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Subject: [AMPS] ACOM Amps
From: (Dick Green)
Date: Sat, 7 Jul 2001 00:08:05 -0400

Sorry I didn't get to replying sooner.

/\  ¿Does the vacuum relay operate from a speedup circuit using a coil PS
of 3 - 5 x the rated coil V, current limited by a series-R.?

I think (strictly from memory) that the coil voltage is 48V. I think there's
a series limiting resistor, but I can't remember (and there are so many
screws in the cabinet that I'm not going in the amp unless I have to!)

/\  ¿Do you have a kaput 3CX800A7 from the Alpha 87A?

No, the tube was still under warranty and I had to send it back to get a
replacement. But the story is kind of interesting:

I got the tube to replace one of the 87A's original 3CX800A7s that had been
causing "low filament current" faults shortly after warmup. The new tube
operated about 100 hours before failing. When installed, it had a somewhat
higher number of new tube "thumps" than I've seen in the past, but then
settled down. At about 80 hours or so, the amp inexplicably blew the two 20A
fuses at power up. It operated fine for several thousand QSOs in the next
contest, then started getting PIN diode voltage faults. I had seen that once
before and had replaced a couple of PIN diodes. I had a couple left and
started to do the troubleshooting procedure that pinpoints (no pun intended)
the bad diode(s). Then I started getting several other fault types
indicating problems with the low voltage power supply. At one point, I saw a
small puff of smoke coming from the LV power supply just before the fault

To make a very long story short, I tore the amp apart and tested both the HV
and LV supplies. In the latter case, I desoldered dozens of components to
check them out. I couldn't find anything wrong with either supply. When I
got the amp back together, it blew the 1A fuse in the step-start circuit. At
that point I realized there had to be a short somewhere and started checking
in the tube compartment. When I found nothing obvious there, I finally
realized that it could be a tube arc. Sure enough, when I removed the new
tube, the amp came up fine. My theory is that air had been (very) slowly
leaking into the tube, resulting in more and more severe arcs, until it was
so bad that even the low plate voltage during the step start caused an arc
which blew the step start fuse. Any other ideas out there?

I still had the old tube that had caused the low filament current trips, so
I put that one in and -- lo and behold -- the amp came up and operated
perfectly. I ran it through the WPX CW contest, making thousands of QSOs,
with no problems at all. I have a couple of theories of why the old tube
works now. One is that during troubleshooting I happened to replace the op
amp on the LV power supply board that measures filament current. Maybe it
was bad. Another theory is that when I removed the tubes when I replaced the
PIN diodes, some dust or gunk got down in the tube socket, compromising the
connection with a filament pin.

One more interesting note: when packing up the bad tube for return to Alpha
(then to Eimac), I stupidly rested the tube on its side. It rolled off the
table onto the floor, denting the outer ring and cooling fins. I figured
once Eimac saw that, they'd refuse the warranty claim. I told Alpha about
it, and said I'd be willing to swear in writing that the tube had been
faulty before the accident. Scott Ehrhorn at Alpha said he'd appeal to Eimac
for me, and they ended up replacing the tube. Good service from both

73, Dick WC1M

>----- Original Message -----
>From: "2" <>
>To: "Dick Green" <>; "Amps" <>;
>Sent: Sunday, July 01, 2001 2:10 PM
>Subject: Re: [AMPS] ACOM Amps
>> >
>> >Tom,
>> >
>> >I'm sure ACOM is alive and well. Last I heard, they had shifted a lot of
>> >their production to the new "1000" line of manual tune amps and are
>> >a lot of them. As I understand it, they underestimated the demand for
>> >2000A and as a result are not making enough of them. I would also
>> >that their margins are better on the 1000. I guess they decided they
>> >need to advertise either one.
>> >
>> >73, Dick WC1M
>> >
>> /\   Dick -- I used to work for the Technical Support Dept. of The
>> Pacific Missile Range at Point Mugu, CA.  Our dept. repaired Collins
>> 208-U10 autotune linear amplifiers.   When a 208-U10 was in good working
>> order, it was like Godzilla rising from the ocean.  50mW in would result
>> in 15kW out. However, on average, roughly half of the 208-U10 amplifiers
>> on the Range were broken, awaiting parts and repair.   It seems that mo'
>> complicated is not always mo' betta.     An interesting parallel is the
>> Antilock Brake System on automobiles.  Even though ABS is way more
>> complex than garden-variety brakes - because it's computer controlled,
>> ABS is supposed to prevent wheel locking, thereby reducing the chance of
>> a crash.  However, Insurance company statistics show that ABS equipped
>> autos have a higher accident rate than autos equipped with ordinary
>> brakes.  My guess is that this has something to do with the ABS'use of a
>> computer.  Do computers ever Crash?
>> -  Perhaps the margin on the model 1000 ACOM is better than the margin on
>> the autotune 2000A due to the difficulties in troubleshooting autotune
>> amplifiers?
>> cheers
>> -  R. L. Measures, 805.386.3734,AG6K,
>> end
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