Korg LP-10 LP10 problem: all sound distorted

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Korg LP-10 LP10 problem: all sound distorted

Postby upunkt » Tue Oct 16, 2018 4:23 pm

Hi!

I got a KORG LP-10 recently found in a barn outside of Berlin in former East Germany, dirty, rotten and completely out of function. I managed to get it back to work. Unfortunately, all sound is a bit low in level and distorted. Both built-in speakers are in good shape and technically ok, btw.

I got a copy of the original service manual directly from Korg in Germany, they're helpful regarding old equipment. It contains descriptions of the circuit boards, a signal flow chart and a parts list. It does not contain details about the power supply and voltage on the rails in general, which is a pity.

What I've done so far:
The transformer (JA-471C) may be used with different voltages. By experimenting I found out that it was set to be used with 220 V, by connecting the yellow wire I set it to 240 V since the general voltage in Germany was raised from 220 to 230 V in the early 1990ies. And I recapped the main board KLM-314.

This did NOT change the sound, unfortunately. The e-piano sounds exactly like before. The distortion is not obvious when you play monophonic, but becomes evident once you press two or more keys at the same time.

Can someone give me a hint where I should start checking for the cause of the distortion?
There is a power regulator NEC 14312, I suppose the output should be 12 V, it is only 11.6 V though. Do you think this could have an influence? I checked ripple also it's about 0.4 Volts.

Thanks a lot in advance,
cheers,

upunkt
Last edited by upunkt on Sun Oct 28, 2018 5:34 pm, edited 1 time in total.
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Re: Korg LP-10 LP10 problem: all sound distorted

Postby Mooger5 » Tue Oct 16, 2018 7:34 pm

Hi.The interesting part about the 0.4V ripple is that 0.4 + 11.6 = 12V. And feeding 230 into the 240 primary will not give the expected voltage at the secondary. It´s a matter of ratio: if 240~12 then it´s 20:1 ratio. Less 10V because 240-230, means a drop of 0.5V at the secondary, or 11.5V. Which meets give or take the voltage you measure at the regulator output IF the smoothing capacitor IS way out of spec. The secondary usually outputs more than 12VAC for a DC of 12V, such as 12*1.4 or 18VAC usually. But it´s Korg, so you never know.

So did you replace the caps in PSU? If so, we can discard this hypothesis. The 230 VAC isn´t fixed anyway, as it´s usually lower during the day and higher at night.
Did you measure the same ripple when the mains were connected to the 220V tap? If so, and with new caps, then it could be the regulator is defective. I think it could be replaced by a common 7812, but didn´t check.
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Re: Korg LP-10 LP10 problem: all sound distorted

Postby upunkt » Tue Oct 16, 2018 10:11 pm

Ah, thanks, this is the kind of reply I like, with technical detail that I could not come up with.

I did not measure voltage nor ripple before I set the transformer to 240 V. Just looked at the transformer and was sure it could be set to be used with different voltages so this was just the first thing to do. There are two more wires at the transformer, brown (117 V) and orange (220 V), it's easy to solder it back. I also got a big 230 -> 220 V transformer last weekend, I could check with the Piano plugged in that also. I'll be trying to get a new regulator anyway (damn cheap) and will report back in a couple of days.

The secondary outputs 17 something, btw. All PSU caps are replaced.
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Re: Korg LP-10 LP10 problem: all sound distorted

Postby Mooger5 » Tue Oct 16, 2018 11:22 pm

OK. The datasheet says 3-pin regulator but I didn´t check the pinout so please verify the compatibility. Better safe than sorry.
If the high ripple persists it´s worth looking for another culprit in the PSU, as the distortion heard when more than one note is played could be a sign the circuits ahead are underpowered.

Good luck.
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Re: Korg LP-10 LP10 problem: all sound distorted

Postby upunkt » Tue Oct 16, 2018 11:46 pm

Thanks, yeah will check that.
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Re: Korg LP-10 LP10 problem: all sound distorted

Postby Mooger5 » Tue Oct 16, 2018 11:52 pm

Just thought of something: that regulator may not be a 7812 equivalent of fixed voltage, but an adjustable reg like the LM317, which is also 3-pin. I´m not seeing a 7812 delivering all the current that something like the LP12 probably needs, if it´s the only device in the PSU doing all the regulating duties. A scan of the schematics would help.
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Re: Korg LP-10 LP10 problem: all sound distorted

Postby upunkt » Wed Oct 17, 2018 12:31 am

I can get an ST L7812 at a local store. Schematics and specs look pretty much the same as NEC μ PC14312 which seems to be fixed voltage regulator, and pins in same order. I'll give that a go, after further measurement. Thanks a lot for your input so far.
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Re: Korg LP-10 LP10 problem: all sound distorted

Postby upunkt » Sun Oct 28, 2018 5:34 pm

Just for the record: The LP-10 works again. I did two things, finally. I substituted the voltage regulator NEC μPC14312 with an ST L7812. This resulted in a nearly clean signal, also much louder than before. Since it was not 100% perfect I doublechecked all steps I'd done before. I found two caps that I actually hadn't replaced, just because they looked like the new ones I used (with a different capacity though) so I assumed these were already replaced ones. Should keep track of that more detailed next time. One of the caps (looked corroded btw, all green underneath) gave me some signal in the component tester but it was quite odd. So replacing this brought back the Piano. Thanks for help again.
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Re: Korg LP-10 LP10 problem: all sound distorted

Postby Mooger5 » Sun Oct 28, 2018 9:16 pm

You´re welcome. Congrats!
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