[Amps] AL1500 PSU design

[Karl-Arne Markström]

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 at free.fr>
To: <amps at 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
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> _______________________________________________
> Amps mailing list
> Amps at contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
> 
> 
> -- 
> No virus found in this incoming message.
> Checked by AVG Free Edition. 
> Version: 7.5.472 / Virus Database: 269.8.13/844 - Release Date: 2007-06-11 17:10
> 
> 


-- 
No virus found in this outgoing message.
Checked by AVG Free Edition. 
Version: 7.5.472 / Virus Database: 269.8.13/844 - Release Date: 2007-06-11 17:10