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Re: [Amps] UM4001CR pin diode source - or Alpha 86 vac relay mad

Subject: Re: [Amps] UM4001CR pin diode source - or Alpha 86 vac relay mad
From: Mark - N5OT <>
Date: Sat, 8 Aug 2020 07:19:36 -0500
List-post: <>
Hi Chris,

I have an Alpha 86.  It's been a fantastic amp, literally loafing at 1500 watts output with headroom to spare.  For like 20 years. But now it is soft-FAULTing on transmit.  I am pretty sure it's the diodes, because I think it did not protect itself quite enough when I was doing some boneheaded antenna adjustments on 160 meters.  I'm pretty sure what actually failed is the protection circuitry itself because that was not the first time I had beat on that amp.

For comparison I have also had the failure N6TV describes - the fuse is an easy fix, and the difference in the symptoms is you don't get the FAULT light when the little fuse blows.

I have a paper around here somewhere that a fellow wrote that describes in great detail how to convert our amps from PIN diodes to vacuum relays.  I am considering doing that, since QSK has fallen out of fashion at N5OT and vacuum relays would be fine. It's funny to hear guys say "I don't mind the noise of the vacuum relays."  How things change!  They used to be the epitome of silence.  But yeah, PIN diodes make less sound than vacuum relays.

I think there may be a professional amp repair-modification person on this reflector who does this exact modification.  I can't remember who it is.  I wanna say "N4-something."

Cloud of Wisdom - can you remind me where to find this paper, and who the amp repair person is?  Chris might like to know it, and I would benefit from being reminded.

Great amp.  Too bad the tube isn't more popular or plentiful, but mine are original, strong, and are only being limited by the fault protection at the moment (which may be why they have lasted so long already!).

73 - Mark N5OT

On 8/7/2020 6:11 AM, Chris Wilson wrote:

   07/08/2020 12:07

I have an Alpha 86 here with suspected TX / RX switching pin diode
failure. The amps TUNE function is suspect, and switching between
standby and operate gives a vey attenuated RX signal in standby mode.
In TX it seems to operate, TUNE display aside, normally.

The manual's schematic shows them as stud mounted UM4001CR devices. I
am unable to source these, does anyone know of a current source for
them please? Alternatively does anyone have instructions for
conversion to using two vac relays? Fast CW is not required, and the
noise of the vac relays not really a concern. Thanks.

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