Fluke 123B/S Service Manual - Page 127
Reference Voltages, 5.10 Buzzer Circuit, If not correct check/replace V301, R307, C3112
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7 Corrective Maintenance 7.5 Miscellaneous Functions Press to open the SCOPE OPTIONS ... menu, and select: SCOPE MODE: I NORMAL | WAVEFORM MODE: I NORMAL 7. Supply a 15.6 kHz square wave of 20V (+10...-10V) to Input A, and Input B. 8. Check: a. TP308 (TVOUT) for 15.6 kHz, -0.8...+0.6V pulse (square wave) bursts (see figure below). 15.6 kHz ≈600 ms If not correct, N301 may be defective. b. TVSYNC, on R392/R397, for 15.6 kHz, +2.6...+3.3V pulse bursts. If not correct, V395 may be defective. c. TP311 (ALLTRIG) for 15.6 kHz, +3.3...0V pulse bursts. If not correct, N301 may be defective. d. TP433 (TRIGDT) for 0...+3.3 pulses. If not correct, TRIGQUAL may be not correct. e. TP338 (TRIGQUAL) for 0...+0.6V pulses, width 70 µs, frequency about 2 kHz. If not correct, D471 may be defective. 7.5.9 Reference Voltages 1. Check: a. TP306 for +3.3V, TP307 for +1.23V If not correct check/replace V301, R307, C3112, P-ASIC N501. b. TP301 for +2V TP303 for -1.23V TP302 for +0.4V TP304 for +3.3V TP310, see figure below (in ROLL mode TP310 is zero). If not correct, check/replace REFERENCE GAIN circuit and T-ASIC N301. +1.2V TP310 ≈ 800 ms -1.2V 7.5.10 Buzzer Circuit 1. Press and select MEASURE on A : CONT ))) 2. Short circuit Input A to COM. The buzzer is activated now. 3. Check TP496 for a 4 kHz, 0...3V square wave during beeping (0 V if not activated). 4. Check TP495 for a 4 kHz +3...-30V square wave during beeping (TP495 is -30V if the beeper is not activated). 7-15