[sdiy] Korg SDD-3000 5 volt supply problems

Adam Inglis 21pointy at tpg.com.au
Wed Nov 14 02:35:43 CET 2018

I think this unit is about 30 years old.
It’s a bit noisy but it has mostly worked well since I’ve had it since the late 90s. At one point, I recall I replaced some electrolytics to cure a constant voltage on the LED headroom meter. There was some internet mythology that ALL the electrolytics need to be replaced, but I’m pretty sure I didn’t do that. 
Many years ago I repaired some heat damage, cracked joints, on the PS board and it has worked well since.

Anyway it’s developed a new fault: every so often, it resets itself. 
The 5 volts supply is a little low, around 4.7 - 4.8, and slowly fluctuating, then every couple of minutes or so, maybe even 10 minutes may pass, there is a sudden drop to around 3 volts for a few seconds, causing the CPU to glitch and then reset.

The 7805 regulator, when unloaded, seems rock steady at 5.1 volts.
There are two main boards, a mostly-digital and a mostly-analogue board, both receiving the 5 v as well as the +/-15 v supplies. The culprit seems to be on the mostly-digital board.
There were some corroded electrolytics filtering the supplies on the digital board, they were replaced with no effect. I can’t feel any hot tantalums anywhere.
The fault doesn’t appear to be thermal related, it can happen after a cold start.

I’m thinking a clue might be that most of the time it is slightly low and slowly fluctuating - what could cause that?

Any thoughts?

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