HP TM 11-6625-2779-14&P manual Defective Function, Carrier, Troubleshooting Input Amplifier

Section V

Table 5-7. Function Troubleshooting

DEFECTIVE
FUNCTION	VF/Nm	CARRIER
75 UNBAL		A3K2
135 BAL		A1T2, A3K3,
		A3R22,A3R23,
		A3L1, A3R24
600 BAL		A1T2
600 BAL	A1T1
900 BAL	A1T1, A3K1,
	A3R19, A3R20
NG	A1R5 thru
	A1R8, A1C1,
	A1S3
HOLD	L1A/B, A1S1
	S1
DIAL BAT	A1S2, A3R59

a.First determine from the Front Panel Trouble Analysis chart (Table 5-9)exactly which range or ranges are defective.

b.Refer to Table 5-9to determine the changes that take place when switching ranges. Select the attenuator pads and/or gain switching resistors that match your symptom and check them.

5-36. TROUBLESHOOTING	THE	INPUT
AMPLIFIER.

a.Check the dc voltages as indicated in Figure 7-3to determine if a catastrophic failure does exist. If the dc voltages are abnormal (greater than + 10% of the indicated level), check for open or shorted components in the area of the abnormal indication.

b.Check to see that A3K1, A3K2 and A3K3 are operating properly. All relays are de- energized when either of the 600 BAL FUNCTION pushbuttons is depressed. Depress each of the other impedance functions (900 BAL, 135 BAL and 75 UNBAL) to see that A3K1, A3K3 and A3K2 respectively, energize and de-energize properly. If any relay fails to operate

Model 3555B

properly, check the relay and the energizing ground supplied through either pins 1, 2 or 3 on XA1.

5-37. FILTER TROUBLESHOOTING.

a.First determine that the set is operating in the TMS input mode. This bypasses the filters. If the set functions properly in the TMS mode, check each of the filters by applying a 1kHz signal at a 0dBm level to the set. All filters are calibrated for 0dBm indication on the 3555B meter at a frequency of 1kHz.

b.Since all the amplifiers in Figure 7-1are used in C MSG, the loss of any one will obviously cause the loss of the C MSG weighting. However, the bad amplifier can be isolated by checking the other filters. Use the following guide to isolate the trouble to a particular amplifier.

1.First be sure that the filters have the correct operating potential applied. Check the voltage at the junction of A4R49 and A4C33 to be sure that there is -20 volts + 1 volt.

2.If none of the filters work, check A3 in Figure 7-1(A4Q 11 through A4Q14).

3.If the PROG filter does not work but the others do, check A6 (A4Q5 and A4Q6).

4.If C MSG does not work but the others do, check A1, A4 and A5.

c.After the trouble has been isolated to an amplifier, check the dc potentials indicated

on the schematic diagram. This will normally isolate the trouble to a component. If the dc levels are correct but the filter response is out of tolerance, no attempt should be made to change the filter characteristics. Return the filter to your nearest -hp- Sales and Service office listed in the back of this manual.

5-38. TROUBLESHOOTING THE METER AMPLIFIER AND DETECTOR.

a.Inject a 1kHz, 0dBm signal (.775V rms) into the 3555B and set the INPUT switch to TMS TERM, RANGE to 0dBm and the FUNCTION to VF/Nm, 600 BAL. Measure the signal at the input of the meter amplifier (XA3 pin 9). The signal level

should be 6.2mV rms. If not the malfunction is ahead of the meter amplifier (refer to troubleshooting tree, Figure 5-8).

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