891Re: [korg_mono-poly] Re: Troubleshooting Mono/Poly getting better
- Aug 24, 2013Ok, after measuring the power supply transformer, when connecting COMmon to the center tap, I register 18VDC from my analog multimeter and 24.2 VDC from my digital one.
From: willacyman <synth@...>
Sent: Saturday, August 24, 2013 4:32 PM
Subject: [korg_mono-poly] Re: Troubleshooting Mono/Poly getting better
Keith, before doing anything it's vital you check the voltages from the power supply and I would disconnect everything from it before measuring. High PSU voltages are bad news and can destroy ICs.
You're looking for +5V, +15V and -15V with respect to 0V. MP power supplies are only just good enough for the job and should always be checked.
Keyboards are notorious for causing problems. Remove the rubber contact strips and lightly clean the conductive 'pips' with isopropyl alcohol.
Once you know your power supply is OK, you can confidently go troubleshooting knowing you're not going to do more damage. Good luck!
--- In firstname.lastname@example.org, "seekwhence" <seekwhensir@...> wrote:
> Greetings all,
> I received a korg mono/poly in pretty rough shape. The CV in is missing all together, leads appear to have been snipped, dust everywhere, keys do not trigger, but sounds like a key is being held down and the power switch had been removed and the SW leads on KLM-372B have been twisted together.
> I began by detaching the Kb leads from KLM-356D and when powered up a dirtied and improperly distorted audio signal can be heard. The knobs on KLM-353C effect the sound and LEDS strobe to the audio when frequency pot is moved.
> The service manual diagram appears to be incorrect as it labels both mod wheels as control wheel and keyboard. THe wheel on the right appears to properly affect pitch, the wheel on the left does not appear to effect the sound.
> After inspecting all the leads and connectors, I removed all boards attached to the control panel for visual and odor inspection. I did find that 510k Ohm resistor at R6 on KLM-355C appeared to have one of the leads cut. I tested the resistor and because it appears to have no other issue but being severed and because in my extensive collection of carbon film resistors I do not have any in that rating, I soldered the body of the resistor back to the lead. After reassembling and closing it up, I now get full clean audio in stereo.
> While going through some of the controls I also noticed that some random keys do trigger or at least change the sound, but not all of them. So. I'm detaching the kb to check and clean the contacts.
> Still at this point, on only sound I can coax from the M/P range between pac man eating dots and a car alarm that's been going all day and the signal has grown dull. Additionally, I should add that on the center board KLM-354E, none of the LEDS light up from any of the 4 VCOs and none of the pots change the sound.
> I'm working with low budget digital and analog multimeters, but have yet to use them, I've only been inspecting by ear, eye and nose.
> Any input or perspective would be helpful and appreciated.
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