Page 190 - Troubleshooting Analog Circuits

Back to Electronic Circuits . . . I77

Comments on Troubleshooting Table for Single-transistor Amplifiers
This circuit has many things in common with many transistor circuits. It is a good
circuit to practice and sharpen up your troubleshooting skills on. A DVM is not a
useless tool, but a scope does a lot of things better.
Now let’s move on to the Positive (Adjustable) Regulator.

0-L
GROUND GROUND
Figure 14.3. Positive Adjustable Regulator.

Table 14.3 Troubleshooting Positive Adjustabk Regulators
Indication of Trouble Possible Cause Solution
Output V much too low. Input much too low? Check input with scope.
R2 or R3 shorted? Check resistors.
C3 shorted or Check C3 or remove.
reversed?
Load is short-circuit Unit gets hot. Try
or too heavy? disconnecting load.
CRl is shorted, or Check diode.
backwards?
C2 shorted or Check C2 or remove.
reversed?
Leakage at ADJ pin? Check for dirty board.
output -0.8 V? Output shorted to -
supply.
Output V much too high. Pot wiper open? Check voltages on pot.
R 1 too low? Check value.
Input V much too high? Check with scope.
Leakage at ADJ pin? Check for dirty board.
Output oscillates. General: Note the frequency.
Check for CI? C 1 should be 0.1 FF or
more, see data sheet.
Load bounces too much? Add a lot of C2 and
see what helps.
Output too noisy. Input too noisy? Study noise on input.
Increase C 1, C2, or C3.
Load too noisy or Increase C2 and see what
jumpy? happens; also C 1 or C3.