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

Dave Baxter dave at emv.co.uk
Mon Jan 21 12:42:55 EST 2008

Hi again Geoffrey.

If there are other inductors in there of the same type (and from what is
descirbed, it's likely) you should be able to identify what it was by
looking at another part, in another line to the same socket.   You might
not get one of exactly the same physical form/size etc, but you should
be able to source one from somewhere you can fit.

It's not unusual either for a "Leaded" part to be fitted in place of a
SMD part when push comes to shove, with a lot of care it has to be said.

Have Fun..

Dave G0WBX.

> -----Original Message-----
> From: Geoffrey Dick [mailto:wa4ikq at nevets.oau.org] 
> Sent: Monday, January 21, 2008 11:32 AM
> To: aprssig at lists.tapr.org
> Subject: RE: [aprssig] APRS - discriminator direct on FT-2800M ?
> 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
> > 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,
> 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.
> Dave,
> 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
> wa4ikq
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