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TUCoPS :: Radio :: ft726_2.txt

Yaesu FT-726 mods


To      : ALL @ ALLUS
Date    : 900526/0352
Msgid   : BN 63093@WD6CMU, 2821@NM3G $63093_WD6CMU
Subject : TAPR PSK mods for the FT-726R

Throughput improvement for the FT726R - Tapr PSK modem 

On the Lo-19 and Ao-16 digipeaters I was only averaging 1 in 5 UI frames
returned.  Viewing my PSK signal on a scope at the FM varactor driver (Q08)
on the Sat Unit revealed a very distorted and jittery  audio waveform. 
Replacing the Tapr's obsolete 4030 (U21) with a 4070 gave me a steady but
still distorted wave.  My throughput was now 1 in 3; better but not good.

Modifying Tapr's low pass output filter (as suggested by G3RUH for his PSK
modem) and a change in the Tapr's attenuator and alignment has given me
almost 100% throughput.

TAPR Attenuator mod:
Remove R5 (22K) by putting a jumper on JP7.  Replace R6 (5K trim pot) with
a 100K trim pot.  Be careful when unsoldering as those small trace pads come
off very easily.

TAPR filter mod:
Replace R7 (22K) with a 47K resistor and C8 (.047 mf) with a .01 mf disk
FT726R mod:
Find plug J04 on the Satellite Unit (front left corner).  Buy an equivalent
plug and header pins and solder them together with short bare wire,
attaching the center conductor of RG174U coax to pin 2 and the shield to pin
1 of the adapter.  Then plug the Yaesu's plug onto the adaptor's header
pins, and the adaptor's plug onto the Sat Unit board's male header pins. 
You could solder the cable directly to the board or connector, but this way
the board's original condition is retained.

The waveform out of the TAPR (ungrounded end of JP8) will now be a somewhat
rounded sawtooth.  Adjust the 100K pot (R6) to mid range.  Set the FT726R's
Mic Gain to the first "tick" (8 O'clock).

Attach your scope to the RG174U coax and slowly increase the Tapr's output
(R6).  You will see the PSK's distorted waveform smooth out and grow to
become more like a sine wave.  Around .6 v pp it will be at it's best.  As
you continue to increase the PSK output distortions will reappear as the FT-
726R's FM limiter clips the signal.  Back off the audio until you get the
least distorted wave shape.  It will be almost a sine wave.

When you set your transmit power lever, back off so your ALC is not active.

Assuming you have all your software switches and parameters set properly you
should now have greatly improved throughput.  Let me know how this works for
you or if you improved your throughput in other ways so we can write it up.

Good luck.  73,
Howard Sodja  W6SHP @ wd6cmu

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