Greetings all. I'm mains electrician with very little electronics experience but have been sucked into troubleshooting old electronic equipment after a job change. I'm realy struggling with a small encoder board and would appreciate some suggestions.

The board is fitted into the nosepiece of a microscope, where the small servo motor rotates the turret to bring the different objective lenses into play. There are two sensors on the board to make sure everything stops in the right place and these are connector via afew resistors and small value capacitors to an NEC C393C 431B ic. There are two led's on the board which light to indicate when the sensors are picking up a slot or shiny surface respectively. The first sensor is a photo interupter straddling a slotted wheel attached to the rotating output of the servo and this seems to be working fine as it lights the led indicator on the board everytime the slot passes. The second sensor is a reflective surface sensor that references to black and shiny marks on the inside of the turret. I found that the sensor's IR led wasnt lighting so I replaced it with a like component and the IR led is now working. Problem is that the sensor seems to have lost it's sensetivity. Before and after I fitted the replacement the sensor will pick up a bright IR source like a tungsten halogen torch bulb or telly remote but doesnt register i'ts own reflected IR led as it should. Could this be an Issue with IC? I've checked out the resistors on the board and they all seem ok. I've not tested all the Cap's but the one i've looked at seem ok too. Im stumped!
Please help!
Peter

 

Dear Peter,

It does seem deeply stumpmaking .. I'm not a trained electronics engineer so you should take what I say with a pinch of salt. I did once build a circuit to "read" the number of a camera magazine with a bank of IR sensors similar to the setup you have here .. However if someone comes up with better advice take it ! Also if my advice is patronizing I apologize .. but this is just what i found out !

The IR receiver part of the sensor is basically the same as a transistor without the base (the "Base" being the received IR light). A usual connection would be a current limiting resistor from the +ve to the emitter and then the collector to ground. The connection from the emmiter is usually where you take the signal.

So if the reciever has two connections .. one could be ground and one could be (in effect) output. Because what you are doing is in effect making an analog signal digital there is a point called "Hystersis" voltage. The IR sensors I used had a built in Schmitt trigger so when the voltage went over a certain level the out was 5v but that level it was zero.

What I suspect is that the c393c is a comparator to give a digital out when the hystersis voltage is reached ..

So looking at the data sheet ..http://www.alldatasheet.com/datasheet-pdf/pdf/6755/NEC/C393C.html

This is a dual one so it may be that one is unused ..

The hystersis (or switching) voltage is put into the chip at pin 3 and pin 5 and the analog input on pin 2 and pin 6 ..

So I would measure the voltage between pin 4 and pin 3 (ground and reference) and pin 4 and 5 (second comparitor)

Write down or remember this voltage.

Then measure the voltage at pin 2 .. keep the test gear connected and run the equipment as it's supposed to be run .. keep eye on this voltage as the turret is turned and the reflections come and go. What is the range of this voltage ?

If the voltage range is around the Hysteris (or reference voltage) (ie the reference is 3.5V dark is 1V light is 5V ) then that side of the electronics is working fine .

If the output on pin 1 or 7 is going high and low around the reference voltage and everything else is operating normally then you can give the sensor and the comparator (c393c) a clean bill of health. (But I suspect you will have a problem at this point).


Were the slotted switch parts all one unit or were they separate? Did you just replace the illuminating LED? or did you replace the whole thing ?

If you just replaced the illuminating led and the rest of the circuit is fine .. and the output is on the low side (ie moves a little but never hits the hystersis voltage) but will with a big light source then I think the LED thats lighting the unit may be at fault (assuming the glass is clear etc .. ) It may be the wrong one or the ballast resistors may be wrong or faulty ..

Another possibility (this has NEVER EVER happened to me .. honest !! ) that while you were replacing the led a small bit of solder has made it's way to short the pins on the comparator so that the supply voltage and reference voltage are the same.

If the sensor has been replaced with another one that's slightly different that may cause the fault you are seeing ..

Waiting for the grownups to give an answer !