Troubleshooting Chart; Introduction; Current Draw Section - QSC III Series Technical & Service Manual

Table of Contents

(5-1-85)

Introduction

This troubleshooting chart refers to parts by their function in the circuit, and also by the part numbers for our popular

models 3350 and 3500 amplifier. The circuits for the 3200 and 3800 are basically the same but the part numbering

is different; please determine part numbers by matching the same location on the 3500 schematic.

1. Current Draw Section

The amplifier draws abnormal current when the Variac is first turned up (with signal but no load). This means there

is a short in the power circuit; the problem is locating it.

Current Draw with No Output Signal (even on speaker bus, before the relay)

1. Possible causes of stable, hard current draw (increases rapidly at only a few volts AC):

Reversed or shorted bridge rectifier B1 or B2

Mass solder shorts on board

Both supply clamping diodes D7 and D10 are reversed or shorted

Both sets of output transistors are of the same polarity or the drivers are shorted

Possible causes of fairly stable, medium-hard current draw (increases more slowly; can go to about 30

VAC before current becomes excessive):

One driver is of the wrong polarity or one output is shorted

One supply clamping diode, D7 or D10, is reversed or shorted

Open or missing bias diodes, D5, D6, trim pot, TR1 and R14

Shorted mounting of D21 or D22

How to determine which devices are shorted

Determine the polarity of a shorted device by noting which rail is being clamped to ground. Positive side shorts (Q1,

Q3, Q5-Q10) clamp the positive rail low, negative side shorts (Q2, Q4, Q11-Q16) clamp the negative rail low. Raise

current draw to several amps and check the low rail voltage and output transistor pins: a hard short (solder short?)

will read virtually zero volts. A reading of 0.6-1V above ground indicates a reversed diode, or shorted output. A

reading of 1-2 volts could mean a bad driver but outputs may be good.

Check for shorted mounting of the switching diodes, D21 and D22, by measuring voltages of both pins. A pin with

zero voltage (usually the pin connected to the flange) may be shorted to ground by a bad mica or PEM stud.

Determine which individual devices in a parallel bank are shorted by measuring the voltage across emitter resistors

on the side with the low rail (the faulty side); the shorted devices will hog the current causing higher voltages on

their resistors. (The good devices on the opposite rail will all be conducting pretty equally.) If the base voltage to a

group of outputs measures zero, the probable cause is a solder short. Put a voltmeter on the most sensitive setting

and probe individual base voltages; change the one with a voltage closest to ground first. If this does not clear the

short or if there is more than 1.5V between the base and emitters, change all the devices in that group rather than

continue poking around.

Check for driver short by measuring across its emitter resistor R25, R26; if voltage is nil, there is no current here

which eliminates possibility of driver short.

Don't forget to check for open circuit resistors (burned open by shorted device) while you have the devices out.

Current Draw but Amp Operates (signal is present)

Possible cause of increasing (runaway), soft current draw (can get 30-40V AC):

One classic cause is reversed filter capacitor. CAUTION: Capacitor may vent explosively.

Possible causes of soft current draw above 60 volts AC (may pass signal):

Severely misadjusted bias circuit or defective diodes D5 or D6

Severe oscillation (obvious on scope) causing current drain

Also, check step diodes, high-level drivers, step circuit (below)

Series Three Power Amplifiers