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Elka VP-10 pedal - specs?

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Re: Elka VP-10 pedal - specs?

Postby The Elf » Wed May 06, 2020 6:54 pm

One day I may find a VP10 and see if it works, but for now I've done as much as I feel able.

I do thank you guys, though, for your time and trouble. You're good'uns. :thumbup:
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Re: Elka VP-10 pedal - specs?

Postby Wonks » Wed May 06, 2020 9:45 pm

Hugh Robjohns wrote:Well, no, neither can I... but just trying to cover all possible benches.

I guess it is possible that those specific inputs to the ADC chip are blown from some previous event, but hard to see how.

Well, it has got a built-in multiplexer (the kit I worked on back in the 80s with an A/D had an external one), so it may be that those particular multiplexer inputs no longer work, whilst the A/D itself still does. The chip doesn't cost much, so if it's socketed, it could be an easy swap. Though it is a vulnerable CMOS device, so you'd need to be earth strapped when handling/fitting it. It's a DIP-28 package version, so a fair bit of de-soldering to do if there's no socket. But I've removed worse in my time.

You would of course remove the old chip and fit a socket if there wasn't one.
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Re: Elka VP-10 pedal - specs?

Postby The Elf » Tue Sep 01, 2020 9:55 am

A helpful chap on another forum has given me this information:

Just a quick update. I have found the schematics for the MK88ii and the MK76 (I have it on paper) but I have seen on the net that you already found it…unfortunately I have no schematics for the VP10.

I confirm that it is an optical pedal (there is a small lamp an probably a photo-resistor mounted on a small board…). Anyway, what I have done is measuring the voltages directly on the plug and here is the result (voltages relative to ground, taken on the plug itself; controller values from Midi-Ox monitor):

TIP: constant at about 4V
RING:

0.6V produces a hexadecimal value of 1E (decimal 31)
3.3V produces a hexadecimal value of 7F (decimal 127)
I can’t obtain values less than 31, at least with this pedal. Measured voltages are fluctuating by a few tenths of a Volt, I have given an approximate value.
Hope that helps.

Errata Corrige: for the 0.6V value, the fluctuation is within 0.1V.
Update: I have taken a third point: 1.83V corresponds to decimal 77. The three points fall pretty much on the same straigth line, so I would say the response is linear with a slope of about 35 per Volt.


This has set me wondering if it might be possible for a clever person to create a box that would sit between a 'normal' expression pedal and the MK76's strangeness to emulate the VP10 pedal. If so, you know where I am!
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Re: Elka VP-10 pedal - specs?

Postby Hugh Robjohns » Tue Sep 01, 2020 11:16 am

His numbers are entirely what I'd expect, and what we've discussed before.

The implication is that the VP10 can be emulated with a simple linear pot... which you did try once before with a 100K pot, and got absolutely nothing....

Although I find it hard to believe, I guess it is vaguely possible that the high impedance pot could supply enough current for the input stage... so it might be worth trying again with a 10K linear pot instead.

As before, one end to ground (sleeve), t'other end to the tip (4V), wiper to give variable output voltage on the ring.

The other thing to try, if you have a good variable bench power supply to hand, is to try injecting various DC voltages between 0.1 and 3.3V on the ring terminal to see if it responds to that at all...

If none of these things work, in all likelihood your keyboard has a blown input to the A-D (which was always a definite possibility).
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Re: Elka VP-10 pedal - specs?

Postby The Elf » Tue Sep 01, 2020 12:01 pm

Thanks, Hugh.

Well the numbers don't mean much to me, but if I find a 10k linear pot I'm prepared to give it another try with the croc clips. That's the extent of my electronics abilities!

I find it hard to believe that anything is blown in there, especially since there are two pedal inputs and neither has shown any sign of life. That's surely too coincidental.
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Re: Elka VP-10 pedal - specs?

Postby Hugh Robjohns » Tue Sep 01, 2020 3:55 pm

From looking at the schematics, and from the measurements made by your other correspondent, the volume pedal arrangement appears to be a straight-forward DC control voltage that works just like countless other keyboards.

Assuming the connections are made correctly, I can't see any technical reason why it shouldn't work... which means either the keyboard's control inputs are broken, or the keyboard isn't configured to use them correctly... There are no other options I can see...
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