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Need help figuring out a work around

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Tino Volpe

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Hi I'm brand new to this site and I have to admit I know very little about electronics circuits and so forth. I'm a metallurgist but I'm hoping someone can help me with an issue. Hope i can even explain it properly. I have a piece of polishing equipment. This equipment has a sensor to detect polishing head position. It always tells me the head is out of position when it isn't . I looked at the little board which has two sensors to detect when metal crosses their field. A led light goes off to tell you the sensor has detected something. Well one LED doesn't light which I thought told me the sensor was bad. I changed both of them with same result. The only other thing on this board is 4 resistors. I'm wondering if I could jump out the sensors entirely. I've attached the schematic. Can someone read this and tell me if I can bypass these sensors to get this machine to run? I've tried to attach a pdf of the schematic, which as I said, I haven't a clue. Hope it is there

Thanks
 

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  • leco board schematic.pdf
    35.5 KB · Views: 157
I would not short out the resistors, doing so could destroy the LED's or the sensors. What do the SW11 and SW12 terminals connect to. If you replaced one of the LED's you may have connected it wrong. The LED's have a polarity. The cathode is the negative terminal and the anode is the positive terminal. Reversing the leads to the circuit board should correct that.
 
Need help in figuring out work around

Thanks for the quick reply. I did not touch the LED's I worded it wrong. I meant I replaced the position sensors. Is it possible a LED is burned out or bad and could that be it. Can I jump the sensors so the machine thinks they are sensing the head is in position?

SW11 goes to something called "head down" on the CPU and SW12 is labeled "kill" on the CPU. Does SW12 tell the machine to stop because it's sensing the head out of position (my problem) and SW11 tell the machine to lower the head for polishing because everything is ok?

Thanks for your help. If you need more schematics I can .pdf and attach.
 
I believe your description of the sensor operation is correct.

I don't think it's the LED. If it were open, there would be a low signal at the sensor and the machine would think everything was okay. If it were shorted the sensor and machine would still work, but you'd just have no indication.

How exactly do the sensors detect the position?

You could bypass the sensors by connecting the sensor side (resistor bottoms on the schematic) of R1 and R2 to ground. But I wouldn't do that if the sensors are for safety or to prevent damage to the machine.
 
Carl,

The sensors work by detecting when the rim of the post the head swings on, passes between the slot (see attached picture. Bypassing the sensor won't compromise safety because the head is permanently locked in position so it no longer swings between two polishing platens.
 

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  • leco board front.JPG
    leco board front.JPG
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I don't recommend trying to bypass the sensors, it may be easier to just fix the sensor circuit.

It could be that there is just a dry joint or crack somewhere on the sensor pcb. Do you have a basic multimeter to check all the circuit connections for continuity?
 
I'm guessing you have slotted photo-interruptor sensors, with some kind of tab that blocks the gap?
If you can remove the connector from the machine, and apply a 5v supply to the board (battery pack will be OK) the led should light until the sensor is blocked. The voltage at the bottom of R1 and 2, should be approx 3-3.2 volts when the sensor is blocked, and 0v when not blocked. With the board removed, you could safely short across C and E, and the LED should light. If not, you either have a bad resistor or LED. If this is OK, then you must have a problem elsewhere. maybe a bad connector, as somehow the signal is not reaching the controller.
 
Thanks guys. This is exactly the type of help i was looking for. That's why I searched for this kind of site. Let me try these suggestions and get back to you with the results. Once again thanks for all the quick replies.
 
They are slotted opto-switches in the picture, these quite commonly fail in VCR's etc.

I suggest simply replacing them would most probably cure your problems. You 'could' test them and find which one is duff, but better to change them both, as the second will probably fail shortly anyway.
 
Unless I misunderstood, I think the OP said he replaced the opto devices already.
 
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