Service manual

Page 12
motive relay, to open, which removes power. When the comparator outputs are both high, pul-
lup resistor R14 assures that Q2 will be on, enabling the relay to turn on. The relay cannot actu-
ally turn on unless the on/off switch is pushed because one side of the relay coil is routed
through the switch to DC power.
Fuse F1 protects the radio from overcurrent conditions. This is a 9A Resettable fuse. When it
warms up enough due to overcurrent conditions, it suddenly develops a high resistance, keeping
high current from flowing. As it cools, it automatically resets.
When Q2 and the on/off switch are on, DC voltage appears at pad W3, which provides power
for the 100W amplifier. Since this is a high current lead, it is located as close as possible to the
relay to minimize voltage drop. A short cable connects this point to the amp. When the tray is
rotated up for service, an extension cable is required. This extension cable will have noticeable
voltage drop when the 100W amp is in use, so performance could suffer. As a result, it should
not be used except for testing.
On/off switch LED
Comparator U6 is used to detect when the DC voltage going to the regulators has dropped to a
low enough level that they are about to shut down. When the voltage gets to a little over 10V,
the LED inside the on/off switch is turned from green to red. This is useful when operating the
radio from batteries.
Regulators
The 5V regulator, U8, is a low-drop-out regulator, which means that it does not require much
more voltage going in than it generates. However, excessive voltage going in would make it run
very hot, so its input voltage is dropped by pass transistor Q1 and regulator U9 from the input
of 11V-15V. Both regulators have input and output protection. They are mounted to the tray,
providing excellent heatsinking, and the dual fans in the compartment below the tray help keep
the tray cool during normal operation. During service, with the top open, the cooling effect of
the fans is greatly reduced, and the tray can get quite hot. Operation with the top off is not rec-
ommended for long periods of time.
U10 is a MAX765 negative voltage generator. It is a “chopper”, which turns the DC input volt-
age into an AC voltage which is then converted back to DC and output as a negative DC volt-
age. This negative voltage is used by the receiver to allow high performance dual supply op-
amps to be used. RFC1 helps keep the switching transients off the line, which could induce
noise into the sensitive receiver circuits.
The raw DC input is also fed to the receiver, controller and transmitter. The receiver uses the