Great info, John. I had to replace a pair of PIN diodes in my 7 year old 87A
once, and now I'm almost certain the cause was too much RF from the second
radio/amp in my SO2R contest station. Although I use bandpass stubs to
prevent high RF levels on harmonics, at various times I've had multiple
antennas for the same band in the system (e.g., a monobander and a
tribander), and this can allow both radios to be on the same band at the
same time. I'm always careful not to let this happen, but occasionally
mistakes do occur. My guess is that the PIN diodes bit the dust due to such
a mistake. Perhaps they saved the rig front-end!
We don't get much static buildup here in the Northeast, so I don't think
that would have been the cause. I have Polyphaser suppressors on every
cable, but don't know if they would shunt a static discharge. Anybody know?
I haven't had a problem since replacing the PIN diodes, and now will be a
lot more careful about keeping the radios off the same frequency.
73, Dick WC1M
> -----Original Message-----
> From: amps-admin@contesting.com [mailto:amps-admin@contesting.com]On
> Behalf Of W0UN--John Brosnahan
> Sent: Thursday, January 02, 2003 9:33 AM
> To: Ragnar Otterstad
> Cc: amps@contesting.com
> Subject: Re: [Amps] Alpha 87A reliability problem
>
>
>
> Hi, Rag--
>
> I have three 87As that I have used fairly hard in the 1990s.
> Two of the amps are about ten+ years old and one is five+
> years old. I have never had a PIN diode failure in any of them!
>
> From the failures that I have seen in other amps, it appears
> that the PIN failures most often come from system (station)
> design. Not always, of course, but most often.
>
> There are two areas that need to be studied to determine
> station problems. I think the most common issue has
> to do with antennas that are not DC grounded. If you take
> an ohmmeter and test from the center pin to the shield of
> the coax there should be a DC ground. The problem (based
> on my experience with the locations of the failures in areas
> of high lightning activity and most often with dipoles and other
> non-DC-shorted antennas) seems to be static buildup on
> an ungrounded antenna--so that when the antenna is selected
> there is a large static charge that is discharged into the amp.
>
> Of course the amp has an RF choke that drains the DC charge
> on an antenna--but, if the antenna is left open-circuited and is
> then switched onto the amp, the static spikes due to the time
> constant of the RF choke. First thing I would do is to check
> all of your antennas with an ohmmeter to make sure they
> are all DC-shorts. If they are not then I would add a shorted
> 1/4 wave line to each--if they are mono-banders. If they are
> multi-band antennas then an RF choke on each one should be
> installed directly on the feedline--this prevents the charge from
> ever building up--even when the antenna is not being selected.
>
> The effect of this static discharge over time can even eat away
> the insulation on a PL-259 coax connector if left ungrounded.
> I have seen this on my old phased array of 160M wire
> verticals that was suspended between two towers. The insides
> of the connectors just "disappeared" over time. So a solid state
> device like a PIN diode is really subjected to abuse when a
> large static charge is introduced into the amp when an ungrounded
> antenna is selected.
>
> The second most common thing I have seen is in multiple transmitter
> situations where close antennas allow an 87A in the RX mode to be
> subjected to large amounts of RF from a second transmitter. This RF,
> if large enough, can damage the PIN diodes (or possibly even confuse the
> microprocessor to sequence improperly). Additional filtering to reduce
> the coupling of RF into the 87A while in the receive mode can help
> here. Again, shorted 1/4 wave stubs can provide some protection
> depending on the specific relationship of the two bands in question.
>
> Easiest thing to check in a multi-TX station is to see if you see
> reflected RF on an 87A when it is not transmitting, while you are
> transmitting on another band with another amp. If you see reflected
> RF of 20 or 30 watts or more getting into the amp when it is in the
> RX mode there is little protection to the PIN diodes. Station (system)
> design is again the key element to work on.
>
> Obviously I don't know what your station setup is--so I don't know
> if these issues apply. But I have seen this same sort of problem in
> a commercial station that is using multiple 87As and was having
> occasional PIN diode failures and once these two issues were
> addressed the failures ceased.
>
> And my own experience with multiple 87A amps in contest service
> shows (at least to me!) that the amps can perform for many years
> with NO failures. I have not experienced any failures of any sort
> with my three 87As--except for one instance when I did something
> really STUPID! ;-)
>
> To demonstrate the ruggedness of the PIN diodes I have pulled off
> the dummy load while running 1500 watts and transmitted into an
> open circuit and never seen a PIN failure. The protection circuitry
> in the 87A is fast enough and good enough to protect the amp
> when transmitting into an infinite VSWR. So that is why I feel that
> the first two scenarios are the most likely for scenario for those
> experiencing repeated PIN failures.
>
> I'll be happy to continue the discussion either on-line or off-line if
> you think I can help.
>
> 73 es gl John W0UN
>
> BTW You don't need lightning for a large charge to build up on an
> antenna. In dry climates you can get a very large charge built up on
> large antennas even during clear skies. I once disconnected the
> grounded balun from a large log-periodic to do some maintenance
> in it. I reminded myself that the antenna was ungrounded and
> to be careful. The day was cloudless and there was virtually no
> wind. But in a relatively short time that log periodic built up enough
> static charge to jump out and knock me to the ground even though
> I was wearing rubber-soled shoes and hadn't touched the antenna.
>
>
|