HP 3455A Operating And Service Manual page 163

Model 3455A
SERVICE GROUP F
8-F-1, QSUTGUARD LOGIC TROUBLESHOOTING (SCHEMATIC 8, 9, 10).
8-F-2. Outguard logic troubleshooting should be done using the Signature Analysis Routines (SA) in
Figures 8-H-20 to 8-H-27. If any incorrect signatures are observed, the following checks may be
helpful.
a. If any incorrect signatures are observed check for a 1 ¢F capacitor across A1U49. Install one if
missing (part number 0180-0291). The capacitor should be installed to the underside of the Al
motherboard, with the + terminal to pin 14 of U49 and the - terminal to pin 7 of U49.
b. If no stable signature can be located and the A3 board has been replaced, check the IC signals.
Make sure they are toggling with good logic highs and lows (approximately 4 V peak to peak).
c, Check for the waveform shown below at the junction of AI1C29 and R42. If this signal is miss-
ing, C29 may be defective. U48 may also cause the missing signal.
ON SSS ea ae eee ee
= 1.4 msec—>|
OV
8-F-3. Main Controller Troubleshooting (Schematic 8).
a. Data lines D@ to D7 (pins 18 to 25 of U3U9) and program address lines PA® to PA9 (pins | to 8
of U9) should have voltage levels approximately + 4.5 V peak to peak. Check for any circuits that
may cause loading. :
b. Sharp peaks of 3 V to 3.5 V may be observed on some lines of the processor (A3U9). This is
normal. The peaks are present when the processor is in a tri state mode.
c. Check the back gate bias voltage (G.B.) of the outguard processor (A3U9). If the voltage is dif-
ferent from the voltage marked on the processor (by + .35 V) check for the correct value of the pad-
ding resistor A3R3. If R3 is the correct value and the bias is incorrect, replace the A3 board. The cor-
rect value of R3 is listed in the following table.
-hp- Part No.
-5.0V 0698-3155
-4.5V 0698-4436
-4.0V 0698-0083
-3.5V 0757-0280
-2.5V 0698-3700
8-F-4. Front Panel Troubleshooting.
8-F-5. Front Panel Operation Check. 7
a. Turn the 3455A off. Place the instrument in the SA mode by disconnecting the test jumper on
the A3 board and disconnect the plug from A1J7.
b. Turn the 3455A on. Half of the instruments from panel LEDs should alternately turn on and
off with the other half.
c. At the time the instrument is turned on and half the front panel LEDs turn on, a 0 should be in-
dicated on the left side of the display. The 0 should move one position to the right each time the LEDs
change. When the 0 reaches the far right of the display, a .0 will start at the left and move to the right.
Section VIII
8-73