laboratory mixer / diode blown?

[kuzao]

hello,

i have here a korean made laboratory mixer which is not working. got this from my colleague who doesnt know how it stopped working. i disassembled the mixer, accessed the pcb and checked all tracks for continuity and most components too. all capacitors, potentiometers, resistors and the fuse are OK. i didnt test yet the triacs (because didnt know how to, but will try that today evening).
the only thing what looks broken at the moment is one diode showing a 0.6 voltage drop in one direction, and 0.4V drop in the other direction. (diode marked D2 on the lower right, right next to the large red capacitor)


actually i dont understand how this motor driver board works so i would like to know your opinion if this diode could be the problem.

i have another question just out of curiosity: what is the black round thing in the picture below? i thought it could be a hall sensor but if thats the case could somebody tell me what it does there?


thanks a lot in advance
regards
alex

 

[tunedwolf]

Don't use a multimeter set to ohms on a diode, the test voltage is most probably high enough to read the surrounding circuitry causing inaccurate readings to be taken. If the meterr has a diode check, use that instead. I suspect that the diode is fine. The round thing you mention is a resettable thermal cutout, when it gets too warm, the bimetal switch opens and breaks the circuit. It then closes again when it cools down.

Take close up photos of both sides of the board and provide a decent fault description and maybe folks here can help you get it sorted out

rgds

 

[ben7]

Well the mystery part is a thermal cutoff which disconnects power when whatever the cutoff is on goes above a certain temperature.

-Ben

 

[kuzao]

thank you for your replies

yes i know the fault description is not really good ^^ but unfortunately i dont know anything else. i got this in a non working condition and all i know is that when switched on it doesnt move at all.

actually i checked the diodes in the diode checking mode and the values i mentioned are the reported voltage drop. so this diode is conducting in both directions. after your suggestion that the surrounding circuitry might effect the test result i desolderd the diode and tested again and it is indeed conducting in both directions.

again learned something new i checked the resettable thermal cutoff for continuity and it looks ok.

does anything in the pictures below looks suspicious?


thanks and regards
alex

EDIT: damn.. i checked again and when desolderd the diode is NOT conducting in both directions. so back to square one. anybody an idea what might be wrong?

 

[ben7]

thank you for your replies

yes i know the fault description is not really good ^^ but unfortunately i dont know anything else. i got this in a non working condition and all i know is that when switched on it doesnt move at all.

actually i checked the diodes in the diode checking mode and the values i mentioned are the reported voltage drop. so this diode is conducting in both directions. after your suggestion that the surrounding circuitry might effect the test result i desolderd the diode and tested again and it is indeed conducting in both directions.

again learned something new i checked the resettable thermal cutoff for continuity and it looks ok.

does anything in the pictures below looks suspicious?
View attachment 51029View attachment 51030

thanks and regards
alex

EDIT: damn.. i checked again and when desolderd the diode is NOT conducting in both directions. so back to square one. anybody an idea what might be wrong?

I would test the devices that are on the heatsinks, they are most likely triacs.

Nothing in the pictures looks suspicious to me.

I would start pulling parts out and checking them.
If you don't know what connections are what on the triacs and other transistors, let us know what the numbers are on their plastic case.

-Ben

 

[tunedwolf]

I suspected that D2 was ok...although it's only a small step in the right direction...
I don't see any obvious signs of stress on the board. I would check the presets on the board, these set the minimum speed etc, all resistors, particularly the 100R ones (R3, R5), the transistor (Q1) and the speed control pot on the machine, which I think is going to be the most likely problem. Dirty or cracked tracks on speed controls on lab equip is the most common problem I encountered.

The other possibility is that the bridge rectifier is goosed (Square thing in the corner marked D1). Check also that the main switch/ fuse is good.

My best guess at the devices on the heatsink is that they are SCR's, but a quick type number check will identify them.

rgds