The Craftmans Shop > New from Old
The Sequel - Oh Blimey I bought a CNC Lathe (Beaver TC 20)
awemawson:
So the next job was to remove the electronics and power them up on the bench. The MI12 supplies 10 volts to the OMM so that's what I gave it. To my surprise although the power LED came on, the Yellow 'wake up' LED didn't. (it had in the machine) Investigating further showed me that the 1 second / 250 millisecond / 125 microsecond bursts were actually formed in the MI12 - the OMM just passes them on as IR - (this fact becomes relevant later)
Now what I wanted to do was prove that the IR receiver part of the OMM was (or was not) working. Using a remote control from an LED Floodlight, I clamped one button 'pressed' , arranged it to point at the OMM IR sensor, and poked about in the circuitry with my 'scope probe. Now much of the electronics near the IR sensor is covered by screening cans that are not easy to remove, and it's all a bit tiny being surface mount. However I did find responses to the IR signals that I was sending, so the IR diode must be working.
I then went to the other end of the electronics, where a DS8921M differential line driver talks to the MI12 - no output, and also no input. Now between where I found the response, and this chip, there is more hidden circuitry that I can't get at. But I don't think that there is anything sophisticated enough to be decoding signals, just buffering and level shifting. This is where the relevance of that wake up signal comes in. If the designer put the clever bit of the wake up signal back in the MI12 unit, he probably did the same for the touch received signal (maybe !!!!)
On the board are two miniature LM7805 5 volt regulators (that are working) and an ICL7660 switching voltage converter that is producing a -3.5 volt negative rail, but I have no idea if this the the 'right' voltage of course.
So in conclusion, it's inconclusive :bang: The IR diode is working but I still don't know if the OMM or the LT02 is the actual fault !
Really I need to find another machine equipped with this system and wave my LT02 at it's OMM!!
In the end I put it all back together and took the dogs for a walk !
PK:
The ICL7660 is a switched capacitor circuit, it's commonly used to produce a negative rail for op amps. The output voltage should be a negative mirror of the input voltage, so measure pin 8 vs pin 5 (with pin 3 as your reference) the voltage on 5 should be the negative equivalent of pin 8.... If it's not then you have a cap or over current problem.
PK
awemawson:
Thanks for that PK.
Your response has prompted me to look very closely at the photos I took at the time.
I've spotted what looks to me to be an exploded surface mount capacitor - if it's the same as others on the board it should be 33uF at 16 volts - certainly doesn't look healthy
Of course it's all back together now :bang:
PK:
Yeah, that looks like the bulk cap for the input to the 7805, look for ripple on the little output ceramic in that picture.
Anything more than a few hundred mV is bad.
cnc-it:
Looks promising..if that capacitor is blown chances are it's something on the circuit that has gone out of spec and blown the capacitor rather than it failing all by its self.
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