[sdiy] Korg SDD-3000 5 volt supply problems

Michael E Caloroso mec.forumreader at gmail.com
Sat Nov 17 01:23:38 CET 2018


Not all 7xxx regulators are rated for 1A or 1.5A in the TO-220
package.  Some are only rated for 500mA even for the TO-220 package.
Pay attention to the OEM & *FULL* part number, and check your
databooks.

MC

On 11/16/18, Steve Lenham <steve at bendentech.co.uk> wrote:
>>> 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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