QuadTech 1657 RLC Digibridge, Instruction Manual

Page: 43 / 57

a. Look at the following control signals with a scope
and compare them with the timing diagram:
PBST, at C-U33 pin 10;
PMSR, at U33 pin 12.
If they are normal, skip to step b. If they are inactive, perhaps they
can be stimulated by applying pulses to the power-on reset circuit; see
step aa.
b. Examine each of the following digital feedback
signals and compare it with the timing diagram. If anyone is
questionable, check the circuit from which it is, derived;
F, 2F, and 4F, at U4 pins 2, 4, and 6 (from U17); DMSR, at U4 pin
10 (from PMSR via U33 and U34); DONE, at U25 pin 1 (from RES via
U29 and U35).
Notice that DONE would echo CMP if the latter were provided as
usual, by the integrator and comparator, U44. Otherwise, DONE echoes
the reset pulse provided in step
""
aa. Provide reset pulses in either of 2 ways. Preferably, set
up a pulse generator as follows:
Source resistance: 50
Ω
.
Repetition rate (period): 1 s.
Pulse polarity and duration: positive, 0.5 s.
DC levels: high = 4.5 V; low = 0 V.
Connect from ground to: U6 pin 13.
ab . The alternative method is to short across C5
momentarily (and repeatedly) with a clip lead. Watch the scope
carefully for activation, perhaps for only 1 cycle, of PBST
and/or PMSR, after each application of the short circuit. Notice
t
hat this short must be only momentary and that it must not be
applied while the pulse generator is connected. Find C5 at the
rear edge of the C board, nearest Q1 which is between
U6 and U7.
ac. If PBST and PMSR remain inactive in spite of the
preceding stimulation, the digital control circuitry is at fault:
go to para 5.8.6. Otherwise, proceed to step b, continuing to
use the reset pulses.
SERVICE 5-17