TM 9-4935-601-14-7&P
Model 8640B
INSTRUMENT MODIFICATIONS
7-7.
INSTRUMENT IMPROVEMENT
MODIFICATIONS
7-8. Hewlett-Packard has developed certain
recommended instrument modifications that can be
used to improve the performance and reliability of
earlier versions of the instrument. In some cases,
replacing certain parts requires a modification to make
these instruments compatible with parts now in use (if
the original part is no longer available). These
modifications are outlined in the following procedures
and are keyed to instruments by serial number or serial
number prefix.
7-9.
Improved FM Metering Accuracy (Serial
Prefix 1229A)
7-10. On instruments with serial number prefix 1229A,
and with an A2 Meter/Switch Detector Assembly that
has a 08640-60031 part number, FM metering accuracy
can be improved at high modulation rates by removing
the two 2.37V zener diodes, A2VR3 and A2VR4 (see
Service Sheet 17). The diodes are in series with each
other and in parallel with A2R3 (at the input to A2U1).
7-11.
Elimination of Frequency Shift in Expand
X100 Mode (Serial Prefix 1229A)
7-12.
On instruments with serial number prefix 1229A,
the frequency counter OVER FLOW lamp must be
rewired to prevent RF Oscillator frequency shift when
switching to the EXPAND X100 counter mode. The
overflow lamp causes the +20 volt power supply voltage
to drop, thus changing the RF Oscillator frequency. The
modification rewires the overflow lamp to the +5.2 volt
power supply and removes current limiting resistor
A8A4R4, 220 ohms.
7-13.
Perform the following steps and refer to Figure
7-8 and Service Sheet 20.
1. Remove instrument top cover (see Service
Sheet F for procedure) and disassemble A8
Counter/Lock Assembly to gain access to the
A8A2 and A8A4 Assemblies (see Service Sheet
B for procedures).
2. Replace A8A2R57 (on A8A2 Counter/Lock
Board Assembly with part number
08640-60027) with a 0.47 ohm resistor (HP
0811-1662). The existing value is 0.82 ohms.
R57 is the current limiting resistor for the display
LED's from the +5.2 volt supply.
3. On A8A4 Counter Display Assembly (with part
number 08640-60025), unsolder and discard
A8A4R4, a 220 ohm, 2 watt resistor.
4. Solder a length of insulated wire from top
resistor pad of A8A4R4 (nearest overflow lamp)
to narrow trace running down center of ground
plane.
5. Reassemble counter and test OVER FLOW
lamp operation as shown in Operator's Checks
in Section III.
7-14.
A8A2 Counter/Lock Board Assembly
Improvements (Serial Prefix 1229A)
7-15.
On instruments with serial number prefix 1229A
and an A8A2 Assembly with part number 08640-60027,
the following modifications will improve reliability (refer
to Service Sheets 20 and 21):
1. In the phase lock mode, the pulse width into
U4A and U28 may be insufficient to clear the
stall counter, and the instrument will not lock.
If the problem occurs, replace R48 with a 15 µH
inductor (L2, HP 9100-1620) and check that C12
is 100 pF.
2. In the count mode, the pulse width from the
output of U17C may be insufficient to clear the
counters (U19 to U24), and the count will be
incorrect, usually all zeros. Should the problem
occur, replace R46 with a 24 µH inductor (L1,
HP 9100-1622). Change C11 to 470 pF (HP
0160-3455).
3. In the phase lock mode, a low borrow output of
U24 ripples through gates U13C and D to NOR
gate U25B. The next clock pulse from U25A
clocks the borrow into the countdown input of
U23 (which clocks on a positive going pulse). If
the borrow from U24B is removed too quickly
from U25B, then U23 will not be clocked and the
instrument will not phase lock. Should the
problem occur, add a 100 µF
7-78
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