Testing a Printed circuit Board

[ke1000]

Hello
This my first post to the forum, to say i know nothing about electronics is to exagerate my understanding, so when my Proxxon Mill went bang and stopped working, my first thought was to hit it with a hammer, that always works doesn't it?

Sense prevailed and I have a multimeter and was hoping that there might be a logical step by step testing regime that even I might understand. I have attached 2 pics of the board, Ihope.

Any help will be much apprciated

Regards Bruce Davis

[Mike2545]

Well the one component looks like it has a burn mark on the top of it, From here it looks like a transistor or voltage regulator, is there any part numbers on it?

[sheldonstv]

we need to know what numbers are printed on the device which has obviously been damaged-at a guess it was either a triac or a thyristor maybe but we need the device numbers to be sure....you can set yr meter to read resistance and check for shorts on all pins on the damaged device also check each diode on the pcb for shorts too....but we do need the device part number before going any further

[unclejed613]

judging by a hasty schematic, the device is a Triac. basically it's a dimmer circuit feeding a bridge rectifier, which then feeds the motor.

looks like you got a metal shaving into the power supply box. most likely the triac is bad as well as the diac on the gate. the diodes in the bridge are only 1 amp diodes, this is a pretty small mill, isn't it?

[ke1000]

Hello

Just got online, Wow that was quick, thanks for all the replies, I will get all the numbers off the board tomorrow.

Regards

Bruce Davis

[ke1000]

Hello

I have tested the connections of the little cylinders on Ohms, R1 was 000, R2 was 229.9, D1 was 000, D2 was 000, D3 was 000, D4 was 000, D5 was 4.36. I hope this makes sense.

Also I have attached a drawing of the yellow side of the board, there sure is a lot of info on it. If you can't read it, I'll try again

Regards and Thanks

Bruce Davis

[sheldonstv]

ok the diodes are faulty if they are reading low resistance both ways-they need to be replaced (D1-D4)replace with 1N4007,D5 may be a diac-if it is sc it needs replacing also what are the markings on tr1???

[ke1000]

Hello Again

There is nothing written on TR1 that I can find, it is plastic with a piece of metal attached to it, it has 3 legs although only one seems to soldered through on the other side. It seems to be stuck to the board with resin.
D1-D4 which are black all give a zero reading even when the leads switch ends. D5 which is blue gives 4.36 one way and 9.65 the other. R1 gives zero both ways and R2 gives 229 both ways.
Sorry to be so long winded but I don't know what iis important.

Am I right in thinking that TR stands for Triac and D for Diac or diode?

Regards Bruce Davis

[unclejed613]

D is for diode, and TR is for transistor, and a triac is generally speaking, a transistor (actually it's the equivalent of 3 or 4 transistors). the diodes showing 0 ohms are definitely shorted. replace them with 1N4007 diodes. the diode D5 should be tested out of circuit, and should under normal circumstances read open (infinity or "OL" on a digital meter). D5 is a diac, which is a diode that acts as an open circuit until a certain breakover voltage is reached, and at that point exhibits a "snap action" and acts as a very low resistance until the current is removed or reversed. the breakover voltage is usually somewhere around 30-60V, which you won't supply from a standard ohmmeter. the triac operates on the same principle as the diac, except it can be triggered by applying a current on the gate (also known as a trigger) pin. a diac and a triac are both bidirectional devices, while an SCR acts as a triggerable one-way rectifier. diacs, triacs, and SCRs can actually be made from regular transistors, but to get all of the breakover and triggering voltages correct takes a bit of work. it's a lot simpler and cheaper to just buy the replacement parts. if you want to understand the basics of how this circuit operates, you can read here:
Light Dimmer Circuit Using A Triac

if you substitute a bridge rectifier and DC motor as the load, you can see how this can control the speed of your milling machine motor.

while i think of it, you might want to check your motor and make sure it wasn't burned out by applying AC across it...

[ke1000]

Hello

Apologies for the delay in replying, pressure of weekend work. I have desoldered the little blue one and the reading is OL. I have checked the motor and it seems OK.
I found it very difficult to do, none of my tools were small enough to hold the blue thing, I even tried blu tack on the end of a match and my soldering iron felt enormous against the small connections. I use it for electrical work on Wiring harnesses, it's 40 watt.
Can you suggest a size of soldering iron more suited to this work and something to hold the units for when I get the black ones out.
Also where do I get the replacements from, is it RS, Maplin?

Thanks

Bruce Davis

[unclejed613]

a 15 or 20 watt iron should be about right. the 1N4007's can be ordered from mouser or digi-key, as can the triac.

since your device has 1 amp diodes in the bridge, a triac that will work is Q6004L4.
if you need to search for a triac, the following specs should help you find one:
600V 4A TO-220 package.

you probably should look and see if the machine also has fuses somewhere. i would expect that there are fuses, at least on the AC input.

[sheldonstv]

well u could invest in a 25w iron and some desolder braid which you can use for parts removal from yr pcb...

[ke1000]

Hello Again

I was away until Thursday, so on Friday I ordered the parts from Mouser. Will report back once I have hopefully replaced the broken ones.

Could I ask about the desolder braid, do you melt the old solder and the braid "sucks" it up?

Thanks

Bruce Davis

[ParkingLotLust]

Exactly! You hold the braid to the connection, and heat up the braid and solder with the iron, and the braid wicks the solder into it.

[ke1000]

Hello Again

Sorry to be a pain. but Mouser didn't have the Triac, told ne it was 9 week wait, so I cancelled the order.
My local maplin have the Diodes and a Triac that is described as Type C226M P2(case style) 600V 8A 50mA(IGT max) 30mA(IH max). They don't have a 4A one. Will this do?

Thanks for any help

Bruce Davis