EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: KaiOwei on October 01, 2019, 02:17:26 pm
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Hello,
I recently got a used Table Saw wich was built in 1990. It works perfectly fine, but the braking when turning it off doesn't seem to work at all.
I attached pictures of the board.
All components look visibly fine and i replaced the Thyristor, wich didn't fix it.
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What testing facilities/tools do you have available ?
Have you checked the two fuses? The solder joints for the diode (see photo) look dodgy - this might indicate that the diode has been over heating.
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It it not breaking at all, or just too short. One could do a test without the blade and maybe also the flange. In this case a short breaking phase would still noticeable. The trimmer should be there to adjust the time - so the actual setting is not super critical and one could note the position and more it back and forth. Another possible failure point is a Diac (often looks like a diode) at the thyristor gate - these sometimes fail and no longer trigger.
Checking the 2 electrolytic caps could also be worth a try.
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I found out that the 68kOhm resistor next to the potentiometer was broken and had a value of Infinity.
Also replaced both electrolyte capacitors.
When i plugged in the AC connector, the Motor was humming a little, as if the brake was on.
I pulled the Plug and after connecting it again, it didn't make a sound, but the brake also didn't work.
Edit: I found a post on a German forum wich has the same circuit and a description of all parts.