Circuit overload question

[taco]

I have two two identical circuit boards on different machines running. The board has an input of 110/120V through a transformer to output at DC 32V 0.8 Amp. The output is variable from a control pedal. When the board is powered up it fails to 100% output and the over voltage light comes on. As a newbie how would I trouble shoot this problem?

Thanks

[T3sl4co1l]

What's the relevance of having two boards?

[SmokeTestAlpha]

Let me ask about the unspoken obvious thing first.  When you power up the board, I'm assuming the control pedal is connected?  If not, perhaps the absence of the pedal is creating an open in the reference voltage the board uses to regulate power accordingly.  I've worked as a service tech on some things before, one of the very first gotchas anyone finds is forgetting to test appropriately when circuits are split between boards or devices. 

If that isn't the actual problem, remember that any variable regulator circuit has to have a reference of some kind to keep it in bounds.  Check the schematics and docs you may have and hunt down the reference, many times a voltage, that keeps the power in range.  You may find your problem that way.  Good luck!

[taco]

Sorry the board just got attached twice. I actually have two controllers doing the same thing, which leads me to believe that there is a weak component. When the board is connected and the control pedal is at 0 the unit automatically speeds to 100%. The OV light on the control circuit is illuminated. I suspect there is a voltage regulating component failing, but being a newbie I removed the board from one unit and am asking what to check. I use these units every day (dental hand piece with controller). This particular brand has this fault. I was attempting to repair it myself while learning since I gotten bit by setting up a home mini shop to understand how these things work.

Thanks

[taco]

Just to be clear. Each unit has the ability to be run by foot pedal or hand (pot) control. It does not matter what mode I use, as they both have ended up working fine until one day you start it up and the unit shows OV and the hand piece revs to 100% even though the controller (hand or foot) is set to zero.

[T3sl4co1l]

So it's not a service problem, it's an original design problem?

Never buy that brand again, easy enough

Tim

[divelectservices]

You're not going to be able to fix them unless you understand exactly how they work.
Time for some reverse-engineering!

[taco]

Exactly. That's what I'm trying to do. I'm trying to figure out what component regulates the voltage. I want to desolder the components suspect and learn to test them properly. One of the board components obviously has failed. Where should I start?

[T3sl4co1l]

We can only see the top side in the picture, and not very well (many components are obscured and the IC is unreadable due to lighting angle), let alone the back side.

What can you figure out from how the traces connect the components?

Tim

[taco]

I will post more pictures..thank you for your help.

[taco]

Hope these pics help..

[Skimask]

Well, if you're just going to throw parts at it rather than learn how circuits operate, etc, then I'd swap the opamp and the 2 big transistors first, in that order.
All 3 of those parts are cheap enough to be throw away...

[T3sl4co1l]

Strange, I don't see anything like a small value capacitor, ceramic, film, that kind of thing... if it's completely uncompensated aside from what the op-amp is doing, that might explain its design mistake...

Tim

[taco]

I'm as dumb as a brick in electronics. I have purchased all the equipment to learn with. I usually look for obvious things wrong and just solder a replacement part on. Where should I start in trying to learn basic circuitry? I want to be able to take a board like this and figure out by testing what makes it work. I understand that the 110 hits the transformer and is reduced to DC 32V on the output. But with the provided board I don't know where to start, and how to proceed to troubleshoot this circuit. Or any circuit for that matter. Where should I start (good books or videos) to learn how to do this? It's frustrating to be able to rebuild the mechanical side of things and not the electronic side. I want to learn....