Author Topic: At the end of my rope..need some advice  (Read 14139 times)

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Offline tacoTopic starter

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At the end of my rope..need some advice
« on: February 24, 2015, 07:34:06 pm »
Ok so I'm trying to repair my own tools. I'm learning as I go, by watching all the videos and reading all the beginner materials I can. I have a unit when functioning normally takes mains 110V and goes through an internal transformer to output 17V AC to the board in the picture. The unit has an over voltage light, and reset switch on the front panel. Normally when I set the speed control pot at minimum I get 1.8V DC, speed control at mid point 11.2V DC, and speed control at maximum produces 34.36V DC. Keep in mind also that as resistance is placed on the DC hand piece that the unit will compensate to maintain constant rpm up to and including 45,000 rpm. The unit when turned on was functioning fine, then all of a sudden the hand piece goes to 45,000 rpm and thats all it will do. The over voltage light came on also. The issue I'm having is that the company that repairs these units, says the board is too old, and they won't repair it.

I started by desoldering every component on the board, identifying the component and then researching how to test it after buying a new Fluke 87V. I replaced the BDW93C TO-220 transistor with a NTE54 because it looked overheated on the heat sink side (bluish color). I also replaced the D1047 transistor because I had one in the parts bin. I replaced the LM358N op amp. I checked every other component except the TL431 (because I could not find out how to test it). re soldered everything back in, and when I turned the unit on it outputted 0V DC at min, 10.9V DC at mid, and 12.90V DC at max. When I connected the hand piece, it ran, but when any speed change was introduced, it would cycle up and down. Then as I was testing, it went wide open to 50V DC and the over voltage light came on again.

I want to fix this and learn from it. What did I miss? Any ideas why it's doing this?

Thanks
taco
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Offline max_torque

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Re: At the end of my rope..need some advice
« Reply #1 on: February 24, 2015, 07:54:46 pm »
With a fairly simple board like that^^ the first thing i would do is try to sketch out a rough circuit schematic.  As it's all through hole components and just two layers it shouldn't be much work to do that.  Then, you can look at the schematic and work out what part is likely to be causing the issue.  For example, over voltage on the output is likely to be as a result of just a few components, and you can easily work out where to probe the circuit and what sort of voltages you might expect to see at those points.  That way, and abnormality should become a lot easier to spot ;-)
 

Offline PA0PBZ

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Re: At the end of my rope..need some advice
« Reply #2 on: February 24, 2015, 08:17:31 pm »
What I don't understand is how you can get 34 or even 50V DC from a 17 V AC input. I don't see a DC-DC converter (no inductor) on the pcb, there's just a diode bridge + cap in there...  :-//
Keyboard error: Press F1 to continue.
 

Offline rdl

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Re: At the end of my rope..need some advice
« Reply #3 on: February 24, 2015, 08:48:27 pm »
Voltage doubler is my guess.

What I don't understand is how you can get 34 or even 50V DC from a 17 V AC input.
...
 

Offline bugs

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Re: At the end of my rope..need some advice
« Reply #4 on: February 24, 2015, 10:08:53 pm »
Is there a way for you to check the wiring, pot's switches' etc? It might just be a bad connection or a noisy pot. Some contact spray in the pot might be a temporary solution.
 

Offline tacoTopic starter

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Re: At the end of my rope..need some advice
« Reply #5 on: February 25, 2015, 05:12:47 am »
I've tested each component when de soldered from the board. I agree on making a schematic. Funny how I can run a steam generation power plant, but when it comes to little circuit boards I'm an idiot. I'm trying to learn after 50 and I'm definitely behind the curve:)
I never suspected a bad potentiometer..hmm check that in the morning.

Thanks:))
 

Offline Whales

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Re: At the end of my rope..need some advice
« Reply #6 on: February 25, 2015, 07:21:03 am »
What does this 'unit' power?  It could just a likely be what you are plugging into it that is causing the problems.

TL431 is a voltage reference controlled by the pot.  Provided you are not turning the pot its output voltage should not vary by anything more than a tiny amount.

Offline kripton2035

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Re: At the end of my rope..need some advice
« Reply #7 on: February 25, 2015, 07:32:07 am »
looks to me that C6 (near MCR100) and the big capacitor top left seems both bulky ???
 

Offline tacoTopic starter

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Re: At the end of my rope..need some advice
« Reply #8 on: February 25, 2015, 09:23:58 pm »
I will post more pictures tonight. The hand piece is definitely out of the equation as I have many good ones to interchange with the same results.
 

Offline tacoTopic starter

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Re: At the end of my rope..need some advice
« Reply #9 on: February 25, 2015, 11:54:27 pm »
Ok,
Here are the pictures. Pretty much I tried to show the whole unit from original, to torn down. The pictures are in order of tear down and the reverse sides of the boards. These units are expensive, running around $235-$279 each. My wife and I use them for carving wildlife in wood.
Thanks all for your help.
 

Offline T3sl4co1l

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Re: At the end of my rope..need some advice
« Reply #10 on: February 25, 2015, 11:59:27 pm »
Could you take a full size (say 2048 x 1536 ish), sharp focus, plan view picture of the top and bottom sides?

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Offline CatalinaWOW

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Re: At the end of my rope..need some advice
« Reply #11 on: February 26, 2015, 12:17:35 am »
looks to me that C6 (near MCR100) and the big capacitor top left seems both bulky ???

I would suggest that what krypton is trying to tell you is that the large electrolytic capacitor in the corner near the white connector shows signs of bulging, which in turn implies that it is no longer operating as a capacitor anywhere near the intended value. 

The styling on the case of this item, along with the packaging technology suggest that it is 1970s vintage.  Electrolytic capacitors are notorious for going bad over time.  In your board these are the black cylinders with silver ends with a scribed X, mounted vertically on the board.  Others will tell you that all of them should be replaced.  I have personally often had success only replacing the obviously bad ones.  Look for a minor bulge at the top, or goo leaking out of the bottom.  The argument for replacing all of them is not bad.  They are all likely to fail sooner or later.  You have already got the board out where they are easy to get to.  An these components are generally not expensive. Your choice, depends on your finances and access to time and components.
 

Offline ludzinc

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Re: At the end of my rope..need some advice
« Reply #12 on: February 26, 2015, 12:43:26 am »
Ok,
Here are the pictures. Pretty much I tried to show the whole unit from original, to torn down. The pictures are in order of tear down and the reverse sides of the boards. These units are expensive, running around $235-$279 each. My wife and I use them for carving wildlife in wood.
Thanks all for your help.

If you take a look at the speed setting pot in your last picture, you can see that the black wire has become detached.

Okay, that probably happened during disassembly, but the solder joint on it is poor and an intermittent connection there would explain the symptoms you are observing.

Good luck!
 

Offline kripton2035

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Re: At the end of my rope..need some advice
« Reply #13 on: February 26, 2015, 08:17:07 am »
+1 for the speed pot with bad soldiers.
from the last pictures, the capacitors look better
but always check (old) capacitors with an esr meter, your fluke 87 can measure capacitance but not esr .
 

Offline tacoTopic starter

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Re: At the end of my rope..need some advice
« Reply #14 on: February 27, 2015, 12:21:20 am »
I will replace the capacitors and correct the solder joints to the pot. I will also take better pictures of the boards later tomorrow. I apologize for being slow, as I'm having some medical tests done so I work on this in between.

Thanks again all:)
 

Offline tacoTopic starter

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Re: At the end of my rope..need some advice
« Reply #15 on: February 27, 2015, 12:23:43 am »
Sorry also tested all the caps after taking them off the board with an ESR meter and they tested ok. But I should have just changed them :palm:
 

Offline bugs

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Re: At the end of my rope..need some advice
« Reply #16 on: February 27, 2015, 12:28:11 pm »
You may want to start to make some measurements when it is powered, with an oscilloscope. I think you have exhausted most other options :)
 

Offline tacoTopic starter

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Re: At the end of my rope..need some advice
« Reply #17 on: February 28, 2015, 04:34:29 am »
I wish i owned an oscilloscope and knew how to use it. Anyone in SW Florida (Fort Myers) willing to teach? Just a small plug in..I ordered all new caps, and they should be here hopefully tomorrow.

So here are pictures of both sides of the boards. Hope these help. Once again thanks for everyones help.
 

Offline gevans

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Re: At the end of my rope..need some advice
« Reply #18 on: February 28, 2015, 01:45:00 pm »
What is the voltage on pin 3 of the TL431?. (RH pin when viewed from the front)
It should always be around 2.5 volts regardless of the pot adjustment.

Does the voltage on pin 1 of the TL431 vary when you turn the pot?

I am wondering if this is a shunt regulator configuration.
 

Offline tacoTopic starter

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Re: At the end of my rope..need some advice
« Reply #19 on: March 01, 2015, 06:25:13 am »
No manual. The company won't tell me anything but buy a new unit. I will test those pins of the tl431 tomorrow late. I will not be able to replace the caps until Monday bc they didn't have all of them.

I will post the voltages of the pins late tomorrow.:))
 

Offline T3sl4co1l

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Re: At the end of my rope..need some advice
« Reply #20 on: March 01, 2015, 05:07:10 pm »
Good pictures!  They're still hard to see, mainly of course because there's components on top.  But this looks plausible?  (Ground fill unmarked.)
http://seventransistorlabs.com/Images/MotorControl_Traces.jpg

Tim
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Bringing a project to life?  Send me a message!
 

Offline tacoTopic starter

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Re: At the end of my rope..need some advice
« Reply #21 on: March 03, 2015, 06:49:15 am »
How did you find the board? Are they available for purchase? If so where?

Still waiting on the cap kit from digi key. Expect it any day. Then I will post the voltages of tl431. My dust collection system fried (motor), but I've replaced the bearings and she's up and running. Appreciate the help everyone. I'm also tracing and drawing the board schematic.
 

Offline IconicPCB

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Re: At the end of my rope..need some advice
« Reply #22 on: March 03, 2015, 07:04:03 am »
Tesla,

Did You generate the overlay?

Done free hand or with some software tool assistance? Which one?
 

Offline T3sl4co1l

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Re: At the end of my rope..need some advice
« Reply #23 on: March 03, 2015, 07:23:48 am »
Freehand, old copy of PSP.

Tim
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Bringing a project to life?  Send me a message!
 

Offline bugs

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Re: At the end of my rope..need some advice
« Reply #24 on: March 03, 2015, 04:44:49 pm »
can you make pictures of the PCB before you place the new caps? it may help with tracing
 


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