Output Amplifier - HP 8112A Operating, Programming And Servicing Manual

Offset-vernier
Control-voltage

Output Amplifier

Caution
1 0.4-14 Servicing the Shaper and Output Amplifier
Check the offset-vernier control-voltage, received from the control
board A2 at board connector side of R501 , against Table 10 .4-6:
I
•
Do not operate the HP 8 1 1 2A without the heatsinks fitted on
board Al
•
If you need to replace one or more of the transistors Q505 - Q513,
do not attempt to remove the heatsink and transistor adaptors
together, this is likely to damage the transistors.
1 . Remove all the screws securing the heatsink(s).
2. Remove the heatsink(s).
3. Remove the adaptor( s) from the transistor( s) to be replaced.
4. Replace the transistor( s )
5. Re-fit the adaptor(s) and heatsink(s).
1. Set up the HP 8 1 1 2A as follows:
101 RCL
Output
2. Press the (SET ) key
3. Measure the voltage at CR501 and CR502 (typ . 10 mV):
If it is fully negative (approximately -15 V) check U502, Q502,
Q504 and Q506.
If it is fully positive (approximately + 15 V) check Q50 1 , Q503,
Q505 and U502.
4. Check if Q510/Q5 1 1 or Q512/Q513 have failed (emitter-collector
short-circuit) .
If you need to change any of the output-stage transistors Q508 -
Q513, also check CR505 and CR506. They protect the output
stage at high amplitudes and frequencies by discharging the
base-emitter capacitor of Q510 -Q513.
5. Set the HP 8 1 1 2A to FIXED mode, output ENABLED .
Table 10.4-6. Offset-vernier Control-voltages
HP 8112A Setting Control voltage
HIL LOL
+8.0 v +7.9 v
+5.5 v +4.5 v
+0.5 v -0.5 v
-4.5 v -5.5 v
-7.9 v -8.0 v
.
0
v
- 1
Disabled
at R501 (typ)
-7.45 v
-4.8 v
<
lO mV
+4.8 v
+7.45 v