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Korg 900PS
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LektroiD



Joined: Aug 23, 2008
Posts: 977
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PostPosted: Mon Aug 25, 2008 8:34 am    Post subject: Korg 900PS
Subject description: Ripe for modding/bending
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Hi, I recently managed to get my old Korg 900PS back from a friend. As it is, it's pretty useless, the sounds are weak, so I decided to open it up.

Inside there are 17 variable resistors, and a row of smaller preset resistors. They are all labelled up, most of the labelling is pretty obvious, such as cutoff freq, Q, a few less obvious, such as Balance, fC, etc, but there are more obscure labels on the smaller preset resistors, such as TP1.2, TP3, TP4, etc (Test Points?) these have been position fixed with paint, check the pics. There are a few more variable resistors (not pictured) under the control panel to the lower left of the machine, including a white noise level controller.

I was also thinking of making it semi modular, - similar to the MS range, as all the wiring is clearly listed on the board. As it is, the LFO (vibrato) & envelope seem to only control the VCO, it would be nice to have the option of routing the filter through them. I wouldn't mind putting CV/Gate sockets on too, but not sure where I would need to tap into the circuit.

I do have a bit of experience with electronics, but might possibly need a bit of guidance in some areas, and maybe a schematic if anyone knows where I can get one (preferably free). Basically any help will be appreciated.

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ericcoleridge



Joined: Jan 16, 2007
Posts: 885
Location: NYC

PostPosted: Mon Aug 25, 2008 9:18 am    Post subject: Reply with quote  Mark this post and the followings unread

This is a pretty cool, rare, and probably unusual sounding synth; if I were you, I wouldn't make any destructive mods on it. Considering that even pre-set type analogs like this one (Arp explorer, SH2000, yamaha SY, minikorg, etc), are selling on e-bay for $400 and up-- it doesn't really make sense anymore to start customizing them. Not when you can buy a Soundlab 2VCO monosynth PCB for $50-- and create whatever kind of synth you want.
That's just my opinion; I've modded some synths before--but I mostly find that the limitations of any particular vintage synth are often what make it unique and interesting.

Also, it would be very unusual if the Env and LFO weren't already hardwired to the VCF and/or VCA (and not just the VCO). As you probably know, Korg used a lot of creative, but confusing, descriptions for various functions on their early models. I believe, for instance, that ENV 'Sustain' is called 'singing'.
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LektroiD



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PostPosted: Mon Aug 25, 2008 9:47 am    Post subject: Reply with quote  Mark this post and the followings unread

It might be rare, and I intend to make it rarer, unique. The current sounds are not unusual or interesting, they are at best what I would describe as bland. However, tweaking the internal variables makes a lot of difference, making the machine usable in a studio environment, currently it is not. It needs a CV/Gate for a start.

I'm not interested in selling it for a profit, I intend to make use of it by doing interesting stuff with it, rather than sell it on ebay as an antique ornament, or more precisely, a door wedge.

Incidentally, I have a Jen SX1000, which I added an external input to the filter, now that synth is useful, wheras it wasn't before. I don't care if it's worth more or less from the mod, the point is, it's now useful to me.

If you really don't want to see this 900PS become a semi-modular, I'll happily swap it for an MS series synth Wink

Last edited by LektroiD on Fri Jan 14, 2011 5:49 am; edited 1 time in total
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LektroiD



Joined: Aug 23, 2008
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PostPosted: Wed Aug 27, 2008 4:24 am    Post subject: Reply with quote  Mark this post and the followings unread

So, am I to presume a CV/Gate is a difficult addition to this synth, or has this thread just been missed by the right people?

I read somewhere the CV/Gate is Hz/V. If that means anything to anyone?

Here's some info I found about this synth in a Dutch website (there seems to be very little about this machine n the internet). I don't know Dutch language, but this post seems pretty clear:

Board KLM-25
Solderpoint 123, label TRIG --->
Wire colour: blue
No key pressed: 19.2V
Key pressed: -7.8V
~~~~~~~~~~~~~~~~~~~~~~~~~~
Board KLM-25
Solderpoint 120, label <--- TRIG
Wire colour: gray
No key pressed: -2.9V
Key pressed: 9.9V
~~~~~~~~~~~~~~~~~~~~~~~~~~
3 wires on right side on keyboard:

White wire on C3
No key pressed: 0.01V
Key C0 pressed: 2.41V
Key C1 pressed: 4.82V
Key C2 pressed: 9.64V
Key C3 pressed: 19.32V

Brown wire 19.39V continuous

Black wire 0V continuous

http://www.synthforum.nl/forums//showpost.php?p=837233&postcount=30

Now, I'm wondering if I would need to make any extra modifications to have a usable CV/Gate I/O, or if I can simply tap directly into the circuit.
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andrewF



Joined: Dec 29, 2006
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PostPosted: Wed Aug 27, 2008 7:34 am    Post subject: Reply with quote  Mark this post and the followings unread

There was a Japanese webpage that had excellent instruction for adding CV and gate inputs to a Korg 800DV. Judging by your picture they are similar vintage and both V/hz. so I guess the circuitry will be about the same, but the connection points may differ.
I will post up the info from the original site, seeing it seems to have gone.

Actually, the VCF one is probably from somewhere else. Well done to those responsible!

If these prove useful, let me know, there are a few more


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ericcoleridge



Joined: Jan 16, 2007
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PostPosted: Wed Aug 27, 2008 8:35 am    Post subject: Reply with quote  Mark this post and the followings unread

Brainstormer wrote:

I read somewhere the CV/Gate is Hz/V. If that means anything to anyone?


Hz/per V is the standard by which most old Korg oscillators tune to, I think it's also called linear tuning (as oppossed to exponential). Yamaha and Paia sometimes used this standard also-- but very few other manufacturers. Most others, Arp, Roland, followed Moog's standard of 1 volt per octave. So, it just means that even with the CV out mod, this synth's keyboard CV won't properly control many other synths (without an exponential convertor). It will control them, just not in tune or scale. Unless you have an Korg MS, Yamaha CS, etc. I'm not trying to discourage you anymore, really. Just saying...
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rjd2



Joined: Sep 02, 2007
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PostPosted: Wed Aug 27, 2008 3:02 pm    Post subject: Reply with quote  Mark this post and the followings unread

sure you dont want to sell it or trade for it? seems like there are a lot more common synths in the world that you could use for a guinea pig...i'd gladly buy one of them, and trade it to you for that, if you're just looking for a frankensynth.... i like these old single vco monosynths.....
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LektroiD



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PostPosted: Wed Aug 27, 2008 11:48 pm    Post subject: Reply with quote  Mark this post and the followings unread

andrewF wrote:
There was a Japanese webpage that had excellent instruction for adding CV and gate inputs to a Korg 800DV. Judging by your picture they are similar vintage and both V/hz. so I guess the circuitry will be about the same, but the connection points may differ.
I will post up the info from the original site, seeing it seems to have gone.

Actually, the VCF one is probably from somewhere else. Well done to those responsible!

If these prove useful, let me know, there are a few more


These are very helpful! I haven't yet started the project. I'm waiting to see if I get a good offer or trade, it seems a few people feel it's a sacrilege to modify something so rare. If no offers are made, I will go ahead with the mods in the near future, in the meantime, I'll collect as much data as possible and draw plans ready.

Again, thanks for this, and if you have more, I would love to see them!

ericcoleridge wrote:
Brainstormer wrote:

I read somewhere the CV/Gate is Hz/V. If that means anything to anyone?


Hz/per V is the standard by which most old Korg oscillators tune to, I think it's also called linear tuning (as oppossed to exponential). Yamaha and Paia sometimes used this standard also-- but very few other manufacturers. Most others, Arp, Roland, followed Moog's standard of 1 volt per octave. So, it just means that even with the CV out mod, this synth's keyboard CV won't properly control many other synths (without an exponential convertor). It will control them, just not in tune or scale. Unless you have an Korg MS, Yamaha CS, etc. I'm not trying to discourage you anymore, really. Just saying...


Maybe it's possible to control it with my Nord G2?

rjd2 wrote:
sure you dont want to sell it or trade for it? seems like there are a lot more common synths in the world that you could use for a guinea pig...i'd gladly buy one of them, and trade it to you for that, if you're just looking for a frankensynth.... i like these old single vco monosynths.....


I would consider this, but if I do make a trade, I would need something that is hands on, which is the whole point I am planning on the modifications. No work has been carried out as yet. As I said above, I am collecting data ready, unless I get a good offer, in the meantime my options remain open for a limited period.

***

Thanks for everyone's input so far, it's nice to have found a group with such an enthusiasm for synthesizers.
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andrewF



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PostPosted: Thu Aug 28, 2008 12:49 am    Post subject: Reply with quote  Mark this post and the followings unread

I don't think the nord will be much use - isn't it just Midi( just an assumption)? you could get a midi2cv converter (try synhouse - midijack herz so good) but more expense.
it is not much use adding CV and gate inputs if you have no CV or gate sources.

Maybe just check out what the trimpots do, especially the ones on the filter board, if they seem to have a good effect, replace the trimpot with a pot mounted on the panel.

As for modding an old synth...my 800DV was battered, several broken switches and keys, so I had no qualms about removing the keyboard and giving it all the CV and gate inputs. If it was in good condition, i don't think i would want to mess with it.....well, an extra pot or switch on the front panel and a couple of CV/gate jacks on the back wouldn't hurt....


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LektroiD



Joined: Aug 23, 2008
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PostPosted: Thu Aug 28, 2008 3:55 am    Post subject: Reply with quote  Mark this post and the followings unread

andrewF wrote:
I don't think the nord will be much use - isn't it just Midi( just an assumption)? you could get a midi2cv converter (try synhouse - midijack herz so good) but more expense.
it is not much use adding CV and gate inputs if you have no CV or gate sources.

The G2 is a modular synth, so you can basically route a clock signal out of the audio bus, it works the same. I remember clocking the sample & hold of an SH101 with a G2 to keep it in time with the MIDI clock using this technique. However, sending voltages out of it may be a different story. I'll need to look at getting a Hz/V CV/Gate converter. I think my Jen SX1000 also uses Hz/V, something I'll look into, might be worth modding that too.

andrewF wrote:
Maybe just check out what the trimpots do, especially the ones on the filter board, if they seem to have a good effect, replace the trimpot with a pot mounted on the panel.

I already have, check the opening post. The Q (resonance) makes a lot of difference, as it is off as standard, I actually thought it was a non resonant filter until I went inside.

andrewF wrote:
As for modding an old synth...my 800DV was battered, several broken switches and keys, so I had no qualms about removing the keyboard and giving it all the CV and gate inputs. If it was in good condition, i don't think i would want to mess with it.....well, an extra pot or switch on the front panel and a couple of CV/gate jacks on the back wouldn't hurt....

To be honest, I don't think it would hurt to add CV/Gate sockets to any synth with no external control, as it makes it ultimately more useable.

Thanks again for the latest diagrams, very helpful! Wink
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rjd2



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PostPosted: Thu Aug 28, 2008 6:21 am    Post subject: Reply with quote  Mark this post and the followings unread

at the moment, the one thing i've got that i would trade for it is an elka 490 string machine. if you are interested, pm me. a few thoughts....

1-hz/octave is rare. i wouldnt bother with a cv/gate mod, personally.

2-"its not currently usable in the studio"....just wanted to mention that synths like this are actually desirable studio instruments for some people. i have made very little recordings that didnt involve manually playing a keyboard.

3-if you do mods that involve inputs and outputs, you need to verify that you are doing it correctly. i believe that you will need to put a non-inverting buffer circuit in front of each input/output. noone in here has piped in yet on this, and there are people who understand this much better than i do.

if you want to mod it to make it more useable, thats great, i just would find it a bit unfortunate if it was treated like a disposable experiment piece, that's all....
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LektroiD



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PostPosted: Fri Aug 29, 2008 8:55 pm    Post subject: Reply with quote  Mark this post and the followings unread

rjd2 wrote:
at the moment, the one thing i've got that i would trade for it is an elka 490 string machine. if you are interested, pm me. a few thoughts....


Possibly not, I'm a hands on type of guy. Hence the intent for these mods. Wink

Quote:
1-hz/octave is rare. i wouldnt bother with a cv/gate mod, personally.

Rare, but if it's the only option, I'll have to go with it, unless there's a Hz/CV to 1v/Oct converter circuit kicking about.

Quote:
2-"its not currently usable in the studio"....just wanted to mention that synths like this are actually desirable studio instruments for some people. i have made very little recordings that didnt involve manually playing a keyboard.

I prefer stuff sequenced bang in time, a-la-Raymond Scott Wink

Quote:
3-if you do mods that involve inputs and outputs, you need to verify that you are doing it correctly. i believe that you will need to put a non-inverting buffer circuit in front of each input/output. noone in here has piped in yet on this, and there are people who understand this much better than i do.

I admit I am not a great circuit designer, but I can easily follow a schematic, and yes, I noticed the diode in the examples above. I spent most of my life building advanced aircraft communication systems for the military, and they were fussy bastards, so I have my soldering skills pretty much nailed. I just need a bit of help on the rest of it Smile

Quote:
if you want to mod it to make it more useable, thats great, i just would find it a bit unfortunate if it was treated like a disposable experiment piece, that's all....

It will not be disposed of, I can assure you, if I mod this, it will be a pro job. I too would hate to ruin something as special as this Smile
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nikko909



Joined: Jan 13, 2011
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PostPosted: Fri Jan 14, 2011 5:40 am    Post subject: Reply with quote  Mark this post and the followings unread

A page is missing about S/H GATE UPPER & LOWER OUTPUT (Section C).
Where can i solder this part?


Thanks
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andrewF



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PostPosted: Sat Jan 15, 2011 7:51 pm    Post subject: Reply with quote  Mark this post and the followings unread

sorry forgot this one
here you go


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nikko909



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PostPosted: Mon Jan 17, 2011 1:33 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks thanks thanks !!!!

Nicolas

www.nikko909.fr
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jamecko



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PostPosted: Tue Dec 13, 2011 7:28 am    Post subject: LFO and the korg 800dv, 700s
Subject description: using the vibrato as an filter LFO?
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I've heard a rumuor that it is possible to rewire the Vibrato on the Korg 800dv to control the filter instead of the pitch. that would solve the hassle of trying to get 20 volt input needed to control the filter via the external input as the analogue solutions kit (which using the kenton pro cv interface only provides 12,5 volt max)
Any one who have tried/ done this mod?
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