HP 85662A, Troubleshooting And Repair Manual

Page: 382 / 594

The current through is determined by the difference between two current sources, one
involving Q2 and the other involving (See Figure 2.) The most convenient way to find
the current from each source is to measure the voltage across each emitter resistor (a 1 k
Ohm resistor should be used in series with the voltmeter probe tip to prevent the circuit
from oscillating and giving an erroneous reading). For Q2, the current through R60 must be
included. If results are inconsistent, the emitter resistor should be checked also. Check to see
that the BW5 line is at approximately -0.5 V for bandwidths
Figure 2. Input Buffer Amplifier, Current Sinks
In the LC mode (the four wider bandwidths), the BW5 line goes to approximately $14.8 V
and turns off the current source Q2. The current supplied by Q2 in the crystal mode is now
supplied through CR5 and R15 from the BW5 line. In the LC mode, the current through
can be found by subtracting the current through R5 from the current through R12. C76, L34,
and C77 form a feedback tank circuit to suppress oscillations.
Buffer Amplifier
The Buffer Amplifier is similar to the input Buffer Amplifier. The input is selected by the
BW5 line from CR9 in the LC mode or from CR8 in the crystal mode.
In the crystal mode, the current through Q4 is determined by the difference between the
current by and that sunk by Q5 (about 4 A significant deviation from
this current should be reflected by the gate-to-source voltage of Q4. The source should be
at least 0.4 V more positive than the gate, but not more than 2.0 V more positive. If the
difference is less than 0.4 V, the FET current is too high; if the difference is greater than 2.0
V, the FET current may be too low. In the either case the FET could also be defective. To
determine precisely the current through Q4, the difference between the current through R47
and that through R61 should be subtracted from the current through R41. If the results are
inconsistent, check the above-mentioned resistors.
In the LC mode of operation, current is supplied through R44 and CR11 from the BW5 line
instead of through
The difference between the current through R44 and that through
R41 yields the FET current. C79, L35, and form a feedback tank circuit to suppress
oscillations.
2