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Service
Model 8901B
Power Supply Check
1.
Check test points AlOTP2 through TP7 with a dc voltmeter. The voltages should be within the
limits shown on Block Diagram 5. If a short on the supply is suspected, continue with step 2. If
a regulator is suspected, see Service Sheets 31 and 32.
NOTE
The supplies are interdependent. Often a short o n one supply will shut down
another. All supplies are dependent on the +15V Supply.
The switched +15V Supply line
i s
normally grounded when the instrument
is in standby. The +15V Supply (unswitched)
LED
should remain lighted
when the POWER
is
set to STBY.
2. Remove the plugs connected to A26J6
(RF
SECTION), A26J3 (DIGITAL SECTION), and A26J2
(AUDIO SECTION) one at a time and observe the five power supply
LEDs.
(The plugs are located
on the circuit side of the A26 Power Supply Motherboard.) An extinguished LED will light when
the short is removed from the supply. The assemblies in the faulty section can then be removed
one at a time until the one with the short is discovered.
3. Remove the plug connected to A26J1 (KEYBOARD). (This plug is on the component side.)
Jumper pins 2 and 6 of A26J1 (where the green and black wires of W13 normally connect) to turn
the instrument on. If the short is on the A1 Keyboard and Display Assembly, the extinguished
LED will light. If the short still persists, see Service Sheets 31 and 32 and check the faulty
regulator.
~ ~ - 6 0 Service Sheet BD5
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