Plastic welding machine repair and hints

[conan981]

hi, i have a welding machine with ultrasonics. it's REALLY old
btw since now i could use it without any trouble. now i have the HIGH POWER NPN TRANSISTORS MJ802 that keep shorting after 1-2 hours of using.
this board is powered with a transformer that from 220V give 55v, 6v, 12v

MY question is : why these transistors burn?

i wish someone expert can give me some hints and what to check on the board.
There are few components luckily. i am uploading photos of the board so you can have an idea. if something else is needed i can do measurements or else (no problem in soldering/delsoldering )
thanks in advance

[stj]

if it runs for atleast an hour then it's probably heat.
try replacing any thermal pads/paste - maybe add a fan.

[sam_sam_sam]

How stable is the 55 volts, 12 volts and the 6 volt power supply rail

And which power supply rail powers the MJ802 s

If this has heating elements that the MJ802 s operates are the heating elements drawing to much current

[conan981]

thanks to all for the hints. about heating, that machine have a fan even if i dind't made a photo to it. and last time was working 1 hour BUT not correctly. i checked quite all the passive elements except the inductances that i cannot verify and seems all to be ok. only two big capacitors i found that are 800uF /900uF and have MANY hours of work, but i don't think they are the culprits for this defect

6v and 12v are perfect and stable, what should be 55v really is 61v but is stable too.
how can i check if elements connected to mj802 drawing too much current ?
thanks again for all the answers

[sam_sam_sam]

Originally Posted by conan981

how can i check if elements connected to mj802 drawing too much current ?
thanks again for all the answers

This is where you have to trace the wires and where the output of the MJ802 goes

You need to know if the 55 volt power supply rail power the MJ802

There are two ways to find out what the current that is coming out the MJ802 are

One is if the output of the MJ802 is being powered by the 55 volt rail

Then you either use a DC clamp on meter or use a current meter that you have to use in series with the load of the MJ802

You also find out if it has more than one heater element

If it does have more than one heater element separate them if you can as long as you can reconnect them back together

What you are checking for is if one has more current draw from one heater element versus the other heater element if it is setup this way

I hope this helps

[conan981]

hi, please explain more.. output from MJ802 should be nearly 100v on the COLLECTOR, right? i did some other photos on the main transformer and his plug (10 pin)

on main capacitors, i have 81V DC. (when machine is ON in standby) so i suppose mj802 are inactive

Also this machine have not an heater element but a vibrating element called sonotrode
from wikipedia :
A sonotrode usually consists of a stack of piezoelectric transducers attached to a tapering metal rod. The end of the rod is applied to the working material. An alternating current oscillating at ultrasonic frequency is applied by a separate power supply unit to the piezoelectric transducers. The current causes them to expand and contract. The frequency of the current is chosen to be the resonant frequency of the tool, so the entire sonotrode acts as a half-wavelength resonator, vibrating lengthwise with standing waves at its resonant frequency. The standard frequencies used with ultrasonic sonotrodes range from 20 kHz to 70 kHz. The amplitude of the vibration is small, about 13 to 130 micrometres.


btw i don't know if this could be the culprit i didn't checked the current yet

[R_J]

Are those the same number output transistors that were in it before? It could be they need to be matched or the drive to them has an issue. It cold also be the piezo has a problem (cracked?)

[stj]

or they could be fakes,
lesser parts that have been cleaned and re-printed.

[conan981]

marking code is the same. original ones are from ON semiconductors. I found it to buy too. i wish to understand first if sonotrode is absorming too much current (more than 30A (from specs of transistor)

[stj]

you need to fix a thermocouple to the transistors and see how hot they get,
those heatsinks are not really enough to let you drive the transistors to the maximum.