Repair: Korg LP-10

Repair: Korg LP-10

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ungefähr 5:36 Minuten

The LP-10 is an early E-Piano built by Korg in 1980. It features three on/off combinable sounds (Piano, E.Piano, Clav), full polyphony, an equalizer section with 6 bands adding/subtracting nicely to the sound, and a speed adjustable chorus.

My LP-10 had been found in a barn in a small village outside of Berlin and it was in a rather bad shape. The housing covered over and over with dust, spill and chalklike debris, broken side panels, a broken key and no sound at all on powering up. Well, the power LED lit at least. Fortunately, all knobs and fader caps were still in place so cosmetically only the torn key would matter.

Inside a slightly different picture: dust and debris, combined with remains of small animals.

Getting back sound

Checking the mainboard I did not notice any obvious damage, so I started with the usual cleaning procedure: brushing away the dust with a soft paint brush, vacuum cleaning the inside, spraying all switches and the radial and axial potentiometers with Tuner 600 and moving them forth and back for a while.

Generally speaking this brought sound back. In fact, all of the controllers did what they were supposed to do. Even all of the keys responded without further maintenance. Still, I did a full cleaning.

Sound was back, ok, but the volume was rather low and it felt somehow distorted, though not clipped.

Rewiring the transformer

Before any further measuring I rewired the Power Supply Unit (PSU). The LP-10 had been produced for the international market so the transformer (JA-471C) features wires for the following voltages:

  • 117 V (brown)
  • 220 V (orange)
  • 240 V (yellow)

Since we're using approximately 230 V in Europe I resoldered to the yellow wire. Sound quality was not affected while the secondary output voltage dropped from 17,7 V to 16,2 V.

Why does this keyboard sound distorted?

Next I switched and levelled through all switches and potentiometers to narrow the cause of the distortion. It turned out to be present no matter what I selected or which key I pressed. So it must be something at the very end of the chain in the VCA or something very basic. While checking voltages on the rails it turned out there was relatively high ripple (0.4 V) on the 12 V line while everything else seemed to be just within specs.

Repair: Korg LP10

Replacing all the capacitors was of course due regarding the age of the instrument so this was the next step before blaming the ripple. Since any key produced the same distorted sound I skipped recapping the keybed circuit board at first and started with the main board KLM-314A. Sound did not improve, though.

Ok, ripple. There is a voltage regulator NEC μPC14312 (or UPC14312) responsible for the 12 V. This one is no longer available but comparing specs I decided to replace it with an ST L7812. This helped: sound volume came back and overall distortion was reduced. But it was still not perfect.

Before checking the VCA I went through the recapping list once more to see if I did a wrong replacement and guess what, I had forgotten two caps because they looked exactly like the replaced ones so I had assumed they had been already new. Note to self: better keep track of this process next time. Go get glasses.

One of these two capacitors was corroded (all green beneath) which does not necessarily mean death but it's a good hint. I played "Good Cap – Bad Cap" with the scope's component tester once more and the result was obvious.

Synthesizer Repair: Good Cap - Bad Cap

Broken key and side panels

The broken key ("F") is of the type Korg equipped the keyboards of e.g. MS-10 or MS-20 with. Unfortunately, I could not grab a spare key yet. Fortunately, the plastic of these keys is sensitive to acetone so for the moment I just attached a piece of old LEGO to make the key playable again. Works, looks funny (you can place all kind of LEGO stuff on that key now or attach a long thread to it easily and play the F remotely which is always fun on stage), but this is not an option for the long run. If you have a spare key please contact me.

The side panels had broken off screw supports. The plastic used here is either aging too fast or the instrument had been having a hard time on the road. All supports were broken or even torn, several bits were missing. I could not use the acetone method here. Instead, I supported the screw holes with paper clip wire, scratched the plastic with a sharp knife and put a lot of hot glue around it which sticks better with scratches. Seems to work.

Repair: Korg LP-10 Side Panel

Fully functional again

So my LP-10 is back to life now and I really enjoy playing on this four octave non-dynamic keyboard. I like the Piano and the E-Piano sound which are really nice and warm even without the chorus. The LP-10 is also great for bass sounds with its precise and strong attack and adjustable decay.

The instrument's sound is based on the TMS3615 which was popular in a variety of organs of that time. It is also comparable to the Vermona E-Piano (repair and review here) though some of the features differ.

Feel free to leave questions or comments below.

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