John,
Yes, it's generally not a good idea to use an automatic antenna tuner to
compensate for a bad match between a rig and amp, especially a commercial
amp that's been designed to present a good 50-ohm match. It's important to
determine the cause of the bad match because it usually indicates a problem
that needs to be corrected. Also, some ATUs can get into trouble with a
dynamically autotuning amp like the 87A (if you have AlphaMax) because the
input SWR varies somewhat as the amp tunes. If the ATU algorithm is too
sensitive, either it or AlphaMax will keep hunting for a match and will
eventually fail.
First thing to do is check the SWR of the dipole on the 1000D's meter with
the ATU turned off and the 87A in standby. It should show similar readings
across the range of the dipole as the MFJ-269 and the HI SWR light should
not come on at the antenna's resonant frequency. If you get high SWR
readings and/or the HI SWR light comes on, then the problem is not the 87A.
It could be the dipole, the MFJ-269, the coax between the amp and antenna,
any switches you may have in the path, the coax between the 1000D and 87A,
or even a bad relay in the 1000D. If the problem is the antenna or something
in the path between the amp and antenna, the 87A should be showing high
reflected power when keyed in Operate mode (lots of green LEDs, one or more
yellow LEDs, one or more RED LEDs lit in the reflected power meter.)
If you get low SWR on the 1000D with the tuner off and the 87A in bypass,
confirm that the antenna and path between the amp and antenna are OK by
putting the 87A in Operate and checking the reflected power. Only one or two
green LEDs should be lit in the reflected power meter at the resonant point
of the dipole (presumably 1.3:1.)
This is optional, but if you have a KW dummy load, hook it up directly to
the 87A with a good piece of coax and confirm that the HI SWR problem still
exists on the 1000D. That would be the definitive test.
If you have eliminated the antenna, the path between the 87A and antenna,
the coax between the 1000D/87A and the 1000D itself, then the problem is in
the 87A. The most likely cause is a problem with the input bandswitch wafer
or the 80m input network. It's conceivable that the problem could be the PIN
diodes used for T/R switching, but I don't think it's possible for them to
be in a partially switched state without the amp reporting a fault. Also,
you would be having problems with other antennas unless there was some sort
of RFI problem affecting the PIN diodes (perhaps caused by RF currents on
the coax shield if you are not using a balun with the dipole.) In any event,
I think this is unlikely.
I think it's also unlikely that there's a problem with the 87A's 80m input
network since it's made up of a simple toroid inductor and some caps. It's
possible a marginal cap has failed, but unlikely. My money would be on a
problem with the input bandswitch wafer, which is soldered to the input
board. You can get a high SWR reading if the contacts and wipers on the
input band switch are slightly misaligned. If you know what you are doing
around high voltage and inside amps, there's a way to confirm whether this
is the problem. If so, it can be fixed with a simple firmware patch to
adjust the stepper motor positioning. If you do not have experience working
inside amps, I'd recommend you send the amp back to the factory for repair.
The voltages inside the 87A are lethal and it requires great care to avoid a
potentially fatal mistake.
It's also possible that the bandswitch contacts are worn out or there's dirt
between the contact and wiper in the 80m position. I recently began getting
fault 17 on 10m only, then later on 40m too. After eliminating misalignment
as the cause, I replaced the input board and the problems disappeared.
Examination of the old input bandswitch was inconclusive. The contacts
appeared to be clean and well-seated. The only thing I could see was that
the long strips on which the input and output wipers ride were no longer
fully pressed into the body of the switch and were slightly lifted at one
end. I think this may have eventually bent one of the wipers so it wasn't
making contact in some positions. Hard to say. I've had my 87A for almost 10
years, so it's not surprising that the wafer has worn out -- it's probably
the least rugged component in the amp.
Hope this is helpful. Feel free to ask more question, privately if you wish.
73, Dick WC1M
> -----Original Message-----
> From: John Nelson [mailto:john@crew-green.com]
> Sent: Wednesday, March 09, 2005 9:47 AM
> To: amps@contesting.com
> Subject: [Amps] FT-1KD and Alpha 87A input VSWR oddity
>
>
> My HF station consists of an FT-1000D and Alpha 87A. Having recently
> replaced an 80/40 trap dipole which blew down a few gales
> ago, I noticed
> that even on the dipole's frequency of lowest VSWR and with the 87A
> carefully tuned and loaded into it, the 'HI SWR' lamp in the
> 1KD flashes
> merrily in time with the keying on both bands. The VSWR
> indicated by the
> 1KD's metering under these conditions is about 3:1.
>
> The dipole was tuned up with an MFJ-269 and displays VSWRs of
> about 1.3:1 at
> resonance on both bands. The 1KD's auto ATU has no problem at
> all matching
> the dipole at any in-band frequency. The Alpha 87A appears to have no
> difficulty with the dipole at or near resonance. The 1KD's
> auto ATU will
> easily remove the mismatch if switched in when the amplifier is being
> driven, but Alpha/Power emphasises in the manual that
> automatic ATUs must
> not be used in conjunction with the 87A (it's not clear why).
>
> I suspect this is something trivial and silly, like the
> length of the feeder
> between the 1KD and the 87A, but has anyone had a similar problem?
>
> --
>
> John Nelson
> Crew Green Consulting Ltd
>
> www.crew-green.com
>
>
>
>
>
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