# DRO power supply problem



## rgray (Jan 8, 2015)

I have a 1957 Bigeport mill with a Dynamics Research Corp DRO. I use it at a minimum 4 times a week.
The on switch is a circuit breaker (just learned this) and today after turning it on it tripped off. anic: Thought I was holding my mouth wrong, but after pushing it in with a few different expressions it finally held in again after about 30 seconds and then tripped off in about 5 seconds.....that's when I realized it was a circuit breaker.

I didn't want to keep turning it on and off :bitingnails: but did it enough to figure out the display sill worked. I traced wires and there are only 2 wires from the power supply to the board, and that they were 12vdc power. I unpluged the board to test farther.
I know just enough about electronics to be dangerous.:noidea: But I'm feeling kind lucky on this one.

First thought was the circuit breaker is old and weak....I'll just hot wire it:thinking: . Using one of those cheap plug in watt meters on the plug in from the wall I hot wired it....it was drawing 350 watts... and then letting off slight smoke when I chickened out and unpluged it.. The transformer is where the heat is. There is an SCR (I think) with a large heat sink(it stays cool) Diodes (they stay cool) one of those voltage reg things with three terminal (can't remember what they are called..it stays cool) A large capacitor (it stays cool...it counts down on the volt meter when I connect to it...that's my best capacitor test) There are a few more little resistor things on the board none seem to get hot. There are some small capacitors on a silver box ahead of the transformer (none get hot).

There is alot of stuff it seems to arrive at 12vdc. 

Needed to get my job done so I grabed a 12v charger and hooked it to the terminals on the board where the 12vdc was....the display flickered and coudn't be zeroed...Ok so it needs cleaner power....grabed my jet ski battery (from the fridge) and hooked it up...worked great and finished that little job

Thinking back to the 350 watt draw I put the volt meter on the battery thinking I may also need the charger....the voltage climbed:noidea:..Ok it was warming to room temp and that caused it to gain voltage faster than the DRO used it....apparently the DRO uses hardly any power. It's been hooked up to the display now for 2 hours and the volage is at 12.23....it startd out at 12.02

So the question .....finally the question...Is It likely from what little info I've provided....that the transformer is the only problem or ???? one of the other pieces causing the transformer heat??? Pics are "right inside" left inside" and "DRO running on battery" blue towels are my insulation so I don't arc anything.


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## JimDawson (Jan 8, 2015)

Based on your very good troubleshooting.  I'm going to say the transformer may have failed.  Given that you can power it up from a battery, I would be temped to get a 12 volt wall wart and power it up from that.


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## cathead (Jan 8, 2015)

It's possible the transformer is bad but also something after it  could cause
the circuit to draw so much current.    
If you power the primary of the transformer and disconnect the secondary
and the transformer still gets hot, then it is bad.  Otherwise you have a bad
diode in the rectifier or voltage regulator or possibly some other part.  A shorted diode will not get warm
and neither will a shorted capacitor.  Test the diodes and make sure they conduct one way and not the other and also check the capacitor to be sure it isn't shorted as well.  If you check
out the transformer, the diodes, and the capacitor and they all check OK, then the
regulator or pass transistor may have failed.  From what you said, the capacitor is OK.  
Digital readouts in general don't draw very much current.  You are pretty brave to rip 
into it but you can probably fix it.  I would be very careful turning it on and seeing what
gets hot as when you do that you are only seconds away from destroying the next
weakest part in the circuit.  The best way to fix it is by the process of elimination. 
Eventually you will find the problem.   I wish you good luck.



Just for clarity, the big transistor on the left is the pass transistor with the yellow wire going to it and the voltage regulator is on the top right on the aluminum plate.  It probably has the markings of LM317 on it if that helps.


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## rgray (Jan 8, 2015)

JimDawson said:


> Based on your very good troubleshooting.  I'm going to say the transformer may have failed.  Given that you can power it up from a battery, I would be temped to get a 12 volt wall wart and power it up from that.



I have a cordless trouble light that has one of those. 12volt 1.25 amp. I plugged it to the board and it lights it right up just the same as the battery did.

Seems to easy.... All that very large equipment is just because it's old??....the wall wart has the same stuff???


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## rgray (Jan 8, 2015)

cathead said:


> You are pretty brave to rip
> into it but you can probably fix it.  I would be very careful turning it on and seeing what
> gets hot as when you do that you are only seconds away from destroying the next
> weakest part in the circuit.  The best way to fix it is by the process of elimination.
> Eventually you will find the problem.   I wish you good luck.




Brave??? I don'y know about that...stupid might fit better:lmao:. Only reason I was that brave was cause there was only the two wires to the electronics board and  I could unplug them protecting the boards (my hope anyway). 
Thanks for the help....I will get that power supply out and see if I can figure anything.
One other thing I did (I forgot to mention)was to switch the meter to ac and test at the 2 plugins to the board (where there was 12v)....my thinking was that if a diode had gone I would see a reading on the ac scale....not sure if that is a legitament test but the meter showed 0.0, it is a fluke 73 III meter.


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## atunguyd (Jan 8, 2015)

rgray said:


> Seems to easy.... All that very large equipment is just because it's old??



Basically yes! Most modern DC power supplies are now switch mode which means they no longer need a big heavy transformer and since they don't use linear regulators they are much more efficient. I would suggest you go into a local Tandy (or radio shack or whatever you have in the US) and ask for a 12v 2A switch mode supply. Will be cheaper than trying to fix your current power supply. 



Sent from my GT-I9505 using Tapatalk


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## Micke S (Jan 8, 2015)

Since the DRO circuits doesn't draw much current it is more probable that something after the transformer is causing a short. Disconnect the large capacitor to see what happens, and test the diodes and transistors. The transformer will probably run cool if you disconnect it on the secondary, or it is fried. I guess you don't have a circuit diagram but if there is one it would be easier to assist.


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## Wireaddict (Jan 14, 2015)

Although it has been suggested several times to disconnect the transformer secondary winding, try this with the wattmeter connected & measure the no-load power consumption, it should be 5 or 10W.  If it still reads over 100W then it has some shorted turns.  I agree that replacing the power supply is probably the easiest & cheapest solution.  Try to find the power consumption in the specs if you have the manual or hopefully that's on the back of the DRO & get a 12V power supply at least that size.  BTW, checking the PS voltages is always a good first step when troubleshooting electronic equipment.


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## lens42 (Jan 14, 2015)

My guess that the problem is not the transformer but some downstream component in the power supply. In any case, the easiest fix is to toss the whole power supply and get a 12V, 2 or 3A switch-mode "wall wart" and wire it in..


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