55 Power Supply Troubleshooting

5-6. Raw DC Supply

With the meter off (but connected to line power), check for approximately 16 V dc (at 120 V ac line voltage) between the ground test point and the cathode of either A1CR2 or A1CR3. If necessary, check for transformer secondary voltage of approximately 24 V ac.

With the meter on, check for 19 V dc from A1U11, pin 8 to the ground test point. This voltage is approximately 5 volts greater than the raw supply voltage when the switching supply is operating properly. The controller IC, A1U11, starts operating when its supply voltage is approximately 2.5 V dc.

Table 5-2. Power Supplies

Supply Name

Measure

Supply Range

From:

To:

Main PCA (In-Guard Circuits)

VDD VSS * +VAC -VAC

A1TP10 A1TP12 +VAC -VAC

COM, A1R2, or A1R3* COM, A1R2, or A1R3 COM, A1R2, or A1R3* COM, A1R2, or A1R3*

4.95 to 5.45 V dc -4.95 to -5.45 V dc 4.7 to 5.35 V -4.7 to -5.35 V

Main PCA (Out-Guard Circuits)

VCC VEE VLOAD

A1TP18 A1TP16 A1TP15

A1TP17 A1TP17 A1TP17

4.85 to 5.35 V dc -5.0 to -6.0 V dc -28.5 to -32.0 V dc

Display PCA (Out-Guard Circuits)

A2U1-4

A2U1-5

A2TP3 or A2U1-42 A2TP3 or A2U1-42 A2TP3 or A2U1-42

4.85 to 5.35 V dc -5.0 to -6.0 V dc -28.5 to -32.0 V

* These points are at 'Common.' 'Ground' is also used in the meter, but in relation to Out-Guard circuits only. For example, the A2 Display PCA uses 'Ground'.

5-7. 5-Volt Switching Supply

Use an oscilloscope to troubleshoot the 5-volt switching supply. Check the waveform at either A1U11, pin 6 (switch transistor collector) or A1T1, pin 2 to determine the loading on the 5-volt switching supply.

The waveform is a square wave with a period of approximately 20 to 25 us and an ON (voltage is low) duty ratio of about 0.35 when the line voltage is about 120 V ac. The amplitude is usually about 15 V p-p. The positive-going edge of the waveform will be "fuzzy" as the duty ratio is varying to compensate for the ripple of the raw supply and the pulsing load due to the switching of the inverter. See Figure 5-2.

The OFF interval (voltage is high) part of the waveform will have a damped ringing sine wave of 2 to 10 cycles.

The waveform is a square wave with a very low ON duty ratio (approximately 0.1.)

If no square wave signal is present, the functioning of the oscillator can be checked by looking at the waveform at A1U11, pin 3. Use the oscilloscope with ac coupling to make this measurement. This waveform should be a sawtooth signal with an amplitude of 0.5 V p-p and a period of approximately 20 to 25 ^s.

The output current of the 5-volt switching supply can be determined by measuring the voltage across the current limit current sense resistors (A1R47, A1R48, A1R49). The current shunt is approximately 0.167 ohms. With line voltage at 120 V ac, typical voltages across the current sense resistors are as follows:

• Meter without options: 42 mV

• Meter with IEEE-488 Interface Option: 72 mV

• Meter with Battery Option Line Operation: 54 mV Battery Operation: 43 mV

• Meter with IEEE-488 Interface and Battery Options Line Operation: 85 mV

Battery Operation: 43 mV

5V

5V

-

-

1

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UUUAM

M

1

A

UUUMAt

-

-

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m

i

-

1

i

1

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k

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î

5ms N

ORMAL LOAD ON 5V SUPPLY

NO L

5ms OAD

OR VERY LIGHT LOAD ON 5V SUPPLY

5V

5V

-

-

-

-

-

-

à

*

P

Î

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*

5ms 5

V SUPPLY OUTPUT SHORTED

5ms

HEAVY LOAD ON 5V SUPPLY

Figure 5-2. Volt Switching Supply

5-8. Inverter

Use an oscilloscope to troubleshoot the inverter. Check for a 9.6 V p-p signal (with a period of approximately 30 to 40 ^s) across the primary winding of the inverter transformer (A1T2). If this signal is measured with reference to ground, the amplitude of the square wave should be approximately 4.8 V p-p.

Note

When making voltage measurements in the inverter circuit, remember that there are two separate grounds. The out-guard ground is the 'GRD ' test point, and the in-guardground or common is the common 'testpoint.

The normal input current to the inverter supply is about 210 mA, or 1.05 V across A1R46.

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