Diagnostic tests on the Ryobi Scroll Saw SC164VS.

My guess(tm) is the triac is dead, probably from mechanical vibration. Maybe add some rubber grommets to mechanically isolate the PCB.

"Ryobi Scroll Saw Fix"

The author replaced what he calls a MOSFET (it's actually a triac) BTA12-600C (600V 12A)

Warnings? Well, the object of this exercise is the fix the scroll saw, not to reverse engineer the design. For complex designs, that probably makes sense, but for a simple controller board, just fix it and move on.

I agree with Jeff. That said, if you want to go through the diagnosis, what is the advantage you get by using a variac? How do you know that 15V is enough to move the motor? Read on for the reason not to use a variac at 15V.

Instead, I would use 120 VAC input. With the triac shorted MT1 to MT2, the motor should spin at full speed. If not, a bad motor or bad bridge or bad wiring/connection.

If the motor spins at full speed, the triac circuit is failing, most likely the triac or the diac. You can use your ohm meter to check the resistors and and a cap tester for the caps. For all of the above your hands and test equipment will not be in the device when 120VAC is applied, so it's safe for you. Shorting the triac wont hurt the equipment during tests.

If it still fails at this point, you have eliminated everything except the triac and the diac and are at the point where I suspect you wanted to use the variac at 15V for safety, so you would be probing with lower voltage. It won't work - it won't reach the breakover voltage of the diac.

Since the diagnosis thus far left you with two parts to check, you might as well replace them. Interestingly, that brings you to the point Jeff made in his post. His way is the most logical straightforward approach if repair is the only objective.

For learning, you could buy extra diacs and triacs and wire up the same ciruit you drew with a lightbulb replacing the motor and the bridge. You can probe that for the learning, taking care that you ARE working with 120 VAC. Theres some thinking (and therefore learning) involved in figuring out what wattage light bulb (minimum and maximum) will "satisfy" the triac.

Ed

Do you test an automobile engine, chainsaw, or other speed controlled devices by "flooring" the throttle or speed control to maximum, and then applying power? Do you fix a hi-fi audio system with the volume controls at full blast? That's exactly what you're doing by shorting the triac. Methinks a better idea would be to leave the triacs and wiring in place, and replace the scroll saw motor with an incandescent light bulb.

Incidentally, please note that my domain name is "LearnByDestroying.com". There's a reason for that name, which I don't want to discuss at this time (dinner will soon be cold). Let's just say that I learned from experience that spectacular methods of self-education are effective, but usually not the best path toward enlightenment.

If you used that method to test an automobile engine or chainsaw, a successful test would be the self-disassembly of the engine after it exceeds the maximum RPM limit. Incidentally, excessive RPM is a problem with engine and chainsaw repair, as well as other devices with an adjustable speed (RPM) control. Maximum control position could easily be higher than maximum safe RPM if the operating RPM requires that the device be under load or if some creative engineer is using some kind of rotation limiter on the control positition as a mechanical maximum RPM limiter.

Ummm, most low end ESR (equivalent series resistance) capacitor testers at designed to test at 100 kHz. There are a few that will test at 60 Hz, but those are not common. In most cases, if the cap tester shows acceptable ESR at 100 kHz, it also be acceptable at 60 Hz. However, I've been suprised by some situations where the test frequency does matter.

Oops. I dropped my calculator on the keyboard and prematurely posted the above partial message. Blundering forward...

On the graph, notice the variations in ESR with frequency for an MLCC X7R capacitor.

Ok, I give up. Dinner beckons.

Of course not.

No, it isn't. The scroll saw is made to run at full speed without self destructing. If he uses a variac set to 15 volts, it won't run at all - the diac won't conduct.

That might be better, I dunno. It may depend on the physical routing and connections of the existing wiring, I figured since he mentioned shorting the triac, that that was easy for him to accomplish.

That's true, but we're not talking about chainsaws or auto engines.

Those caps are too small in value. You're not going to find a bad cap in his circuit with an esr meter. A capacitance meter will show an open cap if its open.

Ed

Where did you find the values of the capacitors? I couldn't see the values in the video: "Ryobi Scroll Saw Fix"

Looking at the caps in the video, they look like metallized polypropylene film capacitors. I would guess 0.1 uF for both caps. You're probably right that 0.1 uF is too small for a valid ESR test.

Instead of a Variac, a "dim light bulb tester" in series with the power line is good enough to prevent motor overspeed. I have several:

I didn't find the value. I just knew from experience with this kind of circuit that they had to be relatively speaking low in value - too low for a typical esr meter to be effective. I didn't see the video you mention below.

If we had full specs (except for the values of c1 & c2) you can work out their values, but were missing values for R5 & 6, and we don't know the breakover voltage for the diac. But a quick look at the schematic and knowing it's a low frequency (60 cycle assumedly) tells you you need a tau something within 8.3 ms. Therefore, relatively low value cap.

Ed

C1 & C2 are marked CY400V, 104JG. Web sites report; 0.1 uF.

For each of C1 & C2, I desoldered one end and connected the old Bob Parker ESR meter. Readings were around 30 ohms. Below the chart line on the meter although stretching the range.

R5 is 100 kohms. R6 is 220 kohms.

The DIAC is marked C702. At least one Web site claims a DB-3 is equivalent.

With a jumper connecting the two terminals of the TRIAC (not the gate), the motor begins to spin at about 8 V on the variac and speed increases with voltage. So the motor seems OK.

With no jumper on the TRIAC and a jumper over the DIAC, no rotation of the motor at any setting of the variac up to 50 V. I expected the TRIAC to close at some voltage and connect power to the motor.

Same results with a locally purchased TRIAC replacing the orginal.

Any value in isolating the TRIAC, connecting the gate and a terminal to an adjustable DC supply and checking with an ohmmeter for the TRIAC to close as voltage is increased?

There must be a way to identify the faulty component. Other tests?

Thanks, ... P.

Jellybean diacs breakover ~32V. Other voltages exist, but are extremely rare and would serve no special function in this case.

ST2, BR100; take your pick.

RL

You can't rely on that ESR meter to test the caps in your scroll saw. Use a capacitance meter to check capacitance, and an ohmmeter to check for leakage.

Looking at your diagram, I'm guessing that R6 is set to establish the minimum speed because it is marked trim, and R5 is the external speed control that lets you set the speed from minimum to maximum. Is R6 mounted on the circuit board and R5 adjustable by the user?

That means the bridge, motor and wiring to MT1 and MT2 on the triac are all good ASSUMING you jumper was directly connected to MT1 and MT2 on the triac.

That leaves some parts untested: C1 and C2 CANNOT be assumed to be properly tested by the ESR meter. At minimum, test them with a capacitance meter AND also an ohm meter. On the ohm meter they should read well into the megohms or open; on the cap meter their values should be close to .1uF . Use the ohm meter to test every resistor including the variables WITHOUT changing the setting of either of them. You don't need to take each one out of circuit - all you need to do is unsolder one end or R3 to make the tests across each resistor. Record the readings.

Next, connect the ohmmeter to p1 and measure path resistance from R3 through R4 and R6. Record the reading. Then measure from R3 through R2, R5 and R1. Record the reading. Be sure to connect the ohmmeter lead such that one of those paths includes R3 and the other one does not include R3.

That's almost all you can do without power applied. A couple more ohmmeter tests: measure for 0 ohms (or very close to 0) from R3 (with R3 soldered back in) to the diac, then from the diac to the gate of the triac.

Assuming no fault is found in any of the above tests, the only components left are the diac and the triac. If new parts were installed and the saw still fails when plugged without using that variac, either the testing was done improperly or you'll need to measure things mains power without using the variac. If that is uncomfortable for you then a possible option is to buy the whole circuit board and replace it.

That is a way one can test a triac, with some caveats: Isolate means use an isolation transformer. Use an incandescent bulb instead of an ohmmeter. Use a current limiting resistor between the supply and the gate of the triac.

^^^ One nasty (at times) gotcha in the above sentence. Sometimes it's more than 1 component. Grumble. Can make troubleshooting hard, and sometimes repair even harder: "Whenever C23 fails, you need to replace it AND Q11" - or whatever for some poorly designed circuit or other.

Ed

At home until June 22 or later. When at the site again will work on the further tests.

When at the site again, will post a photo of the board.

What I marked "trim" is a can about a cm dia with three connections on the bottom and a little blue plastic button on top with a notch for a small screwdriver. Fifty years ago it would have been called a "trim pot". Not accessible when the saw is fully assembled.

Thanks for the detailed explanations.

... P.