Crown OC-150A - Service Bulletin: OC-150 Meter Circuit Modification

SERVICE BULLETPN
m: ALL SERVICE CENmRS
DATE: 23 July 1975
RE: 0C-158 MET= CZRCUIT
Widespread failure cf the metering circuits in the OC-150 has been traced
to secondary breakdown of 43 and 44.
This is caused by the currant surge
raquird to chargo tho timing capacitor, C3.
A sbple modification wi13.
prevent secondary breakdown and inrpmve religbility.
A 100 oh, 114 watt rssistor (CPN 2872) is added between C3 and the emitter
of QS.
Tha enclosad board layout and schamptic illustrate the modification.
93 and/or 9.4 failure syrapta~ts are several.
Typical problms are pe
hold on the macars, and jittery meter move~ents, usually with higher than
namak reading 3,
Any evidence of leakage (in the hold wode) ean be impravt?d by swapping the
2961 (ZN3859A) transistors with 2962 (TZ-81) at 91 ~nd 92,
The 100 ohm resistor modification should be performed to all in-house units, and
xoutin~ly dane go a11 semiee units.