HP 654A - Page 40

Section V
Model 654A
Check the dc voltage at the output of the Buffer
Amplifier ( junction of A2CR24 and A2R44), this
should be 0 +/-. 1V dc.
Check the dc power supply voltages on the A2
board:
+31 +/-. 5V dc at A2 Pin 1.
-26 +/-. 5V dc at A2 Pin 2.
Check the ac voltage at the output of the Buffer
Amplifier ( junction of A2CR24 and A2R44) with
an oscilloscope; this should be a sine wave of 3V
+/-1V peak- to- peak. Notice that for the FAIL
situation there are three possible branches ( NO
SIGNAL, LESS SIGNAL, GREATER SIGNAL).
Take the appropriate branch to the next check
point.
NOTE
If the output from the Buffer
Amplifier is not correct then the
problem is probably in the ALC
loop; following the tree should
be the fastest way of localizing
the problem. If after completing
the tree you still have not local-
ized the problem, then refer to
Paragraph 5-79 which gives a
method for opening the ALC
loop.
A procedure for checking the oscillator circuit is
given in Paragraph 5-81.
FAIL if either ( or both) supply is not present.
Check the dc voltage on the lamp ( A2DSV1) at A2
Pin 5. With a larger than normal signal level in the
Buffer Amplifier the voltage at A2 Pin 5 should be
low, from 0 to + 5V dc.
Check the dc voltage on the lamp ( A2DSV1) at A2
Pin 5, normally this is + 4 to + 6V dc. If the voltage
is less, then the lamp voltage is trying to increase
the Buffer Amplifier signal level and the problem
is in the Buffer Amplifier. If the voltage is greater,
the lamp is causing the low signal in the Buffer
Amplifier. The lamp is probably open if the
voltage is higher than + 15V dc.
0 a. If only the- 26 Vsupply is inoperative proceed
to branch®.
b. Check if the external circuits are loading the
power supply by lifting Al Pin 12 and Al R18
(Schematic No. 2). If the supply operates the
trouble is in the external circuits. If the supply
does not operate proceed to step c of this
Paragraph.
c. The - 26 V supply is referenced to the + 31 V. To
check if the - 26 V supply is loading the + 31 V
supply first lift Al pin 13; if the supplies now
operate the trouble is in the external circuits; if
the supplies still do not operate, lift A1R15*,
Al R6 and Al R7 to isolate the - 26 V supply from
the + 31 V supply. If the + 31 V supply now
operates the trouble is in the - 26 V supply
(proceed to branch 11 ); if the + 31 V does not
operate troubleshoot e + 31 V supply.
Check Ti and the line filter components; also
check A2CR1 thru A1CR4, AlC9 and AICIO.
a. Troubleshoot the - 26 V supply if you have
arrived here from branch C)c.
b. Lift Al Pin 13 to isolate the - 26V supply from
external circuits. If the supply now operates the
trouble is in the external circuits; if the - 26 V
supply does not operate troubleshoot.
Check the dc voltages at the Balanced Amplifier
output which should be 0 +/-. 1V dc at the top of
A2R74 and the bottom of A2R75 (' top' and
'bottom' are as viewed on Schematic No. 2).
Check for open lamp or resistor in the
photo- resistor A2DSV1.
Troubleshoot the Amplitude Control Integrator
(A306 through A3Q9) including the Amplitude
Cur rent Reference and the lamp of the
photo- resistor ( A2DSV1).
Troubleshoot the Buffer Amplifier ( A2Q8 thru
A 2 Q10) also include the resistor of the
photo- resistor, A2DSV1.
Check the ac signal level at the Balanced Amplifier
output with an oscilloscope. The signal should be a
sine- wave of 4 +/- . 5V peak- to- peak at the top of
A2R74 and at the bottom of A2R75, the two
signals should be 180% out of phase with each
other. Note that in the FAIL condition there are
two possible paths.
0 Check the ac signal with an oscilloscope between
the attenuators ( S4) and C9 and C10, the signal at
both points should be 3 +/- IV peak- to- peak. If
signal is not present, check C9, C10, A2R76 and
A2R77.
Check the input signal to the Impedance networks
A4 Pin 1 and A4 Pin 2, this should be 3 + 1- IV
peak- to- peak when the attenuators are set at
+10dBm. If the signal is not present, try all
positions of the attenuators and the impedance
switch.
5-20