Sony LMT-300, Service Manual

Page: 91 / 92

LMT-300
SAFETY CHECK-OUT
After correcting the original service problem,
perform the following safety checks before
releasing the set to the customer :
Check the metal trim, “metallized” knobs,
screws, and all other exposed metal parts for AC
leakage. Check leakage as described below.
LEAKAGE TEST
The AC leakage from any exposed metal part
to earth ground and from all exposed metal
parts to any exposed metal part having a return
to chassis, must not exceed 3.5 mA. Leakage
current can be measured by any one of three
methods.
1. A commercial leakage tester, such as the
Simpson 229 or RCA WT-540A. Follow the
manufacturers’ instructions to use these
instruments.
2. A battery-operated AC milliammeter. The
Data Precision 245 digital multimeter is
suitable for this job.
3. Measuring the voltage drop across a resistor
by means of a VOM or battery-operated AC
voltmeter. The “limit” indication is 5.25 V, so
analog meters must have an accurate low-
voltage scale. The Simpson 250 and Sanwa
SH-63Trd are examples of a passive VOM
that is suitable. Nearly all battery operated
digital multimeters that have a 20 V AC
range are suitable. (See Fig. A)
Fig A. Using an AC voltmeter to check AC leakage.
To Exposed Metal
Parts on Set
AC
voltmeter
(5.25V)
Earth Ground
0.15 μ F 1.5 k
Z