[sdiy] Korg SDD-3000 5 volt supply problems

Steve Lenham steve at bendentech.co.uk
Fri Nov 16 11:47:26 CET 2018


>> That's not right.  7805, TO220 package?  That shouldn't even be close to
>> shutting down at 700mA, unless you mean that the bench PSU is giving up
>> before the 7805?
> 
> 
> Gordonjcp, well spotted, absolutely right - I had this new bench supply machine incorrectly set up. Now, (after removing the 700 mA "over-current protection” limit on the unit, oops!  :::rolls eyes:::)  and running the PSU 5 volt line from that supply with various loads, I can’t reproduce the glitch at all.
> 
> But, once I reconnect the transformer winding to the 5 volt supply, it glitches as before, intermittently.
> 
> With the SDD-3000 hooked up to the bench supply, I plugged in a signal generator and calibrated it as per the service manual. Perfect. And no glitches over a couple of hours.
> 
> I’m wondering, can transformer windings fail like this, intermittently, under load?

1. Did you have any battery leakage on the PSU board? If so, any of the 
semiconductors could be suspect.

2. Are you able to 'scope the input of the 7805 when running from the 
transformer? Failure of the 2200uF bulk smoothing cap or perhaps one leg 
of the 5V bridge rectifier could cause excess ripple that falls below 
the input dropout voltage of the 7805 and cause temporary collapse of 
the 5V rail. Ripple at 50/60Hz rather than 100/120Hz indicates problems 
with the bridge. The trough of the ripple falling below 7.5-8V indicates 
insufficient smoothing.

3. One can't rule out a problem with the transformer, but since each 
winding is effectively just a long piece of wire, it seems unlikely. If 
there is a problem, it is most likely with how the ends of the wire are 
terminated.

4. IIRC the PSU PCB is single-sided. Check the plug-in power connectors 
(esp. those coming from the transformer) for cracked solder joints. I 
see this ALL THE TIME on power supplies.

I hope something in here is useful! Keep us posted.

Cheers,

Steve L.
Benden Sound Technology



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