HP 5305 B - Page 27

Model 5305B
Maintenance
c. Check Loop Amplifier U23. For a 1 kHz input U23(6)
should be about +9V to + 10V. If it is + 11 V or
higher. the loop responds as if the VCO output
frequency is too high. This is true when the loop is
receiving no input at U22(1). If U23(6) is low (about
+4 volts), the loop responds as if the VCO frequency is
too low and tries to increase it. This would indicate a
bad VCO (U24 pin 8) or a bad + 1000 chain (U21,
U20, and U19).
9E-5-36. VOLTAGE-CONTROLLED OSCILLATOR.
To check the VCO, connect an oscilloscope to U21(9).
There should be a square wave at half the VCO frequency.
For a 1 kHz input, the VCO output should be 1 MHz, 1000
times the input. The VCO should run between 25 kHz (U23-
6 at + 11 volts) and 11 MHz (U23-6 at +4 volts). If a square
wave appears at U21(9) from 12 kHz to 5.5 MHz, the
frequency should relate roughly to the voltage at U23-6. If
not, probably trouble is in circuitry associated with Q16,
R83, C42 or U24(9) If there is no indication at U21-9, check
the same circuits. Also, check that U23(6) is above +4 volts
otherwise the VCO tends to shut off. CR17 should prevent
this.
9E-5-37. If there is no indication at U21(9), connect the
scope to U24(9) and check for the following sawtooth
volta
g
e at the output frequenc
y
.
9E-5-38. If the waveform is improper, the VCO is faulty. I
f
the signal is present, and not at U21(9), U21 probably is bad.
If the signal is between 12 kHz and 5 MHz at U21(9), there
should be 1/500 of that at U22(3). If not, U19, U20 and/or
U21 are faulty.
9E-5-39. DETECTOR CIRCUIT. Check U23(3); it should
be at +1.5 volts ±.3 volts. A problem here will saturate U23.
Now test U22, the detector. If U22(1) has a faster frequency
than U22(3), U22(13) should be low part or all of the time at
a rate similar to that at U22(3).
U22(2) should always be TTL high (≈+ 3.5V).
9E-5-8