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Heliwelder IV High current problem and solution
Machine:
Heliwelder IV 1341-0176 code 665
Problem:
Weld current at range maximum regardless of range selection or weld current adjustment
Research:
I obtained a copy of the welder schematic from the forum and found that the weld current control was implemented by altering the excitation of a dc control winding of a saturable reactor. By increasing this pilot current, the ac impedance of the saturable reactor was reduced, allowing a higher weld current to flow. This DC pilot current was controlled by a Printed circuit board by altering the firing angle of a Triac. The problem had to be somewhere in this circuit.
Narrowing the search:
I first removed the connection to the gate lead of the Triac. If the problem remained, it had to be in the Triac. If the problem went away it had to be in the control board. The problem did in fact disappear so the problem had to be in the control printed circuit board.
Diagnosing the board:
It was impossible to find a schematic for the control board, so I did an inventory of the major components. All the board components had manufacturer markings and most are still available. There were no Proprietary components on the circuit board.
The major components were:
MC7812 12 volt regulator 3 legs
LM324 quad operational amplifier 14 legs
LM555 timer 8 legs
4N38 Opto coupler 6 legs
several electrolytic capacitors
Bench check:
I removed the board from the welder and set up a test for the voltage regulator. I applied a variable DC supply voltage to the input leg of the regulator and observed the voltage on the output leg. From the data sheet on the regulator, I knew the output should go no higher that `12.4 volts as the input voltage was increased. In fact the output voltage followed the input voltage past 12 volts all the way to 16 volts. I didn't need to go higher. The regulator was bad and had applied full voltage (~24volts DC) to all the components on the board.
Repair approach:
I knew components had been damaged by the high voltage. I knew all of the integrated circuit components had operating maximum voltages of no more than 16vdc from their data sheets. I ordered the LM324, LM555 and 4N38 IC's and new capacitors (since 35 year old electrolytic capacitors are on their last legs anyway). I replaced them all and installed sockets for the IC's just in case. I also replace the faulty DC regulator with a more reliable voltage regulator board based on a LM2596 IC which has fail safe features.
Results:
The welder now works as described in the operators manual.
I hope someone can use this information,
Joe the tinkerer
Machine:
Heliwelder IV 1341-0176 code 665
Problem:
Weld current at range maximum regardless of range selection or weld current adjustment
Research:
I obtained a copy of the welder schematic from the forum and found that the weld current control was implemented by altering the excitation of a dc control winding of a saturable reactor. By increasing this pilot current, the ac impedance of the saturable reactor was reduced, allowing a higher weld current to flow. This DC pilot current was controlled by a Printed circuit board by altering the firing angle of a Triac. The problem had to be somewhere in this circuit.
Narrowing the search:
I first removed the connection to the gate lead of the Triac. If the problem remained, it had to be in the Triac. If the problem went away it had to be in the control board. The problem did in fact disappear so the problem had to be in the control printed circuit board.
Diagnosing the board:
It was impossible to find a schematic for the control board, so I did an inventory of the major components. All the board components had manufacturer markings and most are still available. There were no Proprietary components on the circuit board.
The major components were:
MC7812 12 volt regulator 3 legs
LM324 quad operational amplifier 14 legs
LM555 timer 8 legs
4N38 Opto coupler 6 legs
several electrolytic capacitors
Bench check:
I removed the board from the welder and set up a test for the voltage regulator. I applied a variable DC supply voltage to the input leg of the regulator and observed the voltage on the output leg. From the data sheet on the regulator, I knew the output should go no higher that `12.4 volts as the input voltage was increased. In fact the output voltage followed the input voltage past 12 volts all the way to 16 volts. I didn't need to go higher. The regulator was bad and had applied full voltage (~24volts DC) to all the components on the board.
Repair approach:
I knew components had been damaged by the high voltage. I knew all of the integrated circuit components had operating maximum voltages of no more than 16vdc from their data sheets. I ordered the LM324, LM555 and 4N38 IC's and new capacitors (since 35 year old electrolytic capacitors are on their last legs anyway). I replaced them all and installed sockets for the IC's just in case. I also replace the faulty DC regulator with a more reliable voltage regulator board based on a LM2596 IC which has fail safe features.
Results:
The welder now works as described in the operators manual.
I hope someone can use this information,
Joe the tinkerer
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