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

Keith VE7GDH ve7gdh at rac.ca
Mon Jan 21 15:17:12 UTC 2008


Geoffrey WA4IKQ wrote...

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

Glad to hear that you found the drawing. If you are going to bypass the
blown inductor, perhaps consider what caused it to go in the first place
before you do that. If the inductor acted like a fuse and went up in
smoke, is some other component going to try and act like a fuse the next
time you fire it up with the inductor shorted out?

I just noticed that Ron said that the blown inductor is L103. There is 8
volts on J10-3 via L103. However the transmit audio path is via L105.
For use with a WXTrak, it shouldn't matter if L103 is blown. Maybe there
is some other damage. If I'm following the circuit correctly, the TX
audio goes from J10-5 thru L105 to J109-9 / J306-9 to C376. You could
verify continuity to that point with an ohmmeter. Then with the power
on, you could check to see if you see 3.29 volts on the other side of
R335. Also you could key the PTT with the MIC and inject some audio
(WXTrak? screwdriver with your finger supplying some 60 Hz hum? signal
generator?) on the MIC side of C376 and see if there is any modulation
on the transmitted signal. I presume you won't get any audio or DTMF
from the MIC itself with L103 blown.

73 es cul - Keith VE7GDH
--
"I may be lost, but I know exactly where I am!"





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