[Amps] AL80A

[Georgens, Tom]

Thanks to all for the responses.

In response to the questions, The amp does have a retrofit step start
installed by a previous owner.  A guest had reported that the amp did
not turn on at all and had verified that the interlock and the fuses
were OK.

My guess was the step start resistor or a bad connection.  When I opened
the unit, the step start resistor was indeed fried.  I replaced it but
the new one failed instantly as well.  I thought that it may be the step
start relay failing so I shorted the step start resistor but the fuses
blew instead.

At this point I disconnected the HV connection at the transformer
secondary, and the primary side of the AC (Fan, step start relay,
lights, etc) all worked fine.  It is then I started measuring the other
stuff and found the blown rectifiers.

Also the diode between HV- and Gnd was also shorted.

>From the feedback, it is probably likely the one diode failed and took
the rest with it.  I will replace them all and bring down a bunch of
spare diodes and step start resistors.

73, Tom  

-----Original Message-----
From: R; Measures [mailto:r at somis.org] 
Sent: Thursday, March 30, 2006 8:25 AM
To: Georgens, Tom
Cc: Amps
Subject: Re: [Amps] AL80A


On Mar 30, 2006, at 7:15 AM, Georgens, Tom wrote:

> Hello -
>
> My AL80A in Barbados failed after years of reliable service.
>
> It looks like all of the diodes on the rectifier board are shorted.  I

> also noticed that one of the diodes had a chunk blown out of it.  I 
> can obviously replace the diodes but I would like to know why they all

> failed.  Is it possible that if one diode fails, they all fail?

Yes, which is the usual case because series rectifiers are like
dominoes.

> Or, should I be concerned about a problem further down stream.

This is a possibility -- especially with an AL-80 because it uses no
glitch R in the +HV to limit fault-I during an anomaly -- such as an
intermittent  +HV arc to ground.

> Rudimentary ohmmeter checks of the capacitor bank/load looked normal

A white deposit near the pressure vent indicates a kaput electrolytic.  
When one or more electrolytics in a string goes bad, it's best to change
out all of 'em.
>
> Thanks for your help.
>
> Tom W2SC 8P5A
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Rich Measures, 805.386.3734, AG6K, www.somis.org