Laminator repair help

[bigone5500]

I have a cheap scotch laminator from target that has stopped working. I am wondering if my rectifier diodes are bad. When I check from N to the + of the half wave I only get 2.1vdc. Maybe it's supposed to be that way but it has me wondering. It has a 78L05 regulator which has me to believe that it will not turn on to give the HT46F47E its power. I only used this thing 2 or 3 times so it may very well be that the MCU is shot.

A1 is BT136 600E
A2 is BT134 600E
C2 and C3 are 470uf and 22uf respectively

I don't know if this is enough info or not but I thought I would try to fix this thing before sinking another $20 into another...LOL!!!

 

[davenn]

OK
where is the AC coming in ?
What is the voltage of the AC coming in
is it from a plugpack ? have you tested it ?
I don't see a bridge rectif ?


Dave

 

[bigone5500]

AC is 120vac. It comes in via a cord connected to a 5 pin plug connector. The rectification is via the two SMD rectifiers on the top left in pic 2. The connector is mounted on the opposite side on pic 2. It is denoted by the white rectangular outline. The mains is connected to L then passes through a 5A fuse then goes to L1 and on to the rectifiers. I checked power from N (neutral) to L1 and have 120vac. I then checked from N to the right side of the top rectifier. I got 2.1vdc. I don't know how I am getting this voltage. I thought it would be higher and be dropped down a bit further to the 5vdc via the 7805.

 

[Harald Kapp]

As D3 is connected between L1 and N, it is probably not part of the rectifier but an overvoltage protection. The rectifier is therefore D4 only.

The black diode labeled ZD... on top (picture 1, next to the red bulky component) is a zener diode which sould limit the voltage at this point. This is a possible point of failure. What happens to the voltage at this point when you de-solder this zener diode?

C2 on top is the main buffer capacitor for the circuit. If this is defect, the Dc voltage will not be buffered correctly. Can you check it with a capacitance meter?

You should also measure the current into the circuit or the voltage drop across R4, respectively. R4 seems to be there to limit the current, here is another possible point of failure.

Note that this circuit is directly connected to mains. Take every precaution that is required to prevent an accident. Note that some components may store charge and display high voltage for some time after the circuit has been disconnected from mains. Wait as long as necessary to discharge every component or force discharge by placing the PCB onto a conductive surface (e.g. black conductive foam as found in IC transport packages).

 

[bigone5500]

As D3 is connected between L1 and N, it is probably not part of the rectifier but an overvoltage protection. The rectifier is therefore D4 only.

The black diode labeled ZD... on top (picture 1, next to the red bulky component) is a zener diode which sould limit the voltage at this point. This is a possible point of failure. What happens to the voltage at this point when you de-solder this zener diode?

C2 on top is the main buffer capacitor for the circuit. If this is defect, the Dc voltage will not be buffered correctly. Can you check it with a capacitance meter?

You should also measure the current into the circuit or the voltage drop across R4, respectively. R4 seems to be there to limit the current, here is another possible point of failure.

Note that this circuit is directly connected to mains. Take every precaution that is required to prevent an accident. Note that some components may store charge and display high voltage for some time after the circuit has been disconnected from mains. Wait as long as necessary to discharge every component or force discharge by placing the PCB onto a conductive surface (e.g. black conductive foam as found in IC transport packages).

Thanks Harald. I will go back into it and see what I can find. It would be nice if I could actually fix it. I don't know what could have happened to make it just not work. The last time I used it, it worked just fine. I laminated a couple full pages and everything went well. I turned it off and placed it back into storage. Oh well...it's only $20.