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

Geoffrey Dick wa4ikq at nevets.oau.org
Mon Jan 21 06:31:34 EST 2008

Ron Tonneson <ron.tonneson at gmail.com> said:
> I think you want the modulator, not the discriminator.  The 
> discriminator is what extracts the audio from the received signal.

Yes, you are right about not needing to wire into the discriminator.
I was incorrectly thinking of the received side of the FM transceiver. 
The 1200 baud for a weather station, into a WxTrak is noncritical, 
because it only looks at noise level of the audio.   The transmitted
output packet string, would indeed, go into the modulator.
> 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
> 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.


Thanks for that pinnout, and the clue that the power supply is +8 Vdc.
Looking at the schematic I had, I suddenly realized, it was for the
wrong radio, a FT-2400 with a +5V supply.  No wonder it did not make sense.  
The 2400 has bypass caps wired directly to each line on the connector.
The 2800 has inductors to each line, a totally different design. 

> From: "Dave Baxter" <dave at emv.co.uk>
> Search the web for a manual for the radio, with schematic (try mods.dk
> for example, there are other sites too that don't need you to register)
> then you'll be able to identify and replace the part restoring the radio
> to full capability.  I'd be surprised if someone hasn't put a book for
> that "on-line" yet.


Thanks for the tip.  I found the schematic on line.  It was not in the
operators' manual, but in the technical supplement.    

> I'd also say dont short out the part as K0QVF suggests, if it was a
> coil, I'd expect the 8V to collapse way before any damage was done.

The component was as Ron stated, an inductor, L103.   It is in the 
configuration of a thin film surface mount resistor.  Burned, it became 
bloated and I could not really identify what it was, compared to the
adjacent inductors.  Do you suppose there might be a ferrite inside?

> You could bypass the mic amp altogether, but you should not perhaps go
> straight to the modulator, unless you are going to run 9600bd packet, as
> you still need to keep the audio filtering and pre-emphasis in line
> (idealy) for 1200bd use.
> Cheers.
> Dave G0WBX.

I will wire across the inductor tonight, and see if that restores 
functionality to the radio.   If that is successful, I will order the
part from Yaesu listed in the Technical Supplement.  Otherwise, I will
explore going into the in-line audio or pre-emphasis filter.

Ron, Dave, 

Many thanks for helping me sort this out.  You have helped make the
aprssig a great resource by taking the time to write.

Kind regards,

Geoffrey Dick

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