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From: ehsjr <ehsjr@verizon.net>
Newsgroups: sci.electronics.repair
Subject: Re: Intermittent fault on Korg SDD 3300
Date: Sat, 25 Jan 2025 15:13:04 -0500
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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


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:
> 
> <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.
> 
> 
> 
> 
> 
>