[sdiy] Korg SDD-3000 5 volt supply problems

Adam Inglis 21pointy at tpg.com.au
Thu Nov 15 10:12:10 CET 2018

Thanks all
Replaced all the tantalums - no fix. 
Barry’s advice reminded me I really should have a current-controllable bench supply (instead of messing around with wall-warts and regulator ICs!), so I got one and did some tests with it.
The digital board seems fine when run from the bench supply.
Disconnecting a few things on the PSU, I supplied the 7805 with 10 volts from the bench supply, and hung a dummy load of 1 w resistors off the 5 v output of the PSU. 

With 7.8 ohm resistance, the 5 volt rail was pretty steady. With 6.7 ohms, the bench voltage shut down. So about 700mA seems to be the breaking point.

That would seem to leave a 1000uF electrolytic, a 0.1 uF disc cap, and a diode that could be suspect. I’ll get back to it tomorrow.

> On 15 Nov 2018, at 1:22 PM, rsdio at audiobanshee.com wrote:
> Good suggestion, and sometimes it’s possible to unlatch an individual pin/wire from a connector for a current measurement like this, and then snap it back in place later. Good as new.
> Another option is to look for any kind of series resistor or other component, then measure the voltage drop there. Not likely that you’ll find such a component every time you need one, but it’s worth a search before breaking the circuit.
> Brian
> On Nov 13, 2018, at 10:52 PM, Oakley Sound <oakleysound at btinternet.com> wrote:
>> The first thing I would do is measure the current that the 7805 is supplying to the digital board. However, this may mean you have to break the wire carrying the +5V to that board. There is a possibility that the digital board is taking too much current and causing the 7805 to go into current limit. This would suggest a fault on the digital board somewhere - a decoupling capacitor is the most likely culprit.
>> That said, it may be the 7805 itself that has gone faulty so it may be just easier to replace the 7805 in the first instance.
>> Tony
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