[sdiy] Korg SDD-3000 5 volt supply problems

Adam Inglis 21pointy at tpg.com.au
Sat Nov 17 16:45:20 CET 2018

> On 18 Nov 2018, at 12:05 AM, Steve Lenham <steve at bendentech.co.uk> wrote:
> On 17/11/2018 12:47, Gordonjcp wrote:
>> On Fri, Nov 16, 2018 at 08:31:14PM +1000, Adam Inglis wrote:
>>> But, once I reconnect the transformer winding to the 5 volt supply, it glitches as before, intermittently.
>> Okay, so that eliminates the transformer, the diode bridge and the
>> smoothing cap, and possibly the loading of the bench PSU would affect
>> any runaway oscillation in the 7805.  
> I'm not sure that is correct.
> As I read it, Adam says that feeding the input of the 7805 with DC from his bench power supply results in flawless operation with no glitches, but that if he reconnects the transformer and powers it from mains, the glitching returns.
> This seems to point the finger at either the transformer, bridge rectifier or smoothing cap (including associated solder joints), not exonerate them.
> Adam, is that right?

Yes, that’s right. I remove the fuse for that particular winding only, then connect the bench supply at the DC side of the diode bridge, to supply the 7805 with 10 volts. The rest of the PSU circuit remains intact, and connected to the transformer to supply the +/- 15 volt rails. 
When run like that, the SDD-3000 is stable, and can be calibrated.
(The 5 volt supply draws about 625 - 650 mA current like this, so presumably the 7805 is not one of those lower-specced ones MC was referring to).

On my to-do list:
With the transformer winding re-connected, it may be useful, as Steve suggests, to scope the input of the regulator, as well as measure the current draw, during one of these glitches..

There’s no battery damage to this board. I replaced that years ago and it still works well.
There were a few cracked joints at the bridge and nearby heat affected areas, but these were re-flowed early in the piece. I haven’t re-flowed the entire board though.

Gordonjcp said "You have confirmed with your
sillyscope that the power supply is not in broad-band shortwave
transmitter mode?”
I’m hoping I’ll get some more detailed instructions as to how to go about that…!!!


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