Heater board troubleshooting

Measurement & adjustment of heater board parametersMJC 2010.05.13

Heater voltages• The reference voltage applied at the heater

is not at ground, but rather at a positive value known as the “Heater Bias”

• The true bias voltage can be measured from test-point-GND to the black pin of the heater plug

• The value on the LCD screen can be calibrated to the measured value using potentiometer R10

• The “heater power” voltage is that across the black and red pins on the plug

• The LCD readout can be calibrated using the process described on the following slide

• The “heater power” voltage should be ~10V with the plug disconnected. If this is not the case, the board needs troubleshooting T

est

poin

t G

ND

(tw

o op

tions

)

Calibrating heater power voltage• Turn pot 1 until voltage measured across white pins matches front

screen• Turn “Heater Power” pot on front to max (fully CW)• Turn pot 2 until reach 2A on front screen• Repeat step 1• Turn “Heater Power” down as appropriate

12

Heater bias is tied to ground• This indicates a short somewhere in

(probably) the TOF• Thermocouple wire touching anywhere on the

assembly would induce this problem• Note that the HB voltage is also applied to the

thermocouple, owing to its placement at the heater tip

Troubleshooting heater power failure1. Base of the large transistor 2N3055

(point 1) should be at 12V referenced to the black pin of the heater plug

2. Pin 1 of the optical isolator (6-pin chip) (point 2 on photo) should be at 5V referenced to TP-GND… when the heater power switch in on

3. The base of transistor Q1 (central pin, point 3 on photo) should be at 5V referenced to TP-GND

1 2

3

4

4. 5V regulator at point 4 should be at 5V ref. to TP-GND

5. Circuit 5V at point 5 should be at 5V ref. to the black pin of the heater plug

5

Verifying potentiometer board power• The pot board should have 5V over the test

points