HP 5065A Operating And Service Manual page 153

Model 5065A
Circuit Diagrams, Theory, and Maintenance
2. If correct, go to item 5 of this paragraph. If
incorrect or not present, cut 1 PPS wire
between A3(1) and A2.
3. Press and release HOLD pushbutton. Mo­
mentarily connect a clip lead from the +12V
supply (A1(R)) to where 1 PPS wire (cut in
previous step) connects to A2 assembly.
4. Recheck waveform at A3(2). If correct, re­
place A3U1. If incorrect troubleshoot A2. See
"Clock Repair and Disassembly" step 3 and
"Circuit Board Troubleshooting", step 3.
5. Connect a counter, set to totalize (manual
gate open) to A3(2).
6. Unplug the white wire (which connects the
clock assembly to the instrument chassis) at
the instrument chassis.
7. Reset counter to zero and momentarily re­
connect white wire removed in step 6. Dis­
connect white wire as soon as the counter
starts counting.
8. Counter should read 51 pulses or multiplies
of 51 (depending on how quickly the white
wire was disconnected). If the counter reads
incorrectly troubleshoot A2. See "Clock Re­
pair and Disassembly", step 3 and "Circuit
Board Troubleshooting", step 3.
9. Check A3(1) for a 1 PPS square wave. If not
present, check for +12V at A3(11). If +12V is
present, replace A3U1. If 1 PPS is present,
troubleshoot A3, See "Clock Repair and Dis­
assembly", step 2 and "Circuit Board Trouble­
shooting", step 2.
b. Display Not Lit
1. Check voltage at A3(5). It should be a few
tenths-of-a-volt less than the voltage at A1 (4).
If incorrect, troubleshoot "AC SENSE" CIR­
CUIT ON A2. See "Clock Repair and Dis­
assembly" step 3 and "Circuit Board Trouble­
shooting", step 3.
2. Check +12V input at A3(11). It should be the
same as measured at A1(R). If not, check
continuity of +12V line from A1 to A3.
3. Substitute a new LED in one of the display
positions.
4. Trouble is in U1, 2 or 3. Check for switching
waveforms at U1(6-12) and (21-26). Check
for switching waveforms at U2 and U3 out­
puts. See schematic for typical waveforms.
A2 50 Hz Logic Check
Troubleshooting checks to this point have isolated most
problems to the failed circuit board. Problems of a more
subtle nature such as clock not keeping correct time, or
display not synchronized to the instrument's 1 PPS out­
put signal, are the type of problems associated with the
50 Hz logic board.
Clock Repair and Disassembly
1. To troubleshoot A1 5V regulator board it is not
necessary to disassemble the clock. The board
may be removed when making repairs, if solder­
ing is required on the backside of the board.
2. To troubleshoot A3 display assembly remove three
nuts which secure the circuit boards to the front
panel assembly. Remove STANDBY DISPLAY
switch from front panel. Remove Clock Circuit
Board Assembly from front panel assembly. Re­
move six spacers from between boards. Reconnect
all wires. Be sure to connect clock circuit ground
(pads under spacers) to instrument chassis.
3. To troubleshoot A2 50 Hz LOGIC do step 2 above,
but do not connect wires. Unsolder A3 from A2.
Connect only the black and red wires (from A1
to instrument). Also connect clock circuit ground
(pads under spacers) to instrument chassis.
CIRCUIT BOARD TROUBLESHOOTING
A1 Power Supply
Since this circuit contains only four active components,
it is relatively easy to troubleshoot. With power discon­
nected check Q1, Q2, CR1 and capacitors. If these are
good, trouble is probably in A1U1.
A3 Clock Display
U1 accumulates time, and drives the LED displays thru
U2, U3 and transistors Q1-6. To troubleshoot, observe
that switching signal originating at U1(6-12) is reaching
U3 outputs, and signal at U1 (21 -26) is reaching collectors
of Q1-6. LEDs can be checked by substitution. If all sig­
nals are correct trouble is in U1.
A2 50 Hz Logic
a. Momentarily press HOLD button.
b. Momentarily connect a jumper from +12V supply
to U3(3). This resets the counters and enables
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