Request for help fixing a motor speed controller

[john.oibrien123120]

I have taken apart a speed controller for a wood lathe in an attempt to fix it. So far I have had no luck, but I'm hoping someone here can point me in the right direction.

The problem is that the speed controller is supposed to vary the DC output in the range from 0 to 115V. Right now, it consistently outputs 168V DC, and that output does not vary with adjustments to the potentiometer.

So far, I have tested the potentiometer (it works fine) and replaced one capacitor that looked damaged (no effect). I have attached a picture of the device. I'm hoping someone here reading this may say "ah, I've seen that problem before, you need to replace [this one component]." Can anyone help me out?

[BeBuLamar]

I would say the SCR is bad. It conducts all the time thus you have the high voltage. When you rectify 120VAC and with a filter capacitor the voltage is 169VDC. So that is what you get when the SCR conducts all the time.

[john.oibrien123120]

You are a saint! Thank you BeBuLamar! Is the yellow thing the SCR? If not, any "tips" on how to identify which component is the SCR?

[HackedFridgeMagnet]

Is it an scr on a dc circuit?
M+ and M- kind of implies DC and also the large brown electrolytic cap.

That board has so many unloaded components, talk about cost savings.

Can you get part numbers from the 3 black semiconductors attached to the heatsink.

Is the one on the right a full bridge rectifier? Has it got four legs?

A good way to check semiconductors for failure is to check for short circuited pins (legs) using ohms or diode on a multimeter. A short circuit often means a fail.
Obviously with any power off.

[Xena E]

The chip is a TL494 so it's most likely driving a transistor or fet in PWM to control the motor.

I'd  agree, dead short in the pass device will be most likely.

As suggested get the type numbers from the devices on the alloy heat sink.

The yellow job is a capacitor for transient suppression.

[Ian.M]

The leftmost power semiconductor's board markings and package suggest an IRFP460 MOSFET.  No SCRs in sight.

[BeBuLamar]

I guess the OP board doesn't use SCR but most DC drives use SCR.

[john.oibrien123120]

Ah, thanks! I'll desolder and test the black things attached to the heat sink. Thanks for all your help! I sincerely appreciate it.

HackedFridgeMagnet, I should have been clear, the circuit takes 120 AC and converts it to 0-115 DC, based on the position of a potentiometer knob (or at least, that is what the circuit would do if it was working). The circuit is part of a wood lathe, which is a tool with a DC motor (in this case) that you plug into a 120 AC wall outlet and use to carve/cut wood objects.

[john.oibrien123120]

So, an update. I'd tried to follow the traces on the circuit board but I get lost too quickly, and there are a bunch of tiny components on the back that I can't identify, so I haven't been able to do it.

I did not desolder all the black components. I did, however, get all their part numbers and look each one up:

KBJ1510 - bridge rectifier
MUR1660 - ultrafast rectifier
RFP460 - power MOSFET

I don't think any of them is an SCR. Is that correct?

When I look at the board, the only thing I can see that looks like it might serve the function of an SCR is the chip, a TL494CN. The internet says that the TL494CN is a PWM control circuit. Is it plausible that the potentiometer connects to the TL494CN, that the TL494CN is defective, and that if I replace the TL494CN (desolder it and solder a replacement TL494CN in it's place) that the DC voltage output would once again vary with changes in the position of the potentiometer?

[Xena E]

The most likely fault is that the RFP460 mosfet has shorted.

This can be tested for with a multimeter.

Identify the drain and source terminals and test on a low ohms range, a test showing a dead short will indicate that it needs to be replaced.

If you undertake to do this yourself take care with handling as these components can sometimes be damaged by static charges.

The tl494 could be damaged though less likely

[john.oibrien123120]

Thanks Xena E!

I desoldered the part next to the RFP460 label on the board. The part number on the part itself is IRFP250N.

When I connect the negative probe of the multimeter to the source pin and the positive probe to the drain pin, the multimeter indicates the circuit is OL.

When I then touch the positive probe to the gate pin, and then touch the positive probe to the drain pin again, the multimeter indicates a voltage difference of about .5 V.

When I short the source and the gate, and again touch the positive probe to the drain, the multimeter again indicates the circuit is OL.

My understanding is that this means that the MOSFET is fine. Is that understanding correct?

[Zero999]

Yes, it sounds like your MOSFET is fine.

It should be open circuit when the gate voltage is zero.

When you touched the gate with the positive probe, you charged the gate capacitance, which meant it conducted, when it was connected between the positive and negative probes. Shorting the gate, discharged it, causing it to become open circuit again. This isn't the most reliable method of testing, since some MOSFETs contain protection circuitry, which will discharge the gate. It's better to use another battery, to turn it on, although your method clearly worked on this occasion.

[john.oibrien123120]

Ok, thanks Zero999.

In that case, is the likely failing component the TL494CN? Should I try replacing that next?

[Xena E]

That's a shame John, it would have been an easy fix.

The chip may be at fault, if you have a spare it may be worth trying next if you're able to remove it from the board easily.

If the mosfet is OK, then it must be having continuous gate drive. It should be getting PWM control.

Do you have a schematic for this board that you could post?

A clear picture of the underside of the board  may help

Regards...

[john.oibrien123120]

I have checked the manual, and unfortunately it does not include a schematic. Attached please find a photograph of the underside of the board.

I don't have a spare on hand. The chip on Mouser is like $.56 but requires $9 in shipping. I can't find any local store that sells it. $10 is still much cheaper than a whole new motor controller, but I am wondering, has anyone found a cheap way to get a specific chip locally (does any store special order them for you or something)?

[BeBuLamar]

Getting chip locally is a tough one. Back in the old days you may be able to find something at Radio Shack or Fry's but now no. Why not getting both the IC and the Transistor? The shipping is the same for both.

[john.oibrien123120]

Good idea BeBuLamar. I tried my best to identify all the components that seemed desolderable and ordered all of them. That way, if it isn't the TL494CN, hopefully I'll have whatever the next suspect is on hand. Thanks for all your help everyone! Exciting!

[Xena E]

Looking at the the last photo I have identified some points on the board you may want to check,  particularly the zener diode D1 15V, that regulates the supply to the control electronics which could be open circuit allowing the supply voltage to rise above the intended ≈15V.

Also the mosfet driver transistor Q1 (8550) that could be shorted, as could the diode D3 (1N4148).