Lathe RPM meter not working

I have a Real Bull Mini Lathe as supplied by Amadeal Ltd of London. The in-built RPM display suddenly stopped working. By which I mean that the display stopped working (no LEDS light) - not necessarily that the sensor wasn't working. Taking the control box off and also looking at the sensor in the gearbox end of the lathe showed no visible reasons for non- operation i.e. severed cables, burnt areas of the control board. All the rest of the lathe functions as normal. Amadeal as always were very helpful, but admitted that they were not electronics experts - but suggested possibly the 5v feed to the display was at fault. And it's true that no combination of the 4 contacts on the control board supplying the display gave anything above 1.2v. The RPM display is a useful item - but not to the extent of buying a new control board. I could just provide an external 5v supply to the display - but without knowing which of the 4 pins provides power - this might do more harm than good. Alternatively I could just add an external Hall effect RPM meter to the lathe - sonething that I have already done to both my Amadeal Milling machine and my Woodworking lathe. Does anybody know the circuit diagram for the control board of these types of lathe - or at least the functional pin-out of the RPM display cable.

If it is the same as most of the rpm meters it is usalyy the 1st 2 connections that are the + & - connection. Google it & ask for wiring diagram for ??? Is this any good.

Regards.

Steve.

I notice that R3 is a 10k resister. wonder if that is ok.

Steve.

Hi Steve, thanks for the information and the circuit diagram . Your comment about the first 2 connections being the + & - makes sense as I was getting 1.2v between the 2nd and 3rd and same between 1st and 2nd - which means that the 2nd connection is probably positive. I will check R3 as well. Thanks again - will keep this thread informed until conclusion.

On a different machine but virtually identical circuit, the 10 uf marked C5 on your diagram had gone short circuit and taken the 78LO5 regulator out. If this is the case then check the 100 mh choke as well.

Also the 10 k resistor value is suspect, I think 1 k would be nearer the correct value !

I=V/R, 0.0005 amp, seems low.

John, 10k might be correct. If the sensor is very high resistance then current will flow through R3 and charge C6 untill C6 was at 5V. If the sensor then went to a lower resistance state the capacitor will discharge through it towards zero volts. This swing in voltage may be used to contribute to the sensor signal.

Martin C

Colin, does your sensor have a LED on it as mine does. If so & that is not lit the supply has gone down. It is from the same source as the read out.

Hi Martin C,

C6 is only 100 pf, not enough to hold a charge of any magnitude, more for spike suppression. C5 is the filter capacitor and will hold far more charge. Its an electrolytic capacitor and more likely to leak and fail. In theory the voltage regulator is supposed to be short circuit protected, but they do fail.

If C6 goes short circuit the high current flow could quite easily kill the voltage regulator. It will be interesting to hear what the OP finds.

Thanks for all the replies. I've discovered that my machine appears to be an exact copy of the Grizzly Model G0765 - for which I've gained a comprehensive 76 page manual.

Specific reply - not sure if John and Martin are talking to me or to each other - so will leave that for now

Steviegtr - not sure if LED or proximity speed sensor. The sensor comprises 2 cube shaped columns either side of a multi-finned disc on the mandrel end as shown in the attached photo The columns do have a thin recessed line halfway down - which could contain a small LED on one side and a detector on the other.

Just a thought of a cheap try it possible fix. The standard USB adaptors for phone chargers are 5v DC output. You could try & disconnect your supply to the DRO & connect one of those.

Steve.

Steviegtr - that's exactly what I plan to do - but unsure of which of the 4 wires going from the Circuit board to the Digital Readout are the +5v and Gnd lines - I don't want to blow the entire control board by getting this wrong. From my initial post - it looks like wire 2 is +ve and either 1 or 3 is Gnd - but not sure. Failing everything else - I've found a youtube video that shows how to put a hall effect sensor chip (of which I have several) and a rare-earth magnet onto the timing belt lower pulley. There again I could do the same onto the spindle behind the chuck - and link up to external Digital Readout unit as I have done for my milling machine.

Hi Colin,

Can you post a picture of the control board ?

Alternatively do you have a multimeter and can you identify the voltage regulator. If so identifying the connections should be straight forward.

PS, you have a slotted vane with a photo optic sensor.

Edited By John Baron on 30/01/2020 19:13:07

Colin

I have the same lathe as you. I have taken a photo of the opto sensor board and measured the voltages.

As you can see, the cables are marked with a '+', a dot and a long dash (left to right) . I have confirmed that they are + 5 volts, ground, and output (left to right again).

Output is about 5 volts when power is on but lathe is not turning. ( the output is biased to 5 volts by a 10 k resistor on this board.) It is pulled down by the sensor when detecting pulses. With the lathe turning, I measured the dc voltage using my multimeter and it was about 2 volts and dropped as the speed was increased. This would be a simple check if you don’t have access to an oscilloscope.

There are 2 resistors on this board. The top one ( nearest the sensor) is 10 k and is connected between + 5 volts and the output to bias the output high ( ie to 5 volts). The bottom one is 300 ohms and is the current limiting resistor in the feed from the 5 volt supply to the LED of the sensor.

I don't know if this is any help, but at least you will know what to expect at this board.

I don't think that the diagram posted by Steve earlier can be the power supply circuit for this type of speed sensor as this would not provide enough current for the LED with a 10k resistor in circuit.

I am going to be dismantling my lathe in the next few days to change the gears and bearings, so if you don't resolve the problem by then I can measure the volts and take some photos on the circuit boards on mine.

Stuart

Colin

Just an extra point to my previous post, with the lathe on but not rotating, the output is just under 5 volts when one of the vanes is between the sensor and about 1.3 volts when a space is between the sensor.

Stuart

Hi Guys,

That makes sense, the opto device is the same as can be found in almost any printer made. From memory the LED in them requires around 5 ma which would mean that the PSU shown has the wrong value for the 10 k resistor if that is the correct circuit diagram.

If the wiring markings are as shown in the picture, is shouldn't be difficult to trace them back to the PSU.

Looking at the schematic posted in the second post the only output seems to be via the 10 K resistor. This would not provide enough current to power the tachometer electronics. I suspect the 10 K resistor is the load resistor for the optical sensor. (If it does use an optical sensor.) There must be some other source of power to the tachometer. For us to be able to help you you will need to provide some more information. Does it use an optical sensor or some other method ? Can you post pictures of all the circuit boards in the lathe and the speed sensor so we can get an idea of how the low voltage to the tachometer is supplied ? Can you include pictures of the component side and the etch side of the auxiliary control board so we are sure it is the same as the one posted.

Les.

Colin

Correction to my previous post, typo crept in . Should be :

With the lathe on but not rotating, the output is just under 5 volts when one of the vanes is between the sensor and about 0.13 volts when a space is between the sensor.

Stuart

Colin

On the Amadeal website, there is a photo of the display board they sell as a replacement for the mini lathe.

The connections are marked on the board in the photo as 5v, GRD, PL and PE.

If this is the same board as yours this may help.

Stuart

Hi Guys,

On my machine the slotted opto sensor gets 5 volts to both the LED and detector via a 1.5 k resistor directly from the voltage regulator. The output to the speed display comes from the top of a 1 k resistor connected between the detector output and ground.

HTH.

Thanks Guys for all the input. You will have to be a bit patient as I go through all these useful ideas and suggestions. While no electronic engineer - I know how to use DVM and oscilloscope (although I only have a PC operated HANTEK). I will let you know of my findings.