Fluke 6080A - FM TROUBLE SHOOTING (A14) 6 E-12.; Frequency Check 6 E-13.

TROUBLESHOOTING AND REPAIR
FREQUENCY AND PHASE MODULATION
FM TROUBLE SHOOTING (A14) 6E-12.
FM troubleshooting is divided into in three parts:
• Frequency Check
• Modulation Check
• Input Signals and Control Input Checks
Frequency Check 6E-13.
Use Table 6E-3 as a guide to check the performance of the FM oscillator for faults in
frequency lock. Note the relationship between the modulation frequency and the
divider frequencies.
Table 6E-3. FM Oscillator Frequency Check Table (Normal Operation)
NOTE: Set SPCL to 909, Freq to 800 MHz. Have EXT FM input equal zero.
FM DEV
EXT FM
Off
50 kHz
100 kHz
400 kHz
FREQ
MHz
80
80
80
80
VOLTS DC
TP11 TP12 TP4 TP2 TP3 TP9
15 0 0 22 6.3 0-1
15 0 0 22 6.3 0-1
15 0 0 22 6.3 0-1
7 0 0 11 6.3 0-1
DIVIDER FREQUENCIES
TP 5,15,6,8 U21
— * 5 MHz
200 kHz 200 kHz
50 kHz —
50 kHz —
DETECTOR
TYPE
Std U21
Std U21
NPI U11,23
NPI U11,23
* TP 5, 15 positive pulses; TP 6, 8 negative narrow pulses
Tolerances TP 2, 11, 12 = .5V; TP 4 = .01 V; TP 3 .2V
1. If the frequency is wrong, check the adjustment of oscillator frequency, L1, and
C9.
2. If the voltages are wrong, (TP11, TP2) check the steering circuit, U5.
3. If the divider frequencies are wrong check the dividers and input drive levels.
4. If the phase detector output at TP12 is high or low check the phase detectors U11
or U21 and current sources U23 and U50 or Q10, Q11 circuits.
5. Check low rate modulation path. Op Amp U38, should have a zero volt on
junction of pin 1 and C70.
6. Check CMOS switches, U32, 35, 36, 39, 43, 47 for proper control voltages.
7. Check associated CMOS switch drivers. Use the Modulation Control Table (Table
6E-1) for logic information. For U36, 39, 43, 47:
• V-on > +10V DC, nominally +13 Vdc.
• V-of < -12V DC.
8. Check for CMOS switch leakage of control to the signal path.
6E-11