Circuit Overview:
A digitally controlled analog subtracive synth which is near identical in capabilities to AX60 but without sliders though it is fairly uncluttered at least and simple to use. Also it has a velocity sensetive 73 key keyboard. Inputs for their samplers to give access to the analog filters...see Chung's page for pinout. VX90 is the rack version.
Service Tips:
The one I got had cracks in the front board. Buttons get stiff and people hit them with the sledgehammer. Had to use goop adhesive to stick the boards together again and clamp til dry then wire over broken connections. Easy common sense repair and some buttons will get dirty. Auto-tune takes care of the 'misplaced note' syndrome so fear not :-). I initially thought something else might be wrong. Otherwise these look like a solidly built unit with of course the CEM3394 synth on a chip. So if you have a voice related problem start by swapping those and monitor signal on the voices to see if the problem follows the chip or the channel.
If there is a problem after the voice chips (eg. the VX90 I'm working which won't tune and has bad sounding noisy output but DOES seem to be trying to generate sound) then note that there are three dual op amps (5218's) near the voice chips. Each section is a current to voltage converter for the CEM3394 outputs. So they just have a feedback resistor in other words and a direct connection to pin 19 of the CEM3394 on the inverting input.
The output of each of these goes to a 10K resistor. Two of the voice outputs, on the VX-90 at least, go to R148 and 149 who sum to section one of IC41, another dual op amp. While the other four outs (odd they didn't do..three and three!?) go to R150, 151, 152, and 153. And they sum to the other section of IC41. So anyway that's a quick way to isolate where problems might be coming from in that section. The power supply connector P4 reverses direction when it attaches to the analog board. But from edge of the PS board going in, we have +15V, -15V (direct from7815 and 7915 on the heat sink) + and - from M5230L's transistor buffers (small heat sinks nearby) and then + and - from the 10 ohm resistors coming from those outs respectively. And ground on the 7th pin.
So the big pair of capacitors on that board filter the +-20V source for the 15 volt regulators which then feed the M5230's inputs. Output buffer transistors are D1189 and B891 for + and - respectively. The supply on the one I was working on had a strange set of measurments. 15V had half a volt of jagged ripple with 14V showing there which would make us thing something is drawing too much current there. The 10ohm resistors show .88V (88ma) on the + side and nothing on the - side. The voltages from the M5230L buffer transistors... +6v and -2.4V. To be continued...(10 years later, lol)
Ok first lots of high ESR caps (all 22uf, 100uF and 220uF). This got me half a volt toward 15V rail...nothing on deficient -6V. Friend Marc on the AX60 and myself on VX90 had same issue. I naturally swapped M5230 to no avail. Marc said his was power transistor. I got ok read B=120 and normal diode drops on 2SB891 unit on neg. 6V. Then later saw high diode drop on B/E!! Swapping in a B/E crosswired 2N6042 darlington I have worked nice. -5.93V out in loaded state. .71V drop on what shows 11 ohm read to -5V point.
Parts:
Let me know if you need a specific part from my listings in the parts section.
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