9 May
2012
9 May
'12
6:02 p.m.
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:
Could=cold=microscopic crack in the solder?
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. 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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