I recently bought a second-hand AL1500, and the PSU blew up when I turned it on
the first time... (the amp was not used since 2002)
On top of the burned 10ohms /10W resistor "starting resistor" in the transfo
primary, I discovered that one of the filtering-capacitor was showing a clear
short-circuit, one 50k resistor on the filtering board was cut, and that one
branch of the rectifier bridge was showing a short circuit.
I replaced all the defective components (5x1N5408), built up a new capacitor
bank with a better safety margin (12x400V - 330UF)and replaced the "starting
resistor".
- 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 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...
Just to note:
- The original capacitor assembly is made of 8x450V which makes 3600V, thus a
very low safety margin...
- 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).
I am surprised to see that despite the obvious design weaknesses, this PA seem
to work in so many shacks and seem to have only "satisfied customers" (i.e Eham
review).
As a first step, I am thinking about replacing the 1N5408 by BY255 (1300V PIV -
3A). Any comment / ideas / suggestions ?
Patrick
F6IRF/CN2WW
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