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board (column 9), through the X-winding to pin 12 on the
top diode board (column
7),
through a diode in diode pack
32 on the top diode board (column 6), to terminal 4 on the
top diode board (column 3) to the X-read
hi
gate, E2Bl2
(column 2).
Likewise, it can be seen that X-write current flows from the
4.1.2.2.4 Exposing Bottom Di.ode Board:
If
an open Y-
drive line exists and the fault cannot be located on the top
diode board, remove the BSM to expose the bottom diode
board.
1.
Disconnect
all
cables to the BSM.
X-write hi gate, S2Dl 1 (column 4), in the reverse direction
2.
Remove all the cards.
through the X-winding, to the X-write lo gate, D2J09
(column 15).
3.
Remove the DSM (weight: approximately 18 pounds
- 8,172 grams) and lay the unit on a table with the
card side down, pin side up.
4.1. 2. 2. 2 Locating an Open Diode: Because of the complex
connections of the isolation diodes, a continuity check is
difficult. To locate an open diode, use the method described 4.
next. The cards named are for the same failing X·address
(000110) discussed in Section 4.1.2.2.1. Refer also to
Loosen the four nuts which hold the array onto the
board. It is now connected by only the drive and
sense-inhibit cables.
Figure 4-5.
1.
Turn off power.
Note: It Is now possible to raise the board separately
leaving the array resting on the table and expose the
bottom diode board, or you may continue.
2.
Remove X gate cards 02 and E2.
s.
Turn the unit over. Support the array since it is con·
nected only by wiring.
3.
Probe the points shown with the ohmmeter; be sure
to observe the polarity of the meter as indicated by
6 .
the+ or-. Expected meter readings are infinity (oo)
Pull the array out vertically and turn it over so that
the top side is down and lying on the card sockets.
The bottom diode board is now completely exposed.
or some resistance (R, unpredictable because of cir·
cuit variations and the meter in use).
An
open or shorted drive line may also be verified by
scoping the current source resistors. See Figure 4-16 for
the waveform of the Y-read current source with either an
open drive line or two drive lines shorted together (see
SR264 ). If either is correct, an open diode is likely. Make
a continuity check to determine which of the two diodes
in the line is open.
If
an open diode exists, the charts of SRI 74 and SR184
will indicate the polarity of the diode to be replaced. See
the bottom of Figure 4-4 for diode locations with respect
to the charts.
4.1.2.2.3 Replacing an Open Diode:
An
individual diode
cannot be removed since it is part of a module containing
16 diodes. Replacement consists of soldering an individual
GY diode (part 2414891) over the defective one. (A
shorted diode calls for replacement of the BSM.)
When replacing a diode, use thermal set compound (part
814007) as a heat sink. Wrap one end of a yellow wire to
the wrap terminal on the diode board and solder the other
end to the diode. Solder the remaining end of the diode to
the solderable pin on the edge of the diode board. After
diode replacement, check for reliable DSM operation.
4.1.2.2.5 BSM Replacement: Most systems supply -30
volts to the BSM with a single 'mini-bus' connector. (See
SR264 for distribution points.) Earlier systems supplied
-30V with jumpers on the board. This includes the associ·
ated DOS ground pins.
When replacing a BSM it is necessary to save the jumpers
for use on the new BSM if your system does not use the
'mini-bus' connector.
For dual DSM systems some wiring changes to the new BSM
may be required. Refer to the SR2XX pages in the BSM
logic manual for card plug charts and wiring add/delete
lists. Note that the new DSM is shipped without a byte
control card and without the terminator cards. These
cards must be removed from the old BSM and used with the
new one. Any reference in these SR2XX pages to "1st
DSM" means location 01 A·D4 and
0
2nd DSM" means
location 01 A·A4.
5410 MM
4·5
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