Help with rectifier identification

Hello, I'm trying to fix an old (c1980s) Oster food processor whose variable-speed motor is stuck at a single speed. I only have a very basic understanding of circuitry, but the board is simple and I'm guessing the rectifier is the most likely culprit. I've searched for the numbers on the rectifier (758-21 J8) and found nothing so I'm hoping someone can help me locate a suitable replacement. Any help would be greatly appreciated. Thanks

The setup is similar to a variable speed drill, but not reversible. One wire of the AC power goes straight to the motor and the other goes to this board (black). The white is AC-out to motor, and red (hidden behind black) and blue (on resistor pad) go to the rotary speed selector dial.

 

Thank you very much Bluejets. I read a little on thyristors and that sounds exactly right. I'm in the US so it's 120v mains. There are no capacitors that I see anywhere unless the white rectangular boxes in the picture are capacitors. This is the only circuit board in the machine. I have not tested the thyristor as I wasn't even sure what type of device it was.

The only other parts to the electrical system that I can see are a push button switch to power to this board (on/off) and a mechanical centrifugal switch which appears to bypass part of this board during high speed operation.

 

Sure, thank you.
General overview:

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Views of the 'centrifugal' mechanism. One contact is on a rod which drags along an offset lobe on the speed selector dial, the other is linked to the end of the motor and moves away from the lower contact as RPM rises. When closed, the motor seems to rev to max.

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Speed dial with lobe inserted and power button.

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Wiring from speed dial to board:

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Board ID number (Oster 208389)

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I could probably tell more if it was functioning properly, but it's hard to decipher what the centrifugal switch triggers with it running full out all the time.

I did some research into testing thyristors and triacs; this would be a triac and not a thyristor because it's AC, correct?

As far as I can tell, triac pinout is standard as (left to right) line1, line2, gate. The tests I found say a healthy triac should have high resistance across 1-2 and 2-gate, but low ohms across 1-gate. This triac seems to be good and tested at 89 ohms across 1-gate and very high resistance across 1-2, 2-gate.

I'll have to look closer at the other components and try to figure out where the power making it to to motor. Just in case I do end up replacing the triac, can you recommend a quality component which would likely hold up under this motor's load? The switch is rated 120v 10A so I expect that's above the motor's draw.

 

I'm always so impressed with how much knowledge people have on complex topics like this! Thank you both so much for this info. Tremendous write up 73's de Edd; if I can't fix it with this info, I'll be ashamed of myself. You're exactly right about why I'm trying to repair it. The Regency model is kind of a universal power base for attachments and great for small kitchens like ours.

The motor turns on with the power/pulse switch and runs a near full speed. Unlike a blender, I think the pulse button is subject to the speed control dial the same as the power button. It just lacks the mechanical latch to lock on.

Your description of the pot's setup sounds exactly right to what I'm seeing.

You mention EMF feedback for speed compensation relative to load- I've been thinking about the centrifugal mechanism and I suspect that's also what it's for. In a variable speed drill, backing off the trigger reduces the motor's speed , but also reduces it's available power. A device like this needs full power even at low RPM so I think the centrifugal switch applies full power to the gate if RPMs drop below the target set by the speed dial. I could be wrong.

"As well, one might expect some gearing down between the motor shaft speed and cutting wheels in order to get down into that low speed STIR function and its required and effective final torque".​

There may well be a gearbox at the top of the motor as it appears to have two output drives- a center high speed shaft and a slower cog ring. I haven't torn it down that far so I may be wrong. The mixer top for this motor-base is a gear reduction box and several of the attachments link through it.

I removed the SCR from the board so I'll put it back and try bridging A to B as you suggest. I hadn't clarified that the motor is running full out all the time, so this may not be decisive if A-B are already shorted?

Will check the gate signal when I get it soldered back together. Thank you so much for taking the time to write up this excellent info!

"...IF the BLACK wire coming in from one motor wire is being the BLACK wire that connects to #2 of the SCR." ​

The black wire goes directly to one of the motor points, and also the centrifugal switch contact which is attached to the motor.

"And would that be Slow-Med or Fast ? and the speed control has NO effect?"​

Fast, and the speed control has no effect
.

"are the GREEN circles being Nylon or Delrin insulated, standard slotted screws"​

No, it's an aluminum rivet which was oxidized to white. The heatsink has no electrical contact to the board though, only held on with a slot on one side a twisted tab (right of C) on the other.

Your signature "$89.95" (ha ha) reminds me of what I always tell people "I'm so poor because I'm always giving people my 2 cents worth".

 

Okay, I did the three tests you mentioned.

"a very fast shorting across the A and B mark up points will cause the same unbridled max speed of the motor."​

I didn't clarify that the motor is always runs fast when on, but I tried this anyways. There was no change in motor speed, though there was a very minor arcing when I completed the jumper so I know I was making good contact.

But, this time I noticed that when I first turned it on, the motor hummed and spun very slowly for about 5 seconds before jumping to full speed. During subsequent off/on cycles it went to full speed immediately. However, if I leave it sit for a minute, it repeats the slow start-up. This reminded me of a video I watched about SCRs (which mostly confused me), but it seemed they lock 'on' even when current is removed from their gate if the load on the cathode is within certain limits. I repeated the "jump A-B" test after leaving it sit for a minute and it went to full speed without any delay. Is it possible that this SCR is designed to lock on open if the gate signal voltage is too low?

"DVM in AC mode and probe between C ground and D and power up to see if there is a . . . .gate drive signal . . . .for you to take a reading"​

The reading is erratic between .2 to .4 volts and shows no notable change in pattern when turning the speed control. I also disconnected the speed dial and tested ohms and I think it's behaving properly with a smooth curve in the middle zone and a plateau at top and bottom.

"move the probes to between E and F to then see if you read the ~25'ish thru 40'ish AC volts, being expected across a DIAC"​

Again, this hovers around 1/2 volt AC. Tried with two digital meters on all these checks to verify results.

So, does this mean the diac is suspect and not the SCR? REALLY, thanks for this help guys. I love learning about such things as well as redeeming a good oldie from the trash bin.

 

I've been researching these 'dimmer' circuits. My background is in automotive so unfortunately my brain is trained in DC but think I've managed to get a basic idea of how the triac/diac circuits are setup. Discussing repairs is a bit outside an 'identify component' forum, but I'll add what little I've learned in case anyone runs into a similar problem.

I mentioned that the motor barely spins when first plugged in, but then pops into high and stay there unless current is removed for about 30 seconds. When the motor is barely spinning, there are 20 volts across the diac, then it drops to almost 0v when the motor pops into high.

I find no evidence that this is a DC thyristor as all schematics I find for this type of circuit use an AC triac. I have no way of knowing the specs for the triac, but I am speculating that the 20v at start up to the gate terminal is the intended gate trigger voltage which would signify that the triac is not responding to the gate signal as designed.

I have read that triacs used in dimmer switches are usually designed to bridge terminal 1 and 2 when they fail (ie turn on high) so I'm guessing that this may be a similar feature making the motor go to high speed if the triac is bad.

As far as I can understand, the diac is designed to trim the waveform to match the minimum voltage requirement for the triac's gate. In conjunction with a capacitor, they effectively become a voltage regulator for the gate. The triac, diac and capacitor are all balanced to work with each other so I'm not sure what will happen if I try a replacement triac with different specs, but I ordered a matched triac/diac set. I don't have a lot to lose by trying it out as the other components on the board are easily identifiable and cheap to replace.

As far as I can tell, if the capacitors on the board don't match the requirements of the new triac/diac set, it will stress the triac and cause premature failure, but shouldn't blow anything up. Then again, I'm a newbie so who knows ;p