrol - I have examples of all three. I don't know if any of them made the small ones in 500k.

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 Post subject: Re: Are Miniature 10-turn Pots Still Manufactured in 500K?
PostPosted: Feb Sun 03, 2019 4:28 pm 
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Hi David,

I have a 1meg 10-turn pot that would work physically, but I don't know if the resistance value would be ok.

If you want it, let me know.

Bob AB1MN


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 Post subject: Re: Are Miniature 10-turn Pots Still Manufactured in 500K?
PostPosted: Feb Mon 04, 2019 6:01 am 
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Location: Lexington, KY USA
Depending on how you are using the supply, having to turn the voltage setting knob so many turns to get from one setting to another may be a serious inconvenience.

The service grade bench supplies that have coarse and fine voltage controls seem to be pretty easy to adjust. This would let you use two inexpensive single turn pots. If you can find a place on the panel for the second one.

It might be worth trying a concentric shaft dual gang pot. I was amazed to find these are still being sold, and they are not too expensive. Mouser lists these in several values. This control would require only the single hole in the panel.

Depending on the mechanical stability of these units, there could be a problem with the coarse gang setting being upset when the fine gang is adjusted. You woud almost have to try one and see.

You would have to fiddle the circuit a little to use two pots, but there are several ways a fine adjustment pot might be added.

Ted


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 Post subject: Re: Are Miniature 10-turn Pots Still Manufactured in 500K?
PostPosted: Feb Mon 04, 2019 7:00 am 
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ab1mn wrote:
Hi David,

I have a 1meg 10-turn pot that would work physically, but I don't know if the resistance value would be ok.slow

If you want it, let me know.

Bob AB1MN


Hi, Bob-

Thanks! I think I'd have to say yes to that. Of course one can parallel a resistor to pull it
down to 500K. What happens, as I remember, is the resistance changes slower in the
middlde, faster at the two ends. And that would be perfectly OK, because even with that
limitation, it is still slower changing anywhere along its rotation compared with a single
turn unit. I'd likely put a mini 1/4in crank handle knob on it and then the 10 turns should
rarely if ever be a problem. BTW the Heath engineers included a B+ On/Off toggle on
the front panel. You can remove B+ pretty quickly if needed. I'll PM you...

David

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Last edited by Clutter on Feb Mon 04, 2019 8:25 am, edited 2 times in total.

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 Post subject: Re: Are Miniature 10-turn Pots Still Manufactured in 500K?
PostPosted: Feb Mon 04, 2019 7:41 am 
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Usually Lurking wrote:
Depending on how you are using the supply, having to turn the voltage setting knob so many turns to get from one setting to another may be a serious inconvenience.

The service grade bench supplies that have coarse and fine voltage controls seem to be pretty easy to adjust. This would let you use two inexpensive single turn pots. If you can find a place on the panel for the second one.

It might be worth trying a concentric shaft dual gang pot. I was amazed to find these are still being sold, and they are not too expensive. Mouser lists these in several values. This control would require only the single hole in the panel.

Depending on the mechanical stability of these units, there could be a problem with the coarse gang setting being upset when the fine gang is adjusted. You woud almost have to try one and see.

You would have to fiddle the circuit a little to use two pots, but there are several ways a fine adjustment pot might be added.

Ted


Hi, Ted-

I think I see what you had in mind with your concentric pot method (two different
values, independent adjustment). Trouble is that I need the vernier action at plus/
minus a few percent around the value of voltage I have dialed in on the big pot-
in other words, 'in the middle'. I think your method puts the second pot at one end
so it's only changing the end-to-end resistance, which won't work. The end-to-end
resistance needs to stay pretty constant like with a single turn pot.

But I've had a two pot scheme in mind all along; it's doable because the C-
control can go, so far as I'm concerned. That frees up a second 3/8in front panel
hole for a second i/4 in shaft pot. So if you get a dual 500K linear pot with the
'grippy' shafts, then both pots turn at the same time-
or some duals haven't got concentric shafts to begin with. You series connect both
elements where each is connected as a rheostat and one decreases with CW
rotation while the other increases. Ends up you always have about 500K end-to-
end and the junction of your two elements now functions as the 'arm'. But actually
right in the middle you break it and insert a second linear pot of about 1/10 the
resistance, i.e. 50K. So you end up with a 550K pot end-to-end and the 50K gives
11:1 vernier action. This is my fallback, with the single pot/ multi-turn method my
first choice. The 50K pot would usually sit at about 1/2 full rotation. Get the output
as close as possible with the dual, then the vernier can put it dead on.

There's one peculiarity with this method- if the big pot is zeroed but the vernier is
not, then you still have some output voltage- up to 1/11th of the full ~380V
output, or about 35V. You have to zero both pots to get zero volts out.

Now if Mouser and Newark etc. had a fancy pot with a dual 500K on one shaft
plus a 3rd 50K element on the other shaft, now that would be fantastic! I bet we
are far from the first to ever think about such a vernier setup, so I do think it's very
possible such controls exist... In this case, I would need the shafts to turn
independently and not be the 'grippy' type...

David

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 Post subject: Re: Are Miniature 10-turn Pots Still Manufactured in 500K?
PostPosted: Feb Mon 04, 2019 1:14 pm 
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Have you considered a 5 turn instead of a 10 turn pot ? Or a counting dial arrangement?

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 Post subject: Re: Are Miniature 10-turn Pots Still Manufactured in 500K?
PostPosted: Feb Mon 04, 2019 4:49 pm 
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Clutter wrote:
I think your method puts the second pot at one end so it's only changing the end-to-end resistance, which won't work. The end-to-end resistance needs to stay pretty constant like with a single turn pot.

I have worked with lab grade supplies that use two single turn pots in series, which are in turn in series with a pullout resistor. The control voltage comes from the junction of the pullout and the pots. For a basic example whose manual explains the principal see the HP 6212B/6214B/6216B/6218B manual at:

https://www.keysight.com/main/gated.jsp ... e=VIEWABLE

Once again, you are trying to make a simple piece of low-cost electronics do something that it really cannot do. If you need more sophisticated operation you need to buy more sophisticated equipment. I don't know what your time is worth to you, but I charge $75.00 per hour bench labor rate. In the last two years I have bought three lab grade power supplies, each for about the cost of 1/2 hour of my labor. To me it is not worth spending time on a piece of electronics if that time is worth more than cost buying a suitable replacement. With that said I will unsub this topic.

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 Post subject: Re: Are Miniature 10-turn Pots Still Manufactured in 500K?
PostPosted: Feb Tue 05, 2019 2:58 am 
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Joined: Jan Tue 16, 2007 7:02 am
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Location: Lexington, KY USA
As a hobby project, many count their time at about five cents the hour, unless that turns out to be too much...

There is no need for a second, fine adjustment, pot to be a particular value. It can be used to inject a little current into the error amp grid through a large resistor. ( Large vs. the 1 Meg grid resistor already there.) The ends of the added pot can be connected to two points in the string of zeners.

A key factor in the two pot scheme is the stability of the coarse adjustment pot. Once set, it needs to be pretty stable.

If you really want accurate setting, tight regulation, etc, you can add a fancier regulation circuit. Solid state would be easiest, nowadays. Actually, if you decide to make some improvements, an adjustable current limit might be even more useful than better voltage setting and regulation. Perhaps a pair of digital meters would be good to have.

You might have more fun with the sow's ear to silk purse exercise, but it's likely to soak up a lot of time. Doesn't the stock supply actually work pretty well?

Ted


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 Post subject: Re: Are Miniature 10-turn Pots Still Manufactured in 500K?
PostPosted: Feb Tue 05, 2019 11:09 am 
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Barry H Bennett wrote:
Have you considered a 5 turn instead of a 10 turn pot ? Or a counting dial arrangement?


Yes, 3 turn/5 turn would work just as well, so long as it's not the vernier
drive type and is a true multi-turn type.

Counting dial? No, too clumsy. I would put a small spinner knob so I
can fast-crank it.

David

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Last edited by Clutter on Feb Tue 05, 2019 8:05 pm, edited 1 time in total.

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 Post subject: Re: Are Miniature 10-turn Pots Still Manufactured in 500K?
PostPosted: Feb Tue 05, 2019 11:44 am 
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Usually Lurking wrote:
As a hobby project, many count their time at about five cents the hour, unless that turns out to be too much...

There is no need for a second, fine adjustment, pot to be a particular value. It can be used to inject a little current into the error amp grid through a large resistor. ( Large vs. the 1 Meg grid resistor already there.) The ends of the added pot can be connected to two points in the string of zeners.

A key factor in the two pot scheme is the stability of the coarse adjustment pot. Once set, it needs to be pretty stable.

If you really want accurate setting, tight regulation, etc, you can add a fancier regulation circuit. Solid state would be easiest, nowadays. Actually, if you decide to make some improvements, an adjustable current limit might be even more useful than better voltage setting and regulation. Perhaps a pair of digital meters would be good to have.

You might have more fun with the sow's ear to silk purse exercise, but it's likely to soak up a lot of time. Doesn't the stock supply actually work pretty well?

Ted


Resistor stability is the other thing I've been suspecting, Ted. I haven't done much
further work with the Heath supply, the past several days. I think a good, quick
check can include swapping in a brand new single turn 500K pot for the original
control- and I have the item in hand. I will do that.

The other thing is to try a new string of 10 zeners, pushed up above the board a
little to allow better cooling. The minor scorching on the Heath PCB speaks volumes.
There may be a hefty TC on the zener string voltages... but first of all I intend to try
to get a sense of what voltages track, so far as the large warmup drift of some 13V
I am seeing. Then I'll dig in and try various things... Is it the zener string? Is it the
resistor string that includes the B+ Adj pot? Soon I'll know better...

Regarding 'does it work'? Actually, yes, pretty well, near as I can tell. It's regulating
to less than the 1 percent figure in the specs and seems to be able to put out the
rated current. So I just need to fix the warmup drift problem. Add in slightly better
setability on the B+ and I'll be as happy as a pig in... well, you know. Yep, it's
always going to be a sow's ear, but I can still do a lot of very useful things with it.
And I am now looking regularly on Ebay to see if I can get a decent supply with
tighter specs. In time I will.

David

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Last edited by Clutter on Feb Tue 05, 2019 8:04 pm, edited 2 times in total.

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 Post subject: Re: Are Miniature 10-turn Pots Still Manufactured in 500K?
PostPosted: Feb Tue 05, 2019 12:08 pm 
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Dale H. Cook wrote:
Clutter wrote:
I think your method puts the second pot at one end so it's only changing the end-to-end resistance, which won't work. The end-to-end resistance needs to stay pretty constant like with a single turn pot.

I have worked with lab grade supplies that use two single turn pots in series, which are in turn in series with a pullout resistor. The control voltage comes from the junction of the pullout and the pots. For a basic example whose manual explains the principal see the HP 6212B/6214B/6216B/6218B manual at:

https://www.keysight.com/main/gated.jsp ... e=VIEWABLE

Once again, you are trying to make a simple piece of low-cost electronics do something that it really cannot do. If you need more sophisticated operation you need to buy more sophisticated equipment. I don't know what your time is worth to you, but I charge $75.00 per hour bench labor rate. In the last two years I have bought three lab grade power supplies, each for about the cost of 1/2 hour of my labor. To me it is not worth spending time on a piece of electronics if that time is worth more than cost buying a suitable replacement. With that said I will unsub this topic.


Thanks, Dale-

Yes, it's true my time is valuable. Thanks for the HP circuit reference. I'd heard
of 'pulldown' resistors; now I know what a 'pullout' resistor does, I'd not come
across the term before.

The Heath will probably do what I need it to do except on rare occasions where
I actually do need a 'stiffer' supply (better electronic regulation). I don't yet know
how often that will be...

It's fun to learn new things, even if it means making mistakes and flailing around
aimlessly at first. I'll graduate to better gear I am sure. You're probably no longer
'listening', but thanks for your contribution to this thread, Dale!

David

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 Post subject: Re: Are Miniature 10-turn Pots Still Manufactured in 500K?
PostPosted: Feb Tue 05, 2019 9:58 pm 
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It turns out that Bob, AB1MN did not have the right value pot-
so I'm still looking for a multi-turn (3-5-10t) 500K miniature pot.

Thanks, David

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