User Guide

PAMS
Technical Documentation
NHD–4
Disassembly & Troubleshooting
Page 47
Original 11/97
• Check the tune voltage on pin 1. It should be between 1.50 and 2.50 V.
If not, check R302, R301, and C302. Check the RX_SLOPE and AFC PDM lines.
Manually adjust them with the PDM controls within Service Software. Read the
RX_SLOPE PDM voltage at R700 and read the AFC PDM voltage at R701. While
tuned to channel 384 the RX_SLOPE PDM should be about 3.00 V. For channel 440
it should be 0.0 V. The AFC PDM will vary between 0.0 V and 3.00 V, generally near
the midrange of 1.50 V.
PLL IC (N301)
• Check the biasing on the PLL IC (N301). Pins 4 and 5 should have
approximately 3.15 V on them. Pin 18 should have about 4.40 V. Check
pin 19, AON, it should have about 2.80 V.
If not, check VRXS at C207 near the CDCONT IC. It should be approximately 4.40
V. Check the pin 14, Iset, for 1.25 V. Check VRX90 at C205 or R310. It should also
be approximately 4.40 V. Check R331, R307, R308, R310, R311, R807, R810 and
C337.
VHF Synthesizer
• Check the tune voltage at the cathode of V301. It should be stable at a
single voltage between 1.50 and 3.5 V.
If not, check to see that the 180 MHz oscillation is feeding back into the PLL IC at pin
15. Continue with the following two checks. Ensure that the varactor diode, V301 is
in the circuit correctly.
• Check the biasing of the buffer amp, V303.
The collector should be biased with 4.30 V.
The base should be biased with 3.30 V.
The emitter should be biased with 2.60 V.
If not, check VRX90 at R806 or at C205 near the CDCONT IC. It should be approxi-
mately 4.40 V. Inspect L303, R316, R318, R322, R323, R324, and R806. Also in-
spect C319, C321, C322, C323, C324, C328, C329, C330, C332, C333, and C340.
• Check the biasing on the oscillator transistor, V302.
The collector should be biased with 2.60 V.
The base should be biased with 2.30 V.
The emitter should be biased with 1.73 V.
If not, check VRX90 at R806 or at C205 near the CDCONT IC. It should be approxi-
mately 4.40 V. Inspect L303, R316, R318, R322, R323, R324, and R806. Also in-
spect C319, C321, C322, C323, C324, C328, C329, C330, C332, C333, and C340.
4.3.2.4 UHF Synthesizer
• Check the biasing on the UHF VCO, G301. There should be
approximately 4.25 V at pin B.
If not, check R808. Check to see that VRXS is 4.40 V at C207.