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[aprssig] APRS - discriminator direct on FT-2800M ?

Ron Tonneson ron.tonneson at gmail.com
Sun Jan 20 16:17:26 UTC 2008


Geoff,

I think you want the modulator, not the discriminator.  The 
discriminator is what extracts the audio from the received signal.

In looking at the schematic for the FT-2800 I see that there is an 
inductor in series with each of the mic connector lines.  If you do not 
have it here is the pinout:

1   SW2
2   SW1
3   +8 VOLT OUTPUT
4   GND
5   MIC INPUT
6   PTT

I would guess that it may be the coil in series with the +8 volt output 
that is damaged.  If so you could just try putting a jumper across it.

Ron - K0QVF

Geoffrey Dick wrote:
> Subject: discriminator direct / or repairing FT-2800M for APRS Wx
>
> This is a question of how to find the input to the discriminator
> circuit, so I can wire a WxTrak directly into it on a Yaesu FT-2800M.
>
> I took a chance and purchased a damaged radio at a tailgate.  The
> radio seems to receive okay, but there is no audio when it keys up.
> The circuit board in the radio head has a burned component on it from
> a short in the microphone cable. I believe worn, twisted wires in
> the cable shorted to 5 volt power in the cable near the connector.
>
> My options are:
> 1) Replace the burned component on the radio head circuit board.  
>    It looks like a diode, or could be a surface mount capacitor, 
>    but is too damaged to read the markings on it. I can take a
>    photo of it and send it to someone who would know how to 
>    identify it.
>  
> 2) Send it into a Yaesu repair center.  With shipping, it would
>    almost cost as much as a new 2M radio with a warranty.
>
> 3) Find someone who has a FT-2800M with a different problem, i.e.
>    damaged final, etc. and salvage the radiohead circuit board.
>
> 4) Use the radio as-is by simply wiring audio directly into the
>    discriminator circuit, and somehow keying up transmit by 
>    another hookup that bypasses this section of the radio head.  
>    I have a schematic, but no component layout or parts list
>    to guide me. 
>
> Anybody on the SIG have any suggestions that might play into
> these 4 options, so I can use the radio for a weather station.
>
> Geoff, WA4IKQ
> Winter Park, FL
>
>
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