Pendular saw control board problem - Page 1
Forum section: electronics / repair

author: carcanhol
author_rank: Regular Contributor
timestamp: November 09, 2015, 01:59:40 pm
content: Hi, a friend gave me this pendular saw and it has a problem in the control board.The motor is DC.I used an hp printer transformer at 32V directly at the motor and it ran fine but slow(need more voltage?)It was too slow to cut wood so i ordered some replacement parts hoping to make it work.The problem was when you turn it on it runs a bit slow and 5 sec later it goes full throttle but more than it should by factory(400 to 1600RPM).Now i swapped the components in the pictures and it doesn't run always when pressing the on button and when it runs it goes full throttle again and stops after 2 sec.Anyone have an idea of what is happening? a friend told me those large ceramic? capacitors break a lot. or it could be the diodes?thanks for any help.

author: SeanB
author_rank: Super Contributor
timestamp: November 09, 2015, 04:24:49 pm
content: Change C4, as it likely has self healed itself down to a low value.

author: bookaboo
author_rank: Frequent Contributor
timestamp: November 09, 2015, 05:22:04 pm
content: Have you tested the electrolytics for ESR and value? May as well just change them while your at it.

author: carcanhol
author_rank: Regular Contributor
timestamp: November 09, 2015, 06:31:07 pm
content: Quote from: SeanB on November 09, 2015, 04:24:49 pmNo good, motor still rotates 1 turn or so and stops after several tries on the power switch.Thx

author: carcanhol
author_rank: Regular Contributor
timestamp: November 09, 2015, 06:32:49 pm
content: Quote from: bookaboo on November 09, 2015, 05:22:04 pmA Fluke 79III could be used to test?Going to change for some nipons i have.Thx

author: carcanhol
author_rank: Regular Contributor
timestamp: November 09, 2015, 06:54:25 pm
content: Changed the electrolytic caps and is the same symptoms.

author: carcanhol
author_rank: Regular Contributor
timestamp: November 10, 2015, 01:40:57 am
content: i have 0.8v on the opamp power pins does that mean some zener is busted a not giving it the voltage needed?

author: Circlotron
author_rank: Super Contributor
timestamp: November 10, 2015, 02:05:23 am
content: Single sided board that has no plated through holes is liable to have cracked solder joints. Re-solder everything then see what happens.

author: bookaboo
author_rank: Frequent Contributor
timestamp: November 10, 2015, 09:25:05 am
content: Quote from: carcanhol on November 10, 2015, 01:40:57 amThat or something's pulling the line down, or the zeners not getting a supply to start with.Sketch a schematic out, that usually makes it easy.

author: carcanhol
author_rank: Regular Contributor
timestamp: November 10, 2015, 06:28:00 pm
content: Quote from: Circlotron on November 10, 2015, 02:05:23 amDone, no change, i'm going to make a schematic, strip all components test them all resolder the clean board and if this is not going to work i kill this bitch.

author: carcanhol
author_rank: Regular Contributor
timestamp: November 10, 2015, 06:29:42 pm
content: Quote from: bookaboo on November 10, 2015, 09:25:05 amUsing the diode test on the MM is good to test zeners?

author: bookaboo
author_rank: Frequent Contributor
timestamp: November 10, 2015, 07:13:44 pm
content: Quote from: carcanhol on November 10, 2015, 06:29:42 pmDepends how high a forward voltage they can put out, if it's lower than the rated zener voltage you wont get a reading. You could try removing the parts on that supply line then testing the voltages at the resistor and zener that way, or remove the zener and use a suitable resistor and power supply to test.Removing the components on that line one by one may isolate the problem if you voltage at the zener returns to normal.