Tait T850 II Series, Service Manual

Page: 63 / 370

M850-00
T855 Initial Tuning & Adjustment B3.11
Copyright TEL 31/09/98
Tune L385 for zero beat.
Note: If a second oscillator is not available, you can connect a frequency counter to
IC710 pin 8 (i.e. after the TCXO buffer) via an oscilloscope probe to measure
the TCXO frequency directly (12.8MHz). At this point the voltage level is
approximately 4V p-p.
Readjust the front end helicals (#FL410 and #FL420) to give the best sinad.
Change the RF signal level to -75dBm and modulate with ±3kHz deviation
(±2.4kHz) [±1.5kHz] at 1kHz.
Connect an oscilloscope probe to SK320 pin 3 (RSSI 455kHz input) and connect
plugs PL210 and PL220 to give a flat audio response (refer to Section 3.5).
Readjust L310, L320, L330, L340, L350, L360, L370 and L380 to give a maximum
amplitude response on the oscilloscope with minimal amplitude modulation.
Note: If you would like a more accurate method of tuning the IF, refer to the sweep
tuning method described in Section 5.5.5.
Further adjust these coils, along with L390, for minimum audio distortion, ensur-
ing that the 455kHz level (on the oscilloscope) does not fall significantly.
Check that the distortion reading is:
wide bandwidth
≤
2%
mid and narrow bandwidth
≤
4%.
If required, reconnect plugs PL210 and PL220 to give a de-emphasised audio
response and check that the distortion reading is
≤
2% (all bandwidths).
Reduce the RF level until 12dB sinad is reached. The receiver sensitivity should
be better than -117dBm (de-emphasised) or -111dBm (flat), assuming that the
audio levels are not being overdriven (refer to Section 3.11).