Re: [LAU] [solved] OT: Behringer PSU UB-SPSU2 for an Eurorack UB2442FX-PRO

On Wednesday, May 09, 2012 11:40:31 AM Ralf Mardorf did opine:

> On Tue, 2012-05-08 at 12:47 -0400, gene heskett wrote:
> > I hope I helped.
>
> You confirmed that it's a good idea to replace the capacitors, regarding
> to the age of the PSU. Yes, you helped.
>
> Btw. the capacitors I unsoldered where still perfectly ok.
> Not all capaictors are tested but we guess all are still ok, we anyway
> will replace them.
>
> We needed a good magnifier to see that a diode (IIRC near the
> opto-coupler) had a could solder point. I couldn't see it with the
> magnifier I own.

Could=cold=microscopic crack in the solder?

> The main issue seems to be that one HER 300 (a 50 ns fast diode) for
> rectification on the secondary side is borked.
> After we found out this, we found out that the Behringer UB-SPSU2 is
> known for this issue. Btw. it's also possible to get the circuit diagram
> in the Internet for free.
>
> The diode does cost nothing, but it's not easy to get here. No equation
> diode. We found out that there's one diode IIRC >= 3A >= 200V <= 50ns
> does exist here, Thomas anyway will order a HER 300. I didn't find it at
> my dealer. So no mixer today, I need to test my Ubuntu Studio by
> directly connecting to the Amp or using the headphone output of the RME
> card.

I'm not familiar with that particular diode. However power rated switching
diodes that fast really stretch the limits of the technology, so I would
expect a higher failure rate for that compared to the 3 to 20 microsecond
recovery times of a more normal power diode.

This recovery time can result in some interesting side effects, which has
nothing to do with your problem. Back in the 70's, I was a tx supervisor
for the Nebraska ETV Commissions KXNE, channel 19 site in northeastern
Nebraska. A GE transmitter, rated at 30kw out, it used a pair of Varian
4KM100LA klystrons for final amplifiers. Those things are hungry, needing
about 19.5 to 20 kilovolts at about 10 amps to run both of them. 20kv*10
amps is 200 kilowatts every hour its running. The supply was a triplet of
240 volt out, 14.4 kilovolt, 75kw rated line powerline distribution
transformers running backwards, which fed a 6 diode full wave bridge. Each
of those 'diodes' was actually about 60 std DO-5 diodes in series, with a
small capacitor and a small resistor across each one to equalize the
reverse voltages when they were off.

But those old diodes had about a 20 microsecond turnoff time, so there was
6 spikes of time during the ac cycle when the still turned on diodes caused
a very effective phase to phase short for that 20 microseconds. That was
relatively minor, but when they all finally turned off, the spike on the
powerline was a sight to behold. Coming out of every wall socket in the
building was the usual 124-127 volts ac, but superimposed were these
voltage spikes about 5 microseconds wide, and as much as 3 kilovolts peak.

Light bulbs and small equipment fuses didn't last as long as they should
have for obvious reasons.

> A note. Thomas has got best equipment from Weller for unsoldering and
> soldering, but even he had issues to unsolder and solder. He was near to
> take it along to his employment to do it with hot air.
> It's possible to do it with my Ersa C25 and a soldering tip that doesn't
> cause tinder, but since it's even hard with best Weller equipment for
> somebody who does such work everyday, it's no fun to repair the PSU.
>
> Thanks,
> Ralf
>
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Cheers, Gene

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