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Old Mill Problem (electrical) Help

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Nick_G19/07/2014 10:44:53
1808 forum posts
744 photos


Hello peoples, I wonder if you can help me help a friend. smiley

A long standing friend of mine has a small jobbing workshop and has a problem with an old mill. Most of his mills are Bridgeports but this one is of Polish origin.

Basically there is a problem with the control circuit.! The machine and it's motors are 3 phase but as is quite common the control circuit voltage is lower.

There is a resistor on the control PSU (seems to be a wire wound one) that has burnt out rendering the machine inoperative. (motors spin up fine if the contactors are manually 'pushed in' ) The machine plate says the control voltage is 220 volts, but I am not sure if this is AC or DC as there are diodes on the PSU that may be configured into a bridge to render the supply into DC

So what I am after is a circuit diagram that will hopefully tell me what value the burnt out resistor was and give me an overview of how the machine is configured electrically. - Does anybody know where I could get such.??? laugh

I am attaching photos that I took with my phone to help with its ID and for the general interest of others. (spot the Myford dumped at it's base in one of the pics)

Denis tells me that although old the machine is a good solid and accurate piece of kit. He is obviously reluctant to trade it in for scrap value for what seems to be a minor issue.

Thanks, NIck

Les Jones 119/07/2014 11:56:30
2162 forum posts
149 photos

Hi Nick,
I think you will be very lucky to find a wiring diagram. I think the control voltage will be 220 volts DC as the two cylindrical things on the top right are probably electrolytic capacitors. Their voltage rating will give a clue to the control voltage. I agree with your assumption that 4 of the stud mounted diodes form a bridge. (I do not know what the 5th one does.) I think the pins on the right hand pair of diodes is the negative of the DC output and the studs of the left hand pair are linked together and are the positive DC. I think the studs of the right hand two diodes will go to either side of the transformer that provides about 200 volts AC. One may go to the transformer via the burnt out resistor. Are these components mounted on a sub panel above the main panel and can this panel be unscrewed and lifted away to see the back of it ? I assume that as you give your location as Cheshire that the machine is also in the Cheshire. If so which area of Cheshire ?


Steven Vine19/07/2014 12:20:21
340 forum posts
30 photos

I don't know if this is the right machine or not, but it looks like have a manual and circuit diagram (£85!)


Billy Mills19/07/2014 12:40:36
377 forum posts

The failed resistor could be a series resistor to a bridge rectifier which would limit the switch on peak current. If one of the diodes has shorted or a contactor coil has gone low resistance then a series resistor could overheat and then go open. Compairing the contactor coil resistances would show a low resistance coil. The seperate diodes can also be checked with an ohmmeter.

Any half decent electronics engineer should be quickly able to sort out what has happened without a diagram once you can see the back of the board and look at all of the components closely. It is also possible that the resistor has died of old age and slight overdissipation over 40 years on our high mains voltage.


Nick_G19/07/2014 17:07:48
1808 forum posts
744 photos


Hi Les,

I am in Warrington. - The machine is between Warrington and St Helens.

Cheers, Nick

john fletcher 119/07/2014 17:56:31
624 forum posts

Regarding the wire wound resistor, it may be possible to connect the two broken ends together then measure the resistance with an ohmmeter to establish some idea of its original value, then obtain a replacement from RS or Farnell. As stated, it maybe some sort of soft start resistor in series with the transformer input to limit the initial inrush current to the diodes. The two large capacitors would initially, for a brief moment in time, take very large charging current. That inrush current could blow the rectifier diodes unless limited, it does happen.. Also have you contact the manufacturer or even the Polish commercial section of their Embassy in UK. I once did (not Polish Embassy) and was pleasantly surprised by the reply, very helpful. Ted

john fletcher 119/07/2014 17:59:35
624 forum posts

Back again, I forgot to mention have you tested the diodes, could be one or more are short circuited, which not unknown.Ted

Clive Foster19/07/2014 19:57:55
2386 forum posts
77 photos

Although £85 seems a lot for a manual if it is the correct one then, from a commercial perspective, its worth it to make fixing a simple read value, purchase and solder in job. Unless the half decent, or even whole decent, electronics / electrical engineer hired to do the job has reasonable degree of familiarity with the circuits likely to be used fair rate labor and call out charges could easily burn up rather more than £85 and that's only the one thing fixed. On a machine of that age other issues must be expected in due course so having the manual "should" make for relatively economical repairs as compared to starting from scratch again.

In my experience all but the most basic machine tool electrical set-ups have their own idiosyncrasies, which probably seemed a good idea at the time to the design team, but are guaranteed to reduce the uninitiated to inelegant commentary until the penny drops.


frank brown19/07/2014 20:17:42
436 forum posts
5 photos

The most unreliable electronic components are electrolytic capacitors - and you have two of them, perhaps four decades old. first thing is to try and read what is on them, should be something like 25 MFD 200 VW - twenty five microfarads, two hundred volts working.. This will give you a clue if they are 100 VW, then 100 V is the maximum voltage for the control circuit. Have a look at the contactors and see if you can find the coil operating voltage (written on the coil?). Try and find a multimeter and see if the capacitors are short circuit, often happens. The excessive current then burns out the rectifiers, or in this case the series current limiting resistor. I wonder why the control fuse did not blow? Are there 50V light bulbs on the machine - could be there is a general purpose transformer lurking some where.


Les Jones 119/07/2014 20:34:52
2162 forum posts
149 photos

Hi Nick,
As you can see from my profile I am only about 20 - 25 miles away so If you have no success yourself I could come to give you a hand. I agree with John that the diodes should be tested as they tend to fail short circuit. If our combined guess is correct that the resistor is between the transformer and the diode bridge then a short circuit diode is a possible cause. Another cause could be one of the capacitors going very leaky or short circuit. I agree with Clive that the manual might be a good investment but I don't agree with just buying a new resistor and replacing it without finding out what the real cause of the fault is. If I am correct that these components are mounted an a panel above the main panel then tracing the wiring on this board would be the first step to see if my assumptions about the way the diodes and the resistor are wired are correct. (You did not say if this is a sub board.) Can you see if you can read the voltage rating of the capacitors and also their value. (The value is less important.) If the diodes and capacitors seem OK then my approach would be to power the control circuit from an external 24 - 50 volt DC supply so that the machine can be isolated from the mains for testing. The contactors would not pull in on this voltage but it could still be confirmed that the control circuit was working without drawing excessive current.


Nick_G19/07/2014 20:38:43
1808 forum posts
744 photos
Posted by Clive Foster on 19/07/2014 19:57:55:

Although £85 seems a lot for a manual if it is the correct one then, from a commercial perspective, its worth it to make fixing a simple read value, purchase and solder in job.


I agree. yes


Les Jones 129/07/2014 10:04:17
2162 forum posts
149 photos

Hi Nick,
Have you made any progress with the problem on the Mill ? Let me know if you would like any help with it.


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