Re: Intermittent fault on Korg SDD 3300

On 1/25/2025 2:58 PM, Dave Platt wrote:

In article <67951f89$1$3620713$882e4bbb@reader.netnews.com>,
bitrex  <user@example.net> wrote:
 

It's annoyingly unpredictable and seems somewhat related to how it's
mounted? I can power it up 25 times on the bench and it works fine
making me think I solved the problem, then slide it back in the rack and
it starts acting up again. I've tried a number of things like adding
bypass capacitance on the display board which is separated from the CPU
by good distance, disconnecting the backlight thinking it might be
interference from the inverter, moving the wiring around, also recapping
the PSU which I was planning on doing anyway. Doesn't seem to help.
>
Here is the upper digital board (the analog board is on the lower
level), the lines to the LCD from the NEC Z80 variant are on the far right:

 I'd be looking for things which could cause an intermittent connection
on that cable and its connectors... maybe a bad or dirty pin or
socket, maybe a hairline crack where one of the pins is soldered to
the PCB.  A bad bus-driver on the main PCB (either a separate chip, or
dedicated pins on the microprocessor) might have a similar effect.
 If the characters being shown were characteristically off by one bit,
it'd point to one of the data lines.  As it is, they seem to be
rather unpredictably garbled, which suggests to me that one of the
clock or handshaking lines might be bad.  Glitchy rising or falling
edges on the "latch your data" signal might result in data being
latched at the wrong time, while the data bus was in transition,
and this could lead to all sorts of nonsense being displayed.
 Might be worth pulling the cables, fluxing and re-flowing the
connecting pins on the PCB, cleaning everything thoroughly.
 If you have a DSO or logic analyzer which has a "look for glitch and
runt pulses" acquisition feature, scoping the data and handshaking
lines at the LCD while actively driving the display, and tapping on
cables and the PCBs, might prove instructive.
 

Thanks for getting back, I'll follow up on those suggestions thank you! No logic analyzer available at home unfortunately but I have one I can probably use for a time if nothing else works..
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