HP A Series - Probable Cause

Tabie 5-5. Current Limit Troubleshooting
STEP
ACTION
RESPONSE PROBABLE CAUSE
1
If unit is equipped with
Option on or 104 crow¬
bar, disconnect crowbar
SCR by lifting one end
of R13or R14 (resistor
in series with SCR) on
crowbar P.C. board.
If unit does not have
crowbar option, proceed
directly to Step 2.
a. Output voltage
goes high.
b. Output voltage
goes to normal.
c. Output voltage
remains low.
a. Crowbar fired due to high output
voltage condition — proceed to
Table 5-4.
b. Crowbar defective — proceed to
Table 5-6.
c. Interna! short or current limit
circuit defective — proceed to Step 2.
2
Remove input power
and connect ohmmeter
across + and — output
terminals.
a. Resistance less
than value of R19
(see schematic).
b. Resistance more
than value of R19.
a. Capacitor Cl 1 or protection
diode CR9 (U5) shorted.
b. Proceed to Step 3.
3
Check for presence
of —7.5V across Zener
diode VR1.
a. Voltage incorrect
b. Voltage correct
a. VR1 defective
b. CR14 shorted, U3 defective.
Table 5-6. Crowbar Troubleshooting (Option Oil or 104)
SYMPTOIVi PROBABLE CAUSE
Crowbar always on, won't reset
a. CRB (or CR6) shorted. If shorted, check Q4
for open.
Crowbar won't trip
a. QTopen or shorted.
b. Q2 (or Q3) open or shorted.
c. R45 (crowbar adjust) open.
d. CR4 (if used) shorted.
e. -i-23V bias missing.
Crowbar trips but won't latch
(oscillates on and off)
a. CR3 (if used) open
Crowbar trip point shifts or is susceptible
to external noise
a. VR1 defective
b. R45 intermittent
a Cl, C2 open
' ' " .I
Crowbar circuit prevents supply from
reaching proper voltage, but SCR is not
fired.
a. Q4 shorted.
5-10