I have a Korg SDD 3300 triple digital delay rack unit that was a bit of
a basket case when I got it. Someone had tried to "fix" it at some point
and it had a number of issues including the wrong voltage regulator
slapped in the negative rail, miswired output jacks, dead EL backlight,
bad SRAM 2032...
Fixed that up and I was able to get a copy of the service manual and use
the three-finger-salute button combo to get its mind back after SRAM
battery replacement as is common on units of this vintage, followed the adjustment procedure in the service manual and everything seems to work
as it should now except one really irritating intermittent fault
remains: the LCD will occasionally garble up at power-on or in use as so:
<https://imgur.com/a/BjxoQDP>
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.
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:
<https://imgur.com/a/DkOs2Pv>
Thinking about just trying a new display with LED backlight at this
point but I've read that this unit can be finicky in accepting
aftermarket LCDs.
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:
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.
On 1/25/2025 12:29 PM, bitrex wrote:
I have a Korg SDD 3300 triple digital delay rack unit that was a bit
of a basket case when I got it. Someone had tried to "fix" it at some
point and it had a number of issues including the wrong voltage
regulator slapped in the negative rail, miswired output jacks, dead EL
backlight, bad SRAM 2032...
Fixed that up and I was able to get a copy of the service manual and
use the three-finger-salute button combo to get its mind back after
SRAM battery replacement as is common on units of this vintage,
followed the adjustment procedure in the service manual and everything
seems to work as it should now except one really irritating
intermittent fault remains: the LCD will occasionally garble up at
power-on or in use as so:
<https://imgur.com/a/BjxoQDP>
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.
When the failure is position sensitive, look for a bad switch or cable
that causes a line to float between a logic 0 and 1.
War story: I had a bad ice maker in the freezer that failed only when it
was installed. Worked 100% on the bench and not possible to troubleshoot
when installed. Turned out to be a bad micro switch that failed (high resistance instead of ~0 ohms when closed) only when I held the thing at
an angle.
Your problem looks like a tough one if it's a switch or cable. :-(
Ed
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