TROUBLESHOOTING GUIDE
If your CT255 does not work at all, recheck the following:
•correct orientation of VR1, U1, and U2 (see PC board layout diagram)
•You should be able to measure the voltage on the input and output termi- nals of voltage regulator VR1. The input should be whatever the supply volt- age is specified at, while the output should be 5 VDC +/- 5%.
•correct polarity of all electrolytic capacitors.
•correct orientation of diodes D1 – D7
•all solder connections
•Jumper bridging pins at the H1 and H2 locations.
If the unit seems to be working, albeit improperly, consult the theory of operation written earlier in the manual. With a digital voltmeter you can observe the cor- rect DC voltages at the supply inputs to the IC’s, as well as the temperature sensor outputs. Have a look at the comparator outputs as well and verify that they are changing states as required by proper circuit operation. If you have ac- cess to an oscilloscope you can observe the oscillator section as well as the counter adding the bits.
STILL HAVING TROUBLE?
While we had hoped that it wouldn’t come to this, if you are still having trouble with your binary thermometer, here are a few additional suggestions.
Use a methodical, logical troubleshooting technique. Most problems can be solved using common sense. A
Have another set of eyes look through your work. Here at the shop we have often run into a “stone wall” of a problem only to have a fellow technician see our obvious error. It is sometimes very difficult to see your own mistake; taking a break can often solve this common problem.
Make sure that you have “checked” all the assembly steps boxes. You may have forgotten one or two of them.
