Commercial scale help repairing it

[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 .
 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 , 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  , 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. 

[BobLab]

1.3Volts, I forgot to mention in the first place that the measurements were made without the battery in the circuit, because the thing is starting to leak.

[BobLab]

The Filter cap on the schematics is the C18, I just forgot to place the value, 2200uF 25v, wich I'm suspecting its  the wrong capacitance because there's signs of this one has been replaced and his size doesn't fit the silkscreen draw.
Also what about the formula on the LM317T datasheet for Vout? (VO = VREF (1 + R2/R1) + IADJ R).
using the reference values and the R7(220Ohms) and R8(2200 Ohms) the output voltage wasn't to be around1,37V?(probably I'm saying nonsense) but just for curiosity.
I gonna replace that filter capacitor, (not sure the value I would use) and the bridge rectifier as well, soon as I can, and I'll let you guys know..
Thanks again for your time!

[fzabkar]

Also what about the formula on the LM317T datasheet for Vout? (VO = VREF (1 + R2/R1) + IADJ R).
using the reference values and the R7(220Ohms) and R8(2200 Ohms) the output voltage wasn't to be around1,37V?

The formula produces a Vout of 7V. Therefore the LM317T is faulty, or its input voltage is low.

[BobLab]

Interesting, May I ask how did you calculated?
I can't find my error, thank you very much!

[fzabkar]

https://www.ti.com/lit/ds/symlink/lm317.pdf

Vref = 1.25V
Iadj = 50uA
R1 = 2.2K
R2 = 10K

Vout = Vref (1 + R2/R1) + Iadj x R1 = 1.25 x (1 + 10/2.2) + (50E-6 x 2200) = 7.04V

https://www.google.com/search?q=1.25+volts+x+%281+%2B+%2810000+ohms%29%2F%282200+ohms%29%29+%2B+50+microamps+x+2200+ohms+in+volts

https://www.google.com/search?q=1.25+volts+x+%281+%2B+%2810+kiloohms%29%2F%282.2+kiloohms%29%29+%2B+50+microamps+x+2.2+kiloohms+in+volts

Sorry, I mixed up the two resistors. Please ignore my rubbish.

[BobLab]

Vref = 1.25V
Iadj = 50uA
R1 = 2.2K
R2 = 10K

Vout = Vref (1 + R2/R1) + Iadj x R1 = 1.25 x (1 + 10/2.2) + (50E-6 x 2200) = 7.04V

I think you switched R2 by R1, Anyway I screwed the schematics, by naming 220R instead of R7 the actual resistance value is 220Ohmsnot 10K, sorry my bad
If my cellphone calculator is right, Vout will be:
VO = VREF (1 + R2/R1) + IADJ R2 -> V0 = 1.25V((1+2200/220)+50*10^(-6)*2200) = 13,89V.

[fzabkar]

Vref = 1.25V
Iadj = 50uA
R1 = 2.2K
R2 = 10K

Vout = Vref (1 + R2/R1) + Iadj x R1 = 1.25 x (1 + 10/2.2) + (50E-6 x 2200) = 7.04V

I think you switched R2 by R1, Anyway I screwed the schematics, by naming 220R instead of R7 the actual resistance value is 220Ohmsnot 10K, sorry my bad
If my cellphone calculator is right, Vout will be:
VO = VREF (1 + R2/R1) + IADJ R2 -> V0 = 1.25V((1+2200/220)+50*10^(-6)*2200) = 13,89V.

Yeah, I made a stupid mistake. That said, it doesn't make sense to charge a 6V battery from a 13V source via a 1 ohm resistor. :-?

[fmashockie]

Vref = 1.25V
Iadj = 50uA
R1 = 2.2K
R2 = 10K

Vout = Vref (1 + R2/R1) + Iadj x R1 = 1.25 x (1 + 10/2.2) + (50E-6 x 2200) = 7.04V

I think you switched R2 by R1, Anyway I screwed the schematics, by naming 220R instead of R7 the actual resistance value is 220Ohmsnot 10K, sorry my bad
If my cellphone calculator is right, Vout will be:
VO = VREF (1 + R2/R1) + IADJ R2 -> V0 = 1.25V((1+2200/220)+50*10^(-6)*2200) = 13,89V.

Are you sure you have the correct value for R8?  Your circuit looks very similar to the application circuit for the LM317T when it is used as 6V current limited battery charge.  Which would definitely make sense based on what you've told us so far (see photo attached).  And didn't you say that the secondary side of your transformer was 7AC?  So how could the LM317 output 13V?  I don't think the problem is with the LM317T.  Again, you should not be measuring 3V across that filter cap C18.

[BobLab]

Yeah, I made a stupid mistake. That said, it doesn't make sense to charge a 6V battery from a 13V source via a 1 ohm resistor. :-?

Yeah it doesn't, but anyway I'm still a newbie  , Looks like this 1Ohm resistor wasn't planned..they just placed there vertically in series with the diode maybe to dissipate some heat, judging his size(fatty)?
I ''circulated'' the resistor area on the picture.

[BobLab]

Are you sure you have the correct value for R8?  Your circuit looks very similar to the application circuit for the LM317T when it is used as 6V current limited battery charge.  Which would definitely make sense based on what you've told us so far (see photo attached).  And didn't you say that the secondary side of your transformer was 7AC?  So how could the LM317 output 13V?  I don't think the problem is with the LM317T.  Again, you should not be measuring 3V across that filter cap C18.

Yes the R8 value is correct, I removed from the board and measured it to make sure, about the 13V output  on the LM317T, I was just messing with the Vout formula on the datasheet, not sure if it's the real output value..
I still need to replace that filter cap, once I do, I gonna post the new measurements here, But I'm still sus about the capacitance value, as I said Looks like it was replaced , it's not the original one it's bigger than the silkscreen draw and there's signs of heat under the trails.
I think the guy that tried to repair once probably changed the value.

[fmashockie]

Hmm what value is that cap you think was replaced?

[BobLab]

Hmm what value is that cap you think was replaced?

It's 2200uf 25v.

[fmashockie]

That value seems right based on the 6V current limited battery charger circuit for the LM317T I showed you (they suggest a 1000uF cap).  And that large 1ohm resistor is a current sense resistor. 

[BobLab]

That value seems right based on the 6V current limited battery charger circuit for the LM317T I showed you (they suggest a 1000uF cap).  And that large 1ohm resistor is a current sense resistor.

I ordered that cap online, I think it will arrive tomorrow, but I couldn't find the bridge rectifier diodes 1N4001,  can I use the 1N4007 ones ?

[fmashockie]

1N4007 should be fine.  Only difference is the 1N4007 can withstand a greater reverse voltage (1000V vs 50V). 

[BobLab]

Well, I replaced the bridge rectifier and the filter cap, now we have around 7,40V across that cap, and the LM317T output is measuring 5,84 ~ 5,90V, the adjust pin is measuring 5,35V. I guess the filtering stage is fixed, but still lighting random segments of the seven segments display every time I turn on and off the board, nothing (smart) going on   

[fmashockie]

Thats great! So it appears you've fixed the power supply and battery charger for the board.  Now you need to troubleshoot the digital side.  I would see if you can locate a datasheet for that large AMTEL micro/pid.  Do you at least have a user manual for the scale?  Maybe it can tell you what that sequence of LED lighting means (if it means anything).  But without a schematic for this, you'll have to rely on things like datasheets for each of the digital chips and make measurements with your DMM to see if everything is making sense.  Maybe the micro has a reset pin that is pulled high or low causing it to be stuck in reset mode.  You'll have to check for things like that.  You are also very limited with the tools you have.  Do you have an oscilloscope?  If so, you can check to make sure the micro's clock generator or oscillator is working (and you check for a number of other things).  But at this point this is where you have to ask yourself if it is worth the time to keep working on it. 

[BobLab]

Thats great! So it appears you've fixed the power supply and battery charger for the board.  Now you need to troubleshoot the digital side.  I would see if you can locate a datasheet for that large AMTEL micro/pid.  Do you at least have a user manual for the scale?  Maybe it can tell you what that sequence of LED lighting means (if it means anything).  But without a schematic for this, you'll have to rely on things like datasheets for each of the digital chips and make measurements with your DMM to see if everything is making sense.  Maybe the micro has a reset pin that is pulled high or low causing it to be stuck in reset mode.  You'll have to check for things like that.  You are also very limited with the tools you have.  Do you have an oscilloscope?  If so, you can check to make sure the micro's clock generator or oscillator is working (and you check for a number of other things).  But at this point this is where you have to ask yourself if it is worth the time to keep working on it.

I will post the datasheet below, I did a quick read at the pins description, looks like it resets with a high logic level on the pin 9 of the microcontroller, also the signal comes from the collector pin of the PNP transistor(BC328), measuring the pin 9 with the DMM it shows 90 ~ 100mV.
About the user manual I don't have it anymore, I just remember when the scale was turned on(when functional), the brand logo c&f appears on the display, unfortunately like you said, I'm limited with just the DMM no oscilloscope.
The other digital IC's are all receiving the supply voltages, there's another atmel smaller chip no idea what's that for.
I'm working on it for fun when I have some time left, so it's ok, thanks again for helping dude, much appreciated!

[fzabkar]

I found the old web site:

https://web.archive.org/web/20100218133427/http://www.cfbal.com.br/paginas_dinamicas/detalhes.asp?iType=82&iPic=641

It seems to have only 1 manual, though:

https://web.archive.org/web/20141222102437/http://www.cfbal.com.br:80/CF_Computadora_2013.pdf

[BobLab]

I found the old web site:

https://web.archive.org/web/20100218133427/http://www.cfbal.com.br/paginas_dinamicas/detalhes.asp?iType=82&iPic=641

It seems to have only 1 manual, though:

https://web.archive.org/web/20141222102437/http://www.cfbal.com.br:80/CF_Computadora_2013.pdf

Thanks, that will help!