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Old 12-16-2013, 03:06 PM
mcclelld mcclelld is offline
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Default UMR2 for Yamaha PS-20

I'm interested in adding the UMR2 to my Yamaha PS-20 Portasound. This 49-key synth is one of Yamaha's last analog Portasound synths from around 1981. The setup seems similar to the earlier PS-3 with the exception that this keyboard has a negative supply voltage of -9 V (measure at about -8 V). Will there be issues with the UMR2 if the ground is on the positive side rather than the negative side of the supply?

I've posted a scan of the 7 x 8 key matrix from the service manual. It's not clear to me whether the N (Note) wires or the B (Bank) wires are the select lines (N is select in the other Yamaha from that era, yes?) and whether they should be active high or active low. I don't have a scope, but from my DMM it looks like the 7 N wires are at around -1.7 V and the 8 B wires are at about -3 V. When a key connects the two, the N side drops slightly (-1.75 V), while the B wire jumps up to around -2.6 V. Let me know if you think this will work and how I might approach this. Thanks!
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Last edited by mcclelld; 12-16-2013 at 03:09 PM.
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Old 12-18-2013, 02:54 PM
mcclelld mcclelld is offline
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Join Date: Dec 2013
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Default N = Select

This is likely not advised, but given the lack of a scope, I routed the N pins to an audio out and confirmed that they generating an audible, high-pitch waveform. The B lines do not. So, I believe N = Select, as on the other early Yamaha keyboards. I think that makes it active high based on the direction of the diodes.

Any thoughts on the compatibility of the negative supply?
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Old 01-09-2014, 08:29 PM
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John John is offline
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Hi Mcclelld,

Welcome to the forum, and sorry for the slow reply.

I must warn against relying on the "listening" test to check for select signals. Sometimes, the data lines can be multiplexed with signal sources other than the keyboard matrix, so they might generate sound, too.

This looks like it might be a tricky install. Assuming that you have identified the select & data signalling correctly, there some work to be done with the voltage level shifting.

You are correct that the UMR2 "V+" can be connected to host ground if the power rail is negative from ground. Of course, the -8V will still have to be regulated to -5V for UMR2 V-.

From there, there is some chance that things would work with direct connections to the keyswitch matrix from the UMR2. However, because of the voltage levels, you'd be jeopardizing the UMR2 and maybe even the host.

Do you intend to use the keyboard as a sound module, as a MIDI controller, or both?
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Old 02-12-2014, 01:46 PM
electrojoe electrojoe is offline
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Join Date: Feb 2014
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Default Yamaha PS-20 Schematics

Hi Mcclelld,

I have a Yamaha PS-20 that has a bad LFO in it and I would like to get a copy of the schematics from you if that would be O.K.

Then I can repair the PS-20.

Gerry Dahl
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Old 08-12-2015, 02:01 PM
dennzo dennzo is offline
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Default Yamaha PS-20 Schematics

Hi, I am also on the hunt for a schematic for the Yamaha PS-20...anybody know where I can find one?

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keyboard matrix, ps-20, umr2, yamaha

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