Repair And Replacement; Adjustment And Calibration - HP 6236A Operating And Service Manual

Table 5-7. +6V or +18V Supply Troubleshooting (Continued)
SYMPTOM
STEP - ACTION
RESPONSE - PROBABLE CAUSE
2. Measure voltage at TP12. a. If voltage at TP12 is positive, check for shorted
R49, open R48, open Z1 between pins 5 and 14,
and defective R46 or U3.
b. If TP12 is
a~prox.
-0.7V, proceed to step (3).
3. Measure voltage at TP11 ..
a. If TP11 is approx. +0. 7V, check CR46 for short.
b. If TP11 is approx. +1.4V, proceed to step (4).
4. Measure voltage at TP15. a. If TP15 is approx. -0.7V, replace U3.
b. If TP15 is zero volts, check for open R50, and
shorted CR42 or CR43.
c. If TP15 is approx. +0.7V, check for shorted R41,
open R42, or leaky or shorted C22.
5-40
REPAIR AND REPLACEMENT
because of inclusion in this table.
Series Regulator Replacement
5-46
ADJUSTMENT AND CALIBRATION
5-42 To remove and replace a series regulator transistor:
a. Remove the top and bottom covers from the instru-
ment.
b. Remove the collector screws and unsolder the base
and emitter leads from the board to remove the transistor.
c. To replace the transistor, follow the below reassembly
order, as viewed from the bottom of the heat sink: collector
screws, P. C. board, heat sink, two insulating bushiflgs (in
collector screw holes in heat sink), silicon grease (Dow DC-3
or HP6040-0209), mica insulator, another coating of silicon
grease, transistor, lock-washers, and hex-nuts.
d. Resolder the emitter and base pins to the circuit
board.
5-43 Semiconductor Replacement
5-44 Table 5-8 contains replacement data for the semi-
conductors used in this power supply. When replacing a
semiconductor, use the listed Hewlett-Packard part or exact
commercial replacement if these are available. If neither of
these are immediately available and a part is needed without
delay for operation or troubleshooting verification, the parts
listed in the Alternate column can be tried with a high
probability of success.
5-45 Notice that both the commercial and alternate re-
placements listed in Table 5-8 apply only to the HP power
supplies covered by this manual and their use in any other
Hewlett-Packard instrument is not necessarily recommended
5-11
5-47 Current Limit Adjustment
5-48 ±20V Supplies. Perform the following steps to
adjust the current limit circuit in the +20V or -20V supply.
Potentiometer R6 sets the +20V and R26 the -20V current
limit.
a. Turn the current limit adjustment pot (R6 or R26)
fully counterclockwise to its minimum setting.
b. Connect the test circuit of Figure 5-3 to the output
of the supply to be adjusted. Use a 40n 10W resistor for
R L ·
-c. Turn on the supply and set the ±20V VOLTAGE
control for maximum output (fully clockwise).
d. Turn the current limit pot (R6 or R26) slowly
clockwise until the DVM indicates a voltage drop across
the shunt corresponding to a current of 0.55A ±5%.
5-49 +6V Supply (Model 6236A). To adjust the current
limit circuit in the +6V supply, proceed as follows:
a. Check the setting of the current limit by performing
steps (q) and (r) of Paragraph 5-16. (Be sure to set the
output voltage to 6 volts.) If reducing the load resistance
permits the current to exceed 2.9A, stop, turn R46 slightly
clockwise, and repeat the test. If, instead, the cur-
rent begins to fall before it reaches 2.6A, turn R46 slightly
counterclockwise and repeat the test.
b. Recheck the setting and readjust R46 until the test
shows that the current limit circuit begins to reduce the
current when a decreasing load resistance increases 'it to
2.75A ±5%.