Motorola XTL 1500, Detailed Service Manual, Page: 137 / 452

June 15, 2005 6815854H01-A
4-12 Troubleshooting Procedures : UHF Range 1 (380–470 MHz) Band Main Board Troubleshooting
NOTE: The UHF band radios use high-side injection with an IF frequency of 109.65 MHz; therefore,
the receive VCO frequencies are 109.65 MHz higher than the selected radio channel
frequencies.
NOTE: If a failure is indicated in any of the VCOs, whose operation is defined in the table above, then
the respective circuit should be checked to ensure proper bias is being applied to the
transistor. Specifically, the transistor collector pin should be at 8.2 V and base-emitter voltage
should be on the order of 0.8 V. The steering line voltage should be in the range of 2 V for the
lower frequency end of operation, and on the order of 11V at the upper frequency end of
operation. There could be typ/- 1 V deviation from the above quoted numbers, which
allow for tolerances in the components. A clean, steady, frequency spike should be observed
on a spectrum analyzer of typically -18 dBm at the end of the resistive pad in the collector
circuitry of a properly working VCO.
3. Continue troubleshooting by using the Fail 001 troubleshooting chart in Chapter 5.
Table 4-7. Test Mode Channels UHF Range 1 (AUX 4 is not used for VCO selection in UHF)
Test
Mode
AUX (TX Logic)
TX
VCO
TX Freq
(MHz)
AUX (RX Logic)
RX
VCO
RX Freq
(MHz)
RX VCO
Freq
(MHz)
1 2 3 1 2 3
Chan 1 0 0 0 Q5825 380.0250 0 1 0 Q5901 380.0750 489.7250
Chan 2 0 0 0 Q5825 391.4750 0 1 0 Q5901 391.4250 501.0750
Chan 3 0 0 0 Q5825 408.9750 0 1 0 Q5901 408.9250 518.5750
Chan 4 0 0 0 Q5825 424.9375 1 1 0 Q5903 424.9875 534.6375
Chan 5 1 0 0 Q5826 425.0250 1 1 0 Q5903 425.0750 534.7250
Chan 6 1 0 0 Q5826 436.0250 1 1 0 Q5903 436.0750 545.7250
Chan 7 1 0 0 Q5826 455.8750 0 0 1 Q5905 455.8250 565.4750
Chan 8 1 0 0 Q5826 469.9375 0 0 1 Q5905 469.9875 579.5750
Table 4-8. VCO Frequency Calculation and Switching Logic (UHF Range 1)
VCO AUX1 AUX2 AUX3 VCO Frequency
RX VCO Q5901 0 1 0 489.65 <= f < 519.65
RX VCO Q5903 1 1 0 519.65 <= f < 549.65
RX VCO Q5905 0 0 1 549.65 <= f < 579.65
TX VCO Q5825 0 0 0 380 <= f < 425
TX VCO Q5826 1 0 0 425 <= f <= 470