Hi everyone, I am trying to figure out what is causing my 855 bender to stop working. I have located a bad 7805 VR and replaced it only to have the replacement smoke within seconds of the power being reapplied. The I/O board has been discontinued and I can’t locate even a used one so I am trying to figure out what is going on. I have a schematic and have basic knowledge but not sure what to check next. The 7805 which is affected is labeled as VR2. Any help is super appreciated!

 

You need to trace everywhere that 5V goes. It appears you have short somewhere, or at least an excessive load that’s torching the regulator.

I thought the 7805 tends to shut down to protect itself from a short and overheating, so I’m a little surprised.

 

Do you have a current limited power supply? You can set the supply to current limit at about 1A or 1.5A and see what happens when you try to power the +5V. You might start out a 0V and work up slow. There might be something really dead on the +5V.

Good Luck.

 

Check D3 and D4, too. It looks like D3 is a half wave rectifier or protection from a backwards connector J2. One good way to smoke a 7805 that is wired correctly is to have a negative voltage into it or across it. D4 takes care of the case where the output voltage is somehow higher than the input - usually from a bit capacitor on the output that keeps voltage up when the input drops during power down.

Check the input voltage too. 35V is the max for most but some are lower.

Check the diodes out of circuit. If indeed the problem is negative input volts on the 7805 and you power it up without that shorted regulator to clamp the voltage, the resulting negative volts will take out the polarized filter capacitor C7.

Good luck!

 

Do all 7805's have the same pinout?

 

I also suspect that the input voltage to the 7805 is out of its limits (perhaps going negative).
They are pretty well resistant to output shorts.

 

Is the old and new 7805 in a TO220 package as shown below?
Is it on a heatsink?
Is it fitted the right way round?

 

Check all the capacitors around the regulator- C1, C12, etc. An LM7805 does have thermal shutdown, but even that can only do so much in an overcurrent condition. It's basically disintegrating.

 

A real 7805 has protection from over-temperature and over-current. Maybe you got a fake one from ebay.
Its pin numbers and capacitor values are very important:

 

If the rectifier is bad the 7812 would be toast too. I believe you should have about 14VDC supplying it and 20VAC off before the diodes. If those large caps (C7) are going it could pass enough ac to spike it too high for the 7805 while being just low enough for the 7812.

i would also check the transformer output.

 

Remove D5 and try a new regulator to see if it operates normally. That splits supply from a potential load source. From the results, you have an idea where to look next.

 

D3 and D6 are the input rectifiers; prime suspects to me. You could remove the 7805 and scope the voltage across C7 to confirm.

ak