broken korg ex 800 ?

broken in brain · July 22, 2012, 10:54:03 PM

Hey just found a korg ex 800 for the wacky price of 20$...of course it doesnt work but still i figured for the sake of parts it would be worth it...now the question...anyone have any suggestions for any interesting projects with any of the chips? anyhting interesting in there? still pretty new to all of this but up for a challenge. gotta be somthing crazy in there, right?

Colin

Circuitbenders · July 23, 2012, 12:25:38 AM

why not just get it working again? I doubt you'd be able to do anything with the worthwhile or rare parts that would be better than an ex800. Not without a whole lot of effort anyway.

If it doesn't turn on the problem is usually the power transistor in the PSU regulation circuit, either that or a dodgy power socket.

broken in brain · July 23, 2012, 03:52:48 AM

i see. i just didnt know where to begin..knowing that..so this is a common thing, eh? i guess ill give it a go...who cant use another synth. was playing my modded mc202 today and remembered why i love this shit. i suppose ill have to do the moog slyer mods too..lol i also have a casio ht-700 that is dying for some love too... thanks man

Gordonjcp · July 23, 2012, 03:23:49 PM

Do you know what's actually wrong with it?

broken in brain · July 26, 2012, 02:29:14 AM

i have no idea whats wrong with it. guy said it used to turn on. but now it didnt. for 20 bucks i was tempted. re soldered the power supply jack with my fingers crossed. it looked like maybe a cold solder joint. no luck though

Circuitbenders · July 26, 2012, 03:02:10 AM

Are you getting anything on the 5v rail? You can tell by testing the power pins of a random logic IC. If you're not getting anything then its probably a good thing, as a power problem will be easier to fix than tracking down a problem elsewhere.

Next thing i'd check would be the big diode just behind the power socket. It might be D2. I've seen this blown in half on more than one occasion! If this has gone you probably won't get any power.

Up by the power input there will be a component that looks kind of like a voltage regulator labelled as a B744 or sometimes a B731. You can see the datasheet here http://www.jmnic.com/pdf/2sb/2SB744%202SB744A.pdf .These also die on a fairly regular basis and the Poly800 will be dead without a working one.

nochtanseenspecht · July 27, 2012, 09:26:16 PM

yes ive had that same problem before with a poly800, a broken regulator.
its a funny synth, i liked the sequencer for some reason..
good luck

Gordonjcp · July 29, 2012, 04:39:40 PM

I'm not sure how similar the EX800 is - is it really just a Poly-800 board with the keyboard disconnected? I know that EX800 and Poly-800 ROMs are interchangeable.

Anyway, in the Poly-800 the DC adaptor input is fed through a simple 9V regulator comprising Q1 (2SB744) and a bunch of other components. If this fails you'll get no 9V across C3/C4 which feeds the rest of the circuit. The odd thing is that Q1 is in the negative supply lead - watch out for this!

Circuitbenders · August 05, 2012, 04:43:03 PM

According to this page the poly800 and EX800 boards are identical, but the EX800 didn't have the keyboard routines in the firmware in order to free up space for midi sysex operations.

http://analog.no/cms/index.php?option=com_content&view=article&id=51&Itemid=50

Gordonjcp · August 05, 2012, 09:34:38 PM

Yea, there's also a bug fix which stops the MIDI buffer overwriting the patches when things get busy.