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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