Checking Thermocouples Tc401 - HP 3400A Operating And Service Manual

Model 3400A
circuit parameters, rather only to localize the mal­
function. Therefore. it is quite possible that add­
itional measurements will be required to completely
isolate the problem. Amplifier gain and biasing
may vary slightly from instrument to instrument;
therefore. it should not be necessary to precisely
duplicate waveforms or values described.
Do not use an extender ooard on the
Chopper Amplifier Board (A6).
5-34. CHECKING THERMOCOUPLES TC401 AND
5-35. The following procedure will allowyou to check
the thermocouples for proper operation in the -hp­
Model 3400A RMS Voltmeter.
ThiS check will not yield any informa­
tion concerning thermocouple operation
for either sluggish or overshoot-under­
shoot response on 3400A meter.
a 3400A has the above symptoms re­
place thermocouples.
5-36. To perform these checks. supply 5 rnA of.cur­
rent to the heaters of the thermocouples.
Model 412A DC Vacuum Tube Voltmeter in ohms
function on XI00 range is ideal for this application.
Instruments required for these checks are as follows:
One -hp- Model 412A DC Vacuum Tube
Voltmeter.
SYMPTOMS
1/2 scale readings on all RANGE switch settings
and input voltages.
3 to 5% meter offset on all ranges with shorted
input .
400 Hz calibration low and frequency response
falls off above 50 kHz.
Switching transients exceed 5% of full scale with
shorted input.
Instrument has been overloaded.
Meter j itter exceeds
O.
Full-scale difference from range to range.
Meter pegs full scale.
undershoot or sluggish response on
Overshoot.
meter.
II
The -hp­
Table 5 - 5 . Front Panel Symptoms
5% of full scale.
One DC Voltmeter with 10 mV F. S.
capability such as another -hp-
4l2A or 3440A/3443A Digital
Voltmeter.
If
the thermocouples fail any of the (ollowing checks
replace the thermocouples according to procedures
outlined in Paragraph 5-37.
a. Turn the instrument off and remove the A6
board.
See Figure 6-3 for component
locations.
Connect test leads of 412A, in ohms function
b.
on XI00 range, between ground and junction
of C413 and C415 on A4 board (this step
checks the resistance of heater in TC401).
412A should indicate between 76. 5 to 103. 5
U
ohms. not within limits, replace the
matched set of thermocouples (-hp- Part No.
0853-0003).
c. Leave the 412A connected as in step b (412A
is used to supply 5 rnA of current to heater
of TC401 in this step) .
meter as follows:
Negative lead to pin 1 1 on AS socket.
Positi ve lead to pin 13 on A6 socket.
Voltmeter should indicate between +S. 5
and+9 mV. Note the indication for future re­
ference. U withinthe test limits, leave DC Volt ­
meter connected and proceed to step d page 5-14.
POSSIBLE CAUSE
Chopper Amplifier (A6). Check C612.
R7 (1/10 SCALE ADJ) misadjusted.
C405 (10 MHz ADJ) mi p djusted.
Q401 or Q402 shorted.
Check collector voltage of Q201 (should not ex-
ceed 9. 0 V).
Check Q201. Q401. Q402 and TC401.
Paragraph 5-34 for details on TC401.
Check Q601, Photochopper Assembly (neons)
see Figure 5-9. Verify the value of R606 to he
3 . 3 kll (-hp- Part No. 0683-3325).
Check resistors in second attenuator.
Check thermocouples. See Paragraph 5-34 for
detailS.
Replace Thermocouples.
Section V
Connect a DC Volt­
m.
V
See
See Paragraph 5-37.
5-7