Jiri,
U2 on the High Voltage Power Supply Board is responsible for IP monitoring.
I suspect your U2 has gone bad. The same chip went bad in my 87A, though the
symptoms were not as drastic as yours. The bottom LED would light up even
when no current was flowing. Eventually, it lit up the second LED. I figured
it would eventually go up the scale and cause problems like the one you are
having.
In my 87A, U2 is a Motorola MC1536U 8-pin op-amp. It's possible that
different revisions of the PSU use different op amps and passive components,
so you should check which chip is actually on your board. When mine went bad
a few years ago, I tried to find a replacement but discovered that the chip
is no longer manufactured. Some suppliers still carry it, but you have to
buy in very large quantities. Alpha ordered a bunch of the chips and sent me
one, and I used it to replace the bad chip. Problem solved.
You might want to check with Alpha to see if they have any of the MC1536U on
hand they could send you.
73, Dick WC1M
> -----Original Message-----
> From: Jiri Culak [mailto:jiriculak@aol.com]
> Sent: Saturday, March 17, 2007 2:23 AM
> To: amps@contesting.com
> Subject: [Amps] Alpha 87A
>
> Hi guys,
>
>
>
> ALPHA 87A
>
> Problem -
>
> Turn apm on, leave it heat up and already IP meter is going
> up and down. I suspected just LED board, but nothing wrong
> there. When AMP goes into op.
> mode
>
> IP goes between 0.1 - 1.0A on the meter which adds it to
> total. So if I put 20W drive in and get 250mA of IP on real.
> LED meter shows 1250mA and IP start to flash and 3sec later
> amp trips over. Emailed alpha service and got email back bit
> later that it might be connectors. I tried that, still
> exactly same. Looking on the board as don't have schematic,
> there is OP AMP which suppose to sense voltage across
> resistor in plate circuit. All metering for IP is done
> probably on PSU board.
>
> Any ideas???
>
>
>
> Thanks,
>
>
>
> Jiri M0ITY / OK2IT
>
>
>
>
>
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