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Model 8904A Instrument Changes
~~
0698-0085
0757-01 99
1901
-0880
NOTE
0
2 Fixed Resistor,
261
OR
3
2
Fixed Resistor,
21.5
kR
7
4
Diode
It
is
possible
that
the
22
gauge black wire may break while replacing the new
ground wire.
If
it
does, strip
8
mm
(5116
inch)
of
insulation from it and
properly wrap and solder it to a second new
lug.
10. Replace the screw in the standoff, inserting the screw through the lug with the black wire first
and then through the lug with the new ground wire. Tighten the screw.
11.
Put the main wiring harness next to the standoff and secure both together with a gray cable tie
(HP 1400-0249). Cut off the excess tie wrap lead. Make sure the box on the tie wrap is toward
the inside
of
the instrument.
12. Replace the top cover and make a quick check of the operation of the instrument.
7-8.
MODIFICATION FOR OUTPUT OVERVOLTAGE PROTECTION IMPROVEMENT
(Serial Prefix
2747A
and below)
The output overvoltage protection circuit
of
the HP 8904A Multifunction Synthesizer has a potential
problem. On instruments with serial prefix 2747A and below, excessive leakage
in
four clamp diodes
(A3CR415 to A3CR418) shifts the trip point of the circuit to a voltage too high to protect the output
against inadvertent application
of
high reverse voltage.
No
failures due to this problem have been observed, but all instruments with serial prefix 2747A and
below should be modified.
Modify the instruments as directed below. (Refer to Service Sheet SS4
for
schematic and component
locator.)
1. Order the parts listed in the table below.
2. Replace diodes A3CR15 through A3CR18 with part number 1901-0880.
3. Replace A3R494 and A3R95 (1780s1) with part number 0698-0085 (26100).
4. Replace A3R491 and A3R493 (34.8 kR) with part number 0757-0199 (21.5 k0).
5. If the instrument has Option 002, a second output, the same modification should be made to the
A10 assembly.
Parts
List
I
HPPartNumber
1
CD
I
Qty*
I
Description
I
I
*
If
the instrument has
Opt
002,
order twice this quantity.
I
rev.
15DEC89
Firmware Updates/Hardware Modifications 7-13