Marginal Check Panel; Mc Switches; Mc Meters - IBM 7090 Instruction-Reference

Removal and Replacement
Access to the switches and receptacles is obtained by opening the rear panel. The
Jones plug receptacles are removed by taking off the back retaining plate which is se-
cured by three nuts. Rubber washers are used to hold the plug retaining tabs in place.
For safety reasons,
if
the console DC-off switch is to be removed, the wall switch
must be opened or the emergency OFF switch must be pulled first before the wires are
disconnected.
7.5.00 MARGINAL CHECK PANEL
The marginal check panel contains marginal check (MC) switches, motor Variac*
control switches, and two voltmeters. Normally the panel is covered. The cover is
opened by tripping a lever located on the right side of the Console, under the shelf.
Provision for marginal checking the Console is placed in the MC panel. However, this
feature will not be used unless transistor cards are built into the unit.
7.5.01 MC Switches
Service Check
If an MC switch fails to make contact, voltages will not be varied in the frame or
gate chosen. However, the marginal check voltmeter will indicate just as if everything
were operating correctly. Therefore, if trouble is suspected with any of the switches,
check the voltages on the frame or gate in question with a voltmeter to see if they act-
ually change as they should. Sometimes one can determine by listening whether the
marginal check relays in the power supply have been picked when the MC switches have
closed.
Adjustment
No adjustment is provided on these switches. Usually, if a switch is bad, the entire
assembly must be replaced.
Removal and Replacement
In order to remove the MC switches, the front curved panel of the console must be
removed. To do this, first remove the front base panel by pulling out. This exposes
the bottom row of screws on the curved panel. Lifting up the entry key panel exposes
the top row of screws. Take out all these screws and remove the panel. Now look
underneath the marginal check panel. There is a protective shield located under the
panel and beneath it are two MC switch assemblies mounted by a screw on either end
to mounting pedestals. Once these screws are remQved, the entire assembly can be
dropped. Throw the console DC off before starting.
7.5.02 MC Meters
These two meters monitor the variable voltages used for biasing. The MC meter
is an ac voltmeter and measures only the amount a voltage is being biased
if
the MC
relay for the module has been picked. If there is any doubt as to the accuracy of a
bias limit, the voltage should be checked at the actual gate being biased.
* Trademark of General Radio Company
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