6. SCR Malfunction Instructions
A. Normal SCR Malfunction Conditions
(1) Blown line fuses as the result of a shorted SCR
(similar to a shorted diode)
. A shorted flyback di-
ode will also produce this situation. This is a severe malfunction. See 6B.
(2) If one SCR does not turn on
[(either it is open or the gate signal is not being received by the SCR
(gate circuit open)],
a very small change will occur at the output which will be difficult to notice. The
ripple voltage at the output will increase.
(3) If two SCRs do not turn on, the ripple current will increase and can cause other problems.
(Con-
sult troubleshooting procedure)
B. Severe SCR Malfunction Conditions
(1) In the case of a severe malfunction, such as a shorted SCR or diode, do not turn on the unit. Dis-
connect the leads from the transformer to the heat sink assembly and check with a VOM for shorted
SCRs or a shorted flyback diode.
(2) To eliminate the possibility of a control malfunction, go inside the unit and check the control circuit
board. See the instructions provided for this test. It is important to run through the tests in the order
they are listed. Note that the SCR devices and flyback diode are still disconnected.
C. SCR tests or checks
(1) If nothing is found defective on the board the next step is to go to the SCRs. First of all an open
gate or an open SCR cannot be checked with a VOM. If an SCR is not firing, the AC ripple current will
increase across the filter capacitors, but no fuses blow. Also, the ripple voltage will increase at the out-
put.
(2) The best way of checking for a SCR device or flyback diode which breaks down into a shorted con-
dition because of inadequate voltage withstand capability is to add one component at a time and then
turn on the input power for a short time. When the faulty component gets connected, excessive input
current will flow.
WARNING: ELECTRIC SHOCK AND FIRE CAN KILL! DO NOT TOUCH ENER-
GIZED PARTS. DO NOT LEAVE POWER SUPPLY ON LONG ENOUGH TO
OVERHEAT OR FAIL IN THE FAULTY CONDITION.
(3) The best way to check that all SCR devices are firing and conducting correctly is to connect the
probe of an oscilloscope to the heat sink and the isolated neutral of the oscilloscope to the braid of
the flyback diode. The SCR pulses will show as 6 evenly spaced pulses of about the same height. If
one of the pulses appears to be part of a malfunction SCR device circuit, the gate lead for that device
may be disconnected from the applicable suppressor board point. The lead disconnection which does
not affect the trace is the lead for the SCR device and suppressor circuit in question. However, if
every third pulse is low or missing, check the balance adjustments, R9 and R10, before attributing the
problem to faulty components.
OM-2010
April 10/89 Revised 3-4
Page 3
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