Karl-Arne
Yes you are right: twice the peak AC voltage applies to full wave rectifiers
with transformer middle-tap... so even the original 5x1N5408 should do the
job... my mistake ! Guess that my hypothesis of a few damaged ceramic disks on
the rectifier board is the right one then...
Anyway thanks for your suggestions...
Patrick
Quoting Karl-Arne Markström <sm0aom@telia.com>:
> Since at every instant, the reverse voltage in the Graetz bridge circuit is
> divided over two series connected rectifier elements (2 x 5 1N5408's in this
> case), there is a small safety margin. Substituting BY255's will increase the
> margin somewhat.
>
> Another suggestion is that you also look into substituting "controlled
> avalanche" rectifier diodes which are more robust against reverse voltage
> spikes. One such diode is the Semikron SKa 3/17 which is a 3 A 1700 PIV type.
> http://www.semikron.com/internet/gecont/pdf/918.pdf
> I have had very good experiences of using these for commercial PA retrofit
> purposes.
>
> 73/
>
> Karl-Arne
> SM0AOM
>
> ----- Original Message -----
> From: <f6irf@free.fr>
> To: <amps@contesting.com>
> Sent: Monday, June 11, 2007 2:54 PM
> Subject: [Amps] AL1500 PSU design
>
>
> > 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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> > Amps@contesting.com
> > http://lists.contesting.com/mailman/listinfo/amps
> >
> >
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>
>
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