> - First tried, the transfo alone - OK-
> - then tried the transfo and the rectifier - OK-
> - connected the capas through 1Mohm (3x330k 2W)to make the
> first charge very
> slow. result: Another branch of the rectifier bridge
> blew-up, killing again the
> "starting resistor" ... (the capa bank was mounted
> "floatting" on the bench,
> with no connection to the tube or the chassis).
I don't understand how you could charge the capacitors
through 1M ohm 6 watts when the bleeder resistance is 400k
ohms 56 watts. This would only charge the caps to 1/3
voltage.
Also what was the voltage rating of the 2W resistors? You
would have 1200V across each. Are they rated to handle that?
> I guess that the rectifier bridge has been damaged by the
> first "bang" (noticed
> some yellow marks on a few ceramic disk capacitors), and
> that everything
> has to be replaced on this board before trying again...
That is a VERY rare failure. The reverse voltage is not an
issue since it is essentially the dc output of the supply,
and current reserves are good. Perhaps someone is uing the
wrong fuse. Maybe a low voltage rated fuse, a slow blow
fuse, or too much current?
It is also possible but rare there is a HV winding to earth
failure, but that alone would almost never damage the
rectifiers unless the fuse was the wrong type.
> Just to note:
> - The original capacitor assembly is made of 8x450V which
> makes 3600V, thus a
> very low safety margin...
The original caps are 600V foil that is aged to 525V if they
are still using the same part. They are custom aged and
tested for leakage at the manufacturer, not bulk off-shelf
parts. Of course it is possible they have changed vendors,
but voltage is a small influence on life compared to
temperature. The temperature is low in that area so the life
has always been acceptable. What will ruin a capacitor very
fast is an open equalizing resistor or a backwards
capacitor.
> - The original rectifier bridge is designed for 5000V PIV
> (5x1N5408 in each
> branch), which looks also unsufficient taking into account
> that a normal
> rectifier design would require a PIV of twice the AC peak
> voltage (~6600V).
Not true. It is a FW bridge with capacitor input.
> As a first step, I am thinking about replacing the 1N5408
> by BY255 (1300V PIV -
> 3A). Any comment / ideas / suggestions ?
Yes, find the real problem first. I do not understand how
you tested or charged the electrolytics with a series 1M
resistance. I do not understand how the diodes could blow
running into 1 meg ohm, or even into a short. Perhaps it was
earlier diode damage, but that is VERY rare. The diodes are
one of the least likely failures in that 20 year old design
unless the fuses are the wrong type. Most common is people
use 32V rated fuses, but sometimes they use slow blow or too
much current. Capacitors are only likely when a equalizing
resistor opens.
I have an AL1500 I have been using since 1986. I have been
running the transformer on 250VAC into the 220V winding, and
use 20A 250V fuses. I haven't had a power supply problem in
21 years even though I use it almost every day.
73 Tom
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