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#1 2006-08-02 00:58:52

Jaicen
Centipede
Registered: 2006-07-14
Posts: 461

Making Pots More Accurate

For all those that have been having trouble with their pots i've come up with a pretty easy solution.
I noticed that my pots were often hovering in-between values making it hard to make exact settings. They were also using only a small percentage of the overall rotation (<180 degrees). I tried many different values and tapers of pots, but none seemed to work satisfactorily. This was particularly annoying with the Fine Tune value in MONO.
Eventually, I settled on a 470K LINEAR pot, however I modified the taper on this using a 200K resistor in series with Pin 8 on the connector. This gives a smoother taper over more of the range of the pot, something close to 250 degrees of rotation. It still reaches max values before the end of rotation, which could probably be fixed by adding a larger resisor, but I've not tried that yet. Ideally I imagine that adding a trim-pot in place of the resistor will allow you to tune the taper to suit.
That's just my small contribution, let me know what you think guys.

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#2 2006-08-03 11:13:22

basil faulty
Dan Dare
Registered: 2006-07-19
Posts: 73

Re: Making Pots More Accurate

I have tried this, but no luck. I admit, the range of the pot does increase a bit but you get a problem with the x and y pots 'interfering'. For example, reducing the Y pot to it's minimum value causees the X pot to reduce also. I have tried it with a few value resistors, and by disconnecting pin 8 completely.

I am still stuck with 1/2 travel on all pots, but the travel is smooth.

I read in another post that you can wire resistors in parallel over the pot. How does this work? Has anyone tried this, and can they offer some instructions how to wire it up?

BTW- I have 470K linear pots

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#3 2006-08-03 12:02:47

ron
PROPHET64 GURU
From: berlin
Registered: 2006-07-08
Posts: 502
Website

Re: Making Pots More Accurate

There was an issue in the old yahoogroups forum which hasn't been imported:

yes_robot wrote:

Hi again,

I solved the problem.
I was NOT using a LOG pot, but a LIN one that is now working fine.

I think I don't quite understand the schematics on the Peripherals.pdf
As I understand from that scheme the Pot should have 3 wires, and this was what I
initially did:

Pin 5 from Joy DSUB to mid lug of the Pot (Y used)
Pin 7 to one of outer lugs and Pin 8 to the other.

This way it worked only half of the pot, as I told on the other post.
On the scheme, I don't understand the line going from the middle Pin to the lower
one (next to where it says +5v). I tried to bridge those Pins, but logically, I got instant
0 or 255 values.

Today I just tried again and all I had to do to get it working correctly was to
disconnect Pin 8.
So I have Pin 5 to the mid lug of the Pot, and Pin 7 to one of the outer lugs.
I am only using 2 wires, and it works fine...

Sure it's got something to do with my non-existent electronics knowledge...
Thanks again.

Solved my half-way POT problem too...

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#4 2006-08-03 12:27:46

basil faulty
Dan Dare
Registered: 2006-07-19
Posts: 73

Re: Making Pots More Accurate

Was your problem with one pot, or four? When two pots are connected to the same controller port (X and Y), then you get 'interference' between the two.

If you look at the bars on the port test screen, you will see what I mean. Turn both X and Y pots up to max, and turn the Y pot down slowly. The X pot will also drop a little. Plus the bar is a lot more 'shaky' than if all three wires are connected.

I am surprised disconnecting pin 8 works at all, as the pot is supposed to be working in a 'voltage differential' mode and not as a variable resistor.

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#5 2006-08-03 12:55:41

ron
PROPHET64 GURU
From: berlin
Registered: 2006-07-08
Posts: 502
Website

Re: Making Pots More Accurate

The problem was with one pot, old 'now free' editions. Didn't hook up the 4 pots yet..
I was fiddling around with resistors/trimmers in series before but didn't succeed.

However, my anxcient (german) "C64 Hardware" book says the pot is working as a variable resistor?!? Pretty much the same to be found here:

http://stud1.tuwien.ac.at/~e9426444/sidtech3.html wrote:

The conversion process is based on the time constant of a capacitor tied from the POT pin to ground, charged by a potentiometer tied from the POT pin to +5 volts..

Due to my very basic electronic knowledge I interpret the pot as a variable resistor rather than a 'voltage differential', but maybe i'm overlooking something...

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#6 2006-08-03 13:29:31

basil faulty
Dan Dare
Registered: 2006-07-19
Posts: 73

Re: Making Pots More Accurate

I believe from what you have said, that disconnection from pin 8 would indeed work for just one pot. Four pots seems to need a different solution entirely

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#7 2006-08-04 12:43:43

ron
PROPHET64 GURU
From: berlin
Registered: 2006-07-08
Posts: 502
Website

Re: Making Pots More Accurate

Hmmh... just installed 4 pots the way the "getting started" manual says (470k lo, connected to 7-8-5/9 - cheap pots without Jaicen's resistors).

I get almost the full rotation (approximately 5 degrees "blind"). The pots were quite cheap, no hi-end stuff.. Values are trembling occasionally just a little bit. I'll order a few lin pots and check out Jaicen's  proposal, too to see the difference.

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#8 2006-08-05 09:43:04

tRasH cAn maN
Dan Dare
From: Sweden
Registered: 2006-07-09
Posts: 70
Website

Re: Making Pots More Accurate

Has anyone succesfully connected 4 pots and gotten smooth and full rotation? FirestARTer?

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#9 2006-08-05 11:05:12

www.DOGMATECH.info
Bomb Jack
Registered: 2006-07-08
Posts: 36
Website

Re: Making Pots More Accurate

Me not. See topic: "Poti Trouble"


Join the Nintendohardcoreacidkamikazesamurai army!!!
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#10 2006-08-06 00:39:35

basil faulty
Dan Dare
Registered: 2006-07-19
Posts: 73

Re: Making Pots More Accurate

tRasH cAn maN wrote:

Has anyone succesfully connected 4 pots and gotten smooth and full rotation? FirestARTer?

NO!!!!

I have tried 220K pots. I had one pot reaching almost full travel (perhaps 3/4) but the other axis would never reach more that 3/4 of the bar being full. I have gone back to 470K pots

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#11 2006-08-06 19:49:40

firestARTer
Administrator
From: Germany
Registered: 2006-06-27
Posts: 579
Website

Re: Making Pots More Accurate

I got the pots smooth and with full rotation working.
Only thing that can be that it sometimes shaking between 2 values ( 1 digit only) . But thats a thing of the AD-Converter in the C64, long cables, capacitor quality, resistor quality.


firestARTer - Gameboy - C64 - Atari Music
Hardware developing for your favourite homecomputer musicsoftwares
http://www.sidsyn.com
http://www.myspace.com/firestartermusic

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#12 2006-08-06 23:07:55

tRasH cAn maN
Dan Dare
From: Sweden
Registered: 2006-07-09
Posts: 70
Website

Re: Making Pots More Accurate

firestARTer wrote:

I got the pots smooth and with full rotation working.

Are you using 470K's? no extra resistor?

firestARTer wrote:

[...snip...] long cables, capacitor quality, resistor quality.

Of what? The pot mod or inside the C64?

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#13 2006-08-07 12:23:10

firestARTer
Administrator
From: Germany
Registered: 2006-06-27
Posts: 579
Website

Re: Making Pots More Accurate

Actually i use 100k pots with an extra capacity but that would be too difficult to explain.
But i was going from the original patent and it says 470k.

And check this, probably you connected it wrong. It seems that the manual is a bit wrong. Too bad, i never tried via the manual, only by the original datasheet.

http://www.firestarter-music.de/prophetcart/pots1.jpg


firestARTer - Gameboy - C64 - Atari Music
Hardware developing for your favourite homecomputer musicsoftwares
http://www.sidsyn.com
http://www.myspace.com/firestartermusic

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#14 2006-08-07 12:33:01

ron
PROPHET64 GURU
From: berlin
Registered: 2006-07-08
Posts: 502
Website

Re: Making Pots More Accurate

So this connection goes without pin#8?

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#15 2006-08-07 13:00:11

AxiMaxi
PROPHET64 GURU
From: The Netherlands
Registered: 2006-07-09
Posts: 548
Website

Re: Making Pots More Accurate

firestARTer wrote:

Actually i use 100k pots with an extra capacity but that would be too difficult to explain.

No need to explain how it works, but some schematics for the non-educated would be nice wink


If everything is under control, you're not going fast enough.

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#16 2006-08-07 13:20:21

firestARTer
Administrator
From: Germany
Registered: 2006-06-27
Posts: 579
Website

Re: Making Pots More Accurate

ron wrote:

So this connection goes without pin#8?

Yep, ground is normally not connected at all.


firestARTer - Gameboy - C64 - Atari Music
Hardware developing for your favourite homecomputer musicsoftwares
http://www.sidsyn.com
http://www.myspace.com/firestartermusic

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#17 2006-08-07 13:22:39

firestARTer
Administrator
From: Germany
Registered: 2006-06-27
Posts: 579
Website

Re: Making Pots More Accurate

AxiMaxi wrote:

firestARTer wrote:

Actually i use 100k pots with an extra capacity but that would be too difficult to explain.

No need to explain how it works, but some schematics for the non-educated would be nice wink

But thats not needed at all. 470K or 500K are doing the job very well. There's no chance to make it better.


firestARTer - Gameboy - C64 - Atari Music
Hardware developing for your favourite homecomputer musicsoftwares
http://www.sidsyn.com
http://www.myspace.com/firestartermusic

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#18 2006-08-07 13:38:39

AxiMaxi
PROPHET64 GURU
From: The Netherlands
Registered: 2006-07-09
Posts: 548
Website

Re: Making Pots More Accurate

firestARTer wrote:

But thats not needed at all. 470K or 500K are doing the job very well. There's no chance to make it better.

Awww... come on, if only for my education wink
(I already bought a 500K pot)


If everything is under control, you're not going fast enough.

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#19 2006-08-07 15:30:37

VCFool
Bloody beginner
Registered: 2006-07-09
Posts: 6
Website

Re: Making Pots More Accurate

ron wrote:

There was an issue in the old yahoogroups forum which hasn't been imported:

yes_robot wrote:

Hi again,

I solved the problem.
I was NOT using a LOG pot, but a LIN one that is now working fine.

I think I don't quite understand the schematics on the Peripherals.pdf
As I understand from that scheme the Pot should have 3 wires, and this was what I
initially did:

Pin 5 from Joy DSUB to mid lug of the Pot (Y used)
Pin 7 to one of outer lugs and Pin 8 to the other.

This way it worked only half of the pot, as I told on the other post.
On the scheme, I don't understand the line going from the middle Pin to the lower
one (next to where it says +5v). I tried to bridge those Pins, but logically, I got instant
0 or 255 values.

Today I just tried again and all I had to do to get it working correctly was to
disconnect Pin 8.
So I have Pin 5 to the mid lug of the Pot, and Pin 7 to one of the outer lugs.
I am only using 2 wires, and it works fine...

Sure it's got something to do with my non-existent electronics knowledge...
Thanks again.

Solved my half-way POT problem too...

Did this really solve your problem?

I was the author of that post and it was written with the heat of the moment... smile
Actually, disconecting pin 8/ground only made my pot work a little more (reaching the top value near 3 o'clock, while with pin 8 connected it was reached somewhere near the middle position, 12 o'clock).


firestARTer wrote:

Actually i use 100k pots with an extra capacity but that would be too difficult to explain.
But i was going from the original patent and it says 470k.

And check this, probably you connected it wrong. It seems that the manual is a bit wrong. Too bad, i never tried via the manual, only by the original datasheet.

http://www.firestarter-music.de/prophetcart/pots1.jpg

So I just tried your sheme and it works just the same for me. I must say, again, that my electronics knowledge is really low, and I must use common sense while reading this schematics. As I understand, pin 5 (or 9) goes to the middle lug of the pot and bridged to one of the outer lugs. Then pin7 to the other outer lug. Am I wrong?

As I told before I have pin 5 to the mid lug of the pot, and pin 7 to one of the outer lugs. I tried that bridge of pin 5 and I can't see the difference. Works just the same, top reached near 3 o'clock.

The other day Rabato bought another Linear 470k Pot. It's a metallic one made by a company called PIHER. Again, it does work just the same as the Radiohm that I have used.

I am really waiting to do realtime editing with MIDI CC when the MIDI interface is finished, so I am not concerning too much. But would be nice to find a solution to this neverending story. Maybe that 100k pot+capacitor schematics would be appreciated in here... smile


"There's never enough time to do all the nothing you want." ...

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#20 2006-08-07 17:18:55

firestARTer
Administrator
From: Germany
Registered: 2006-06-27
Posts: 579
Website

Re: Making Pots More Accurate

Hehe, but my 100k solution is really just the same as the 470K thing. I use 100k cause they are no 470 or 500K digital pots around which i need for the midi-interface.
http://www.firestarter-music.de/prophetcart/pots2.jpg

But keep in mind, if the 470K / 500K solution is not working, then you will also have problems with this here.

When your top is reached at 3 o'clock, where do you start? If you start around 7:30 then 3 o'clock is a good position if using 500K pots.


firestARTer - Gameboy - C64 - Atari Music
Hardware developing for your favourite homecomputer musicsoftwares
http://www.sidsyn.com
http://www.myspace.com/firestartermusic

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#21 2006-08-07 18:03:30

basil faulty
Dan Dare
Registered: 2006-07-19
Posts: 73

Re: Making Pots More Accurate

What causes the pot travel problem then?

Why is is that if you have the problem - nothing seems to fix it? It seems from your reply that if you have the problem you are stuck with it.

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#22 2006-08-07 18:13:02

firestARTer
Administrator
From: Germany
Registered: 2006-06-27
Posts: 579
Website

Re: Making Pots More Accurate

Well, you can search for the problem. Replace with another SID, replace the Analogue-switch (4066) inside the C64, replace the 2 capacitors (connected to pin 23&24 of the SID) .
Of course there are alot of things that can be wrong. If the 500k Pots are not running well and you are sure you didn't do anything wrong, you should really search the problem in your 20 year old computer.


firestARTer - Gameboy - C64 - Atari Music
Hardware developing for your favourite homecomputer musicsoftwares
http://www.sidsyn.com
http://www.myspace.com/firestartermusic

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#23 2006-08-07 20:58:52

AxiMaxi
PROPHET64 GURU
From: The Netherlands
Registered: 2006-07-09
Posts: 548
Website

Re: Making Pots More Accurate

Oh, yeah, I almost forgot!
Let's not bother FirestARTer, he's building a MIDI interface big_smile


If everything is under control, you're not going fast enough.

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#24 2006-08-08 13:49:36

VCFool
Bloody beginner
Registered: 2006-07-09
Posts: 6
Website

Re: Making Pots More Accurate

firestARTer wrote:

Hehe, but my 100k solution is really just the same as the 470K thing. I use 100k cause they are no 470 or 500K digital pots around which i need for the midi-interface.
http://www.firestarter-music.de/prophetcart/pots2.jpg

But keep in mind, if the 470K / 500K solution is not working, then you will also have problems with this here.

Thanks!

firestARTer wrote:

When your top is reached at 3 o'clock, where do you start? If you start around 7:30 then 3 o'clock is a good position if using 500K pots.

Oh, sorry. This sure was important...

Yes, placed the pot to start somewhere near 7:30, the ending is near 4:30. Are you telling that the last degrees of the pot are not expected to work? (empty range is from 3:00 to 4:30)


"There's never enough time to do all the nothing you want." ...

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#25 2006-08-08 14:24:19

AxiMaxi
PROPHET64 GURU
From: The Netherlands
Registered: 2006-07-09
Posts: 548
Website

Re: Making Pots More Accurate

My 2 cents: yesterday I measured my 500k potmeter.
It runs from 0 to 448 kOhm.

Adding a resistor also increases the lower end, so how would you be able to only increase the upper end?


If everything is under control, you're not going fast enough.

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