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Re: [Amps] Blowing out finals on a Icom 7300 using a Ameritron AL80B. He

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Subject: Re: [Amps] Blowing out finals on a Icom 7300 using a Ameritron AL80B. Help Please!
From: "Paul Christensen" <>
Date: Tue, 7 Jun 2022 21:12:29 -0400
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> "I suspect that we might need to find a HW 100 or 101..."

Yup.  Until the root cause is determined, I would use a tube-output "beater" 
transceiver as the exciter.  

Since the AL-80B uses a tuned Pi input, that almost rules out a VHF burst or 

Keep in mind too that the IC-7300 hot-switches most amplifiers.  To date, Icom 
has not fixed it in firmware.  In VOX and QSK modes, the 7300's amp SEND line 
releases approximately 3-4 ms before RF stops.  That means: (1) the amp's RF 
in/out relays are hot-switching; and (2) the 7300 is briefly operating into an 
open circuit as the input relay's armature opens with RF present.  

There are several published solutions to the problem, most depend on an RC 
circuit of some form.  I developed a pulse-stretcher that works well.  The 
device ensures that the 7300's amp SEND line opens after RF decays to zero.

Timing is managed by a tiny Adafruit microcontroller and can be adjusted in 
precise 1 ms increments.  RCA input and output jacks are optically-isolated to 
help mitigate RFI and power is supplied by an iPhone wall cube or PC USB port.  
The entire board is smaller than a matchbook.   It's also applicable to the 
Icom 705 and 9700 transceivers which suffer from the same design neglect.

Gerber PC board files, B.O.M., microcontroller code and instructions can be 
downloaded from my QRZ page.  The Gerber PC board is linked to OshPark.  I have 
nothing to sell.  

Paul, W9AC 

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