Electronics > Repair

Commercial scale help repairing it

(1/5) > >>

BobLab:
 Hello there boys and girls! Hope you all are fine.

 So I have this old commercial scale that stopped working a few years ago, Then I decided try to repair it myself without success so far..I have some basic knowledge on electronics but zero experience repairing this things and with just a DMM(a Fluke 114) in hands :P.
 Here we go into details.. When I try to power the scale on from AC nothing happens, dead scale, measuring the secondary of the transformer I have around 7 Vac but on the filter cap its measuring just 3Vdc, this board has two voltage regulators, the first one(LM317T) outputs around 1.3Volts, this voltage I believe goes to the battery terminals, measuring them sometimes the 1.3Volts is there and sometimes not(very weird), The same voltage also goes to the second voltage regulator(LM2940) and same happens(same as the battery terminals), sometimes the 1.3volts its there on the input pins sometimes not, and output nothing even when the 1.3Volt is present on the input pin.
 Without success I tried to power the board direct from the battery terminals with an old 5Vdc 1Amp DLink router power supply, (the scale battery is rated to 6Volts 4 Amps), Now It showed some sign of life, the seven segments display light some random segments but the buzzer is beeping loud and continuously(not behaving as normal did with a single beep when turned on), So I suspected the MCU was bad and removed it from the socket, but the buzzer still beeping loud and continuously and the seven segment display lights even without the MCU on the circuit |O, Looks like this buzzer is wired directly with the voltage regulator output pin.
 I gonna try to draw part of the schematics and post here(hard task cuz the board is dual layer) since I didn't found nothing related about over the internet just some tutorials about how to calibrate them.
 My only hope is you guys  :scared:, maybe anyone have any clue about whats happening? or fixed a scale before...
 I'm Thankful since now, and thanks for your time reading this post.
 I'll post some Pictures of the board below, maybe its gonna be helpful!
 Also sorry about any English mistakes I'm not native  :-+

fmashockie:
Drawing some parts of the schematic would definitely be helpful if you can.  The LM317T is likely measuring 1.25V on output with the microcontroller off.  It's a common circuit with these (an example is shown in the datasheet for it.  5V electronic shut-down regulator).  So once the micro turns on, it will directly/indirectly power on a transistor tied to the adjust pin of the LM317T allowing it to regulate 5V.  That could be whats going on here.  But the LM317T can also be used in battery charging circuits as well so it could be involved with that if you're saying this scale can be powered with batteries.

The seven segment LEDs could still light up with or without the micro.  Its not indication that the micro is bad.

Considering the battery is 6V and you're measuring 7VAC on secondary side of the transformer, it would not surprise me that is the correct voltage for the secondary side of the transformer.  But you should definitely not be seeing only 3VDC on that large electrolytic cap by the full-wave bridge rectifier (the 4 diodes by that connector).  You should be seeing ~10VDC.  There's a good chance that cap has gone bad so it's no longer filtering properly.  I would remove that from the board and test.  Also check those 4 diodes that make up the bridge rectifier.  Use the diode test function on your multimeter.  If you don't have a diode function (and it looks like you don't since you said you have a Fluke 114) then just check for shorts on the diodes.  there shouldn't be any.
 

I would try to trace out the schematic best you can.  It will definitely help you better understand what's going on.

BobLab:
Thanks for your answer @fmashockie, and sorry about the delay, I'm a little short in time lately. As you said I tested the diodes for short, turns out they are not shorted, also there's no short on the filter cap, but I'm limited with this DMM, I can't test diodes neither capacitors with it, I gotta buy one to deal with eletronics but I'm short in money right now for a good meter, unfortunately they are too expensive here in my country.
As promised I tried to draw part of the schematics but many of the traces are going under the IC's, and its very hard to follow, I'll need to Unsolder them in the future..This is what i got so far!

fzabkar:
If you disconnect the battery, what voltage do you measure at the output of the LM317?

fmashockie:
Based on your schematic, it looks like the LM317T is being used as a current limited 6V charger for the batteries (also an example circuit in the datasheet for this component).  Vin for this circuit must be between 9-30V.  So as I said, you likely should be seeing around 10VDC across that filter cap by the bridge rectifier diodes (I don't see that cap in the schematic you drew).  You should be seeing at least 6VDC on the output of that LM317T.

If the rectifier diodes you measured are not shorted, then it is likely that filter cap that is causing the issue.  But without the ability to test the capacitor, it will be hard to verify that.  If it is cheaper for you just to replace the cap (instead of buying a meter with capacitance/diode test functions), then give that a try. 

Navigation

[0] Message Index

[#] Next page