Models 78100A, 78101A Section V
Maintenance
:I 5-33. Board-Level Troubleshooting Extensions. 3. Look at VRl for noise.
5-34. The following procedures are used to continue diag- 4. Remove C6. If noise is gone, check the amplifier
nosis if the board fault is isolated using the appropriate circuitry. If the noise still persists, check C6, Q12, Q13, C7
troubleshooting tree. The procedure numbers are keyed to by substitution, and check VR1 for noise.
corresponding numbers at the end of some of the trouble-
shooting tree branches. If the board is replaced with a 5. Other components to check are: R10, R8, CR2,
rebuilt assembly, check and adjust the replacement board CR3, R47, R48, and R46.
using the Test and Adjustment Procedures, which start at
Paragraph 5-37. After repairs have been completed, com- PROCEDURE 2 Receiver Low Frequency Board ECG
pensatory adjustments must be made to the circuit board. Gain, Bandwidth, Noise.
Refer to Table 5-3 for a list of Transmitter Board adjust-
ments, to Table 5-4 for Receiver HF Board adjustments, a. 3312A Function Generator should be set up as de-
and to Table 5-5 for Receiver LF board adjustments. scribed in the troubleshooting tree to provide a simulated
subcarrier to the LF board.
PROCEDURE 1: Transmitter ECG Gain, Bandwidth, Noise.
b. Observe TPlO (Figure 5-7) for fault by comparing
a. Unseal the transmitter capsule (see paragraph 5-19). scope signal with Waveform A of Figure 5-8. Observe TP15
Insert battery or use DC power supply to supply 8V at for fault by comparing signal with Waveform B. Observe
battery contacts. TP16 for fault; signal should be 2.83Vpp +30% (a range of
1.98Vpp to 3.68Vpp) at the output.
b. Connect a 1 mV rms 10 Hz sine wave or other signal
that simulates the primary fault from the 3312A Function Nore: The 10 Hz modulating frequency may be varied to
Generator into the LA lead. With the 3465A Multi-function measure frequency response or the amplitude may be varied
Meter, inspect for approximately 15 mV rms sine wave on to measure distortion or linearity. The signal will have no
U1 (9), and with scope, check for an approximately low frequency roll-off. High frequency roll-off is controlled
200 mV rms sine wave on U1 (16). If the output on pin 9 is 1 by networks C17, R55, R57, C18, C26 in 50 Hz position of
faulty, inspect Q7, Q8, UlA or UlB. If the output on S4 and C20, R60, R6 1, C2 1.
pin 16 is faulty, inspect UlC. If neither output is faulty,
connect the test input from the Function Generator to the PROCEDURE 3 Receiver LF board with No ECG Output
RA lead and repeat this procedure.
Turn on Transmitter and short all input leads together.
c. For bandwidth problems, check high and low fre- Then check Receiver front panel lamps and perform sub-
quency response at U1 (16) with a 10: 1 probe. procedure a, b, or c.
1. High frequency response. Check amplitude at a. If Range/Battery Lamp is ON:
100 Hz. Look for greater than or equal to 70% (-3 db) of
the amplitude at 50 Hz. 1. Check emitter follower Q4 to ascertain whether
signal is reaching R58.
2. Low frequency response. For standard units, check
at 0.5 Hz for greater than or equal to 70% (-3 db) of the 2. Check voltage on + end of C9, which should be sub-
amplitude at 1.2 Hz. For diagnostic units, check the low carrier signal pp value less 1 volt. If not, check short time-
frequency roll-off at 0.05 Hz. Look for greater than or constant pp detector Q7-Q8.
equal to 70% (-3 db) of the amplitude at 0.14 Hz. If ampli-
tudes not at least 70% check voltage, current drain, and U1. 3. Check voltage on + end of C7, which should be the
Also check R15, R17, R49 and C6. same. If not, check long timeconstant pp detector Q5-Q6.
4. If voltages checked in steps 1 and 3 are OK, check
d. Noise. Possible causes of noise are numerous and many that the voltage on the base of Q6 is greater than the
are beyond reasonable test. Some, however are more obvi-
ous and more probable. The following are general tests: voltage found in steps 2 and 3. If not, try adjusting R100,
then check 46, R21, R20, R19, and R100.
1 1. Assure the board is bolted down tightly. 5. If all of the components just checked are OK, then
check that the voltage on the base of Q8 is less than the
2. Check that C6 is not open or leaky. Check Q7, then voltage found in steps 2 and 3. If not, check Q8. Then
Q8 for noise or leakage current imbalance. adjust R1OO again.
078 100-2 5-1 1 |
ACEC
HEWLETT-PACKARD
