I have an AL-1200. The manual CLEARLY states not to rewire the amp so that
the high voltage is above 3600 volts. I was tempted to do this, as with my
line voltage and configuration, my amp runs about 3450 all the time. That
last 150 volts would make the difference between 1300 and 1500 watts out.
You may not have rewired it, but your line voltage and the configuration of
the amp is enough to push the voltage above the limit.
When the equalizing resistor opened, that cap probably saw too high a
voltage and blew. The high voltage has probably overstressed all the other
caps as well. I'd recommend replacing all of them so the voltage and ripple
current is equalized. You can get the caps from Ameritron, or other sources.
-Tony, K1KP
----- Original Message -----
From: <n9bx@mchsi.com>
To: <Amps@contesting.com>
Sent: Tuesday, October 30, 2007 1:03 PM
Subject: [Amps] Ameritron AL-1200
> Hi: I have a perculiar problem with the high voltage power supply on the
> AL-1200. Here is the sequence of events.
>
> I had the Amplifier on and it was idling and all of a sudden it crackled,
> stopped and about 15 minutes later did this again. High Voltage stayed
> the same ,(meter Indicates 3600-3700 VDC, The whole time. Well I shut it
> down and after the HV was alowed to bleed off I started checking the
> filter cpacitors. this amp used 210 MFD at 450 VDC and Had eight of them.
> Yes I know that 8X450 =3600 Volts, and that's about what my voltage reads
> on the meter but I figured the designers of this Amp knew what they were
> doing and there was an adequate safety margin built in.
>
> Well One of the Capacitors was Bad. In fact the little white Dot that is
> on the Capacitor was bubled up and the Capacitor did not charge or
> discharge. I then checked all of the 50K ohms 7 watt resistors across the
> Caps and sure enough two of them read open. These were the ones that were
> across this Cap and the one Adjesent to it.
>
> So I replaced the Resistors, This Time with 75K OHM Flame proof Resistors,
> all Eight of them with the same wattage and replaced all Eight
> Electrolotyic Capacitors, all I had were 220MFD at 450VDC so Hey A little
> bit more voltage regulation key down but not that much.
>
> Turned the unit back on Everything was fine. Made one QSO, Voltage was
> above 3600 VDC again as before and I thought the Amp was back in Business.
> Well after sitting with no load on it two hours it started crackling
> Again, and one again it does this at about the 15 minute interval.
>
> I am almost sure that I will find the same Bad components agin, but What
> is causing the Capacitors, Or more likely the equalizing resistors to fail
> which causes the Caps to Fail.
>
> Sorry for the Bandwidth but this amp has been in service for 12 Years with
> not a hint of Problems before This Started to Happen.
>
> 73 Bruce N9BX
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