Watlow QPAC Modular SCR Power Control Service - Manual (Page 22)

The QPAC Transformer or Resistor Fuse (R10) is Bad
Check for AC voltage present at the transformer (T1), see Page 34 for transformer
wiring. Be extremely careful! High voltage is present on the transformer connec-
tions. If no voltage is present, verify that the resistor fuse is good by measuring
across R10. You should be able to visually determine whether or not the fuse is
bad. The resistor will usually be black or dark brown if open. It should read a mini-
mal voltage of < 1V. If it reads open circuit line voltage, R10 is bad. See Page 5 for
ordering a replacement transformer.
The QPAC Mother Board is Bad
Check the DC voltages at the J1 connector. Place the voltmeter common on pin 7
or 8. They should read as follows:
J1 Connector Label Without Card With BF Card
Test Point Installed
1 T STAT Not Used 3 to 5V
2 -V -11 to -14VDC -15 to -22VDC
3 FWZ* 11 to 14VDC 11 to 14VDC*
4 +V 15 to 18VDC 15 to 18VDC
5 Q32, connected to Pin 1
Q01 or Q33
Not Used Not Used
6 +5V 5V ±0.1V 5V ±0.1V
7 COM
8 COM (meter common)
9 KEY
10 COM
11 +LED Not Used Pulses possible
12 COM
*See scope pictures on Page 31.
Problem: Full or Partial Uncontrollable Heat
The SCR Switch is Shorted
Remove the control card and power up the QPAC. The output should be OFF. If
the unit is still full ON, remove power and disconnect the load wires and measure
across L1 and T1, and L3 and T3 with an ohmmeter. If resistance is low (5Ω or
less), the SCR is shorted and must be replaced. See Page 5 for ordering a
replacement SCR.
A Heater is Partially Shorted
Disconnect the control card; the output should be OFF. If not, disconnect the power
and heater, and measure the heater resistance. It should match the heater KW rating.
The Control Card has the Wrong Bias Adjustment
The bias adjustment is set too low and needs to be readjusted. See the Bias and
Gain Adjustment procedure with the appropriate control card on Page18.
The Control Card is Bad
First, try to re-seat the control card. Make sure there is a good connection between
the control card and the mother board. Power down the control and remove the
control card. Power up without the control card installed. If there is no heat, the
control card is bad. Replace the control card. See Page 5 for ordering.
An Excessive Input Signal from the Temperature Control
Check the input signal for proper calibration (i.e. 4-20mA at 4mA). The output
should be full OFF. At 20mA the output should be full ON.
22 WATLOW QPAC Service Manual
Troubleshooting, Chapter 5
Q32