I have an EX-800 and couldn't get any sounds from it. I tried to fix it but unable to do so because transistors like B744, B731used in this diy are either expensive or out of production. As a cheap alternative of these components I order few components like BD170, BD190, MJE254, MJE711 and MJE712 from utsource. I hope this will help me to repair my diy. Any advice will be appreciated.
Kind of weird to have transistor failures in a keyboard. Most transistors fail in power amps because the user pulled the 1/4 phone plug part way out and shorted it, or plugged two outputs together in the dark, or a speaker coil turn shorted internally due to heat. Exception, little white plastic transistors in 70's early 80's Japanese products, they were ****. drivers & VAS can blow because the manufacturer was too cheap to put on a heat sink.
I sub transistors by package type and polarity, mainly. Pinout can be a problem in TO92 and TO126, watch for that. In audio, the only critical application I've found is high frequency sources as top octave piano sounds or tinkly bells. Those require Ft >3 mhz in drivers and VAS. Jfets and mosfets aren't in the old stuff I usually work on so no experience. Op amps blow up from current leaking out of the later blown transistor.
More common problems in keyboards, electrolytic capacitors in the power supply or coupling caps drying up and changing value. Tantalum caps whereever used. EPROMs and pmos ROMs that forget the data. Also microprocessors with memory built in can forget. RTL dividers, but that is mostly in organs. Rubber key contacts fracturing or getting compressed. Any microprocessor or memory problem, you need a donor keyboard, amateurs & individual techs can't debug those usually.
I sub transistors by package type and polarity, mainly. Pinout can be a problem in TO92 and TO126, watch for that. In audio, the only critical application I've found is high frequency sources as top octave piano sounds or tinkly bells. Those require Ft >3 mhz in drivers and VAS. Jfets and mosfets aren't in the old stuff I usually work on so no experience. Op amps blow up from current leaking out of the later blown transistor.
More common problems in keyboards, electrolytic capacitors in the power supply or coupling caps drying up and changing value. Tantalum caps whereever used. EPROMs and pmos ROMs that forget the data. Also microprocessors with memory built in can forget. RTL dividers, but that is mostly in organs. Rubber key contacts fracturing or getting compressed. Any microprocessor or memory problem, you need a donor keyboard, amateurs & individual techs can't debug those usually.
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I searched 'Korg EX-800 repair' and came to know that the regulators commonly fail in these model , especially B731 and B744.
I was planning on replacing the 100 uf capacitors that are near the power inlet as well.
Don't know whether TO-126 is an all epoxy package or if it has a thermal slug. I need to know more about it.Thanks for the advice though.
