View Full Version : Thunderbolt 225 ac/dc

03-14-2011, 09:14 PM
I have an old Thunderbolt 225 ac/dc. It does good on ac welding, when I switch to dc it has a hum when i try to strike an arc and will not weld??? What can I do? I have taken the cover off and find nothing visually wrong. How do I check the coils in this welder. I have had this cracker box for a long time and burned many rods with it. I really would like to fix it. PLEASE give me some good advise. Thanks anybody in advance for any info.

03-15-2011, 03:32 AM
Not much to fail in your welder. Manual shows a rectifier that changes AC to DC.

You could have loose, disconnected or broken wires between transformer and rectifier or between rectifier and rotary switch.

Rectifier could have failed and needs replaced. Rectifier might have individual diodes and capacitors that can be replaced.

Rotary switch could have dirty or burned contacts from lots of use or operating switch while welding.

Volt/ohm meter often called a multi meter can help find or confirm problem. area. Google testing rectifier.

03-15-2011, 05:36 AM
Find the serial number and the exact model number of it. It will save time looking for the illustrated manual if you don't have one. go over to the Miller forum site and look for resources in the title header you can down load a PDF manual for it and some other info.

03-15-2011, 10:41 AM
I tried to work on it again last night. As I stated it works very well in the ac mode. It will arc in dc mode but it will not burn into the metal. I have looked for a serial number but there is none to be found. Could be a rectifier problem but I do not know how to check all this. I will post a picture of the welder and maybe someone can identify which one it is.

03-15-2011, 10:47 AM
I do have a copy of the manual and I have tried what very little it says to do in troubleshooting. None of that has helped.

06-02-2011, 08:22 PM
Hi, if'n you are getting low amps on DC, it sounds like the rectifier diodes are at fault.
Firstly, if'n you don't understand electrics...DON'T MESS WITH THE INSIDES, OR YOU'LL GET COOKED.:eek::eek:
That is, there are four diodes in a bridge formation, they take the AC and direct it down one leg of the output wire to get DC to the electrode.
If'n one of the diodes or two are dead you'll only get half wave rectification and so half the amps, or at worst no amps at all in DC mode.
If'n the rectifier is a sealed block with just two wire connectors for the input and two wire connectors for the output..... you're stuffed for testing with a multimeter. :mad:
But if'n you have seperate diodes on a heat sink, then disconnect the in and out wires to the diodes (mark them with masking tape for in and out) and then apply a multimeter to each diode to check if'n you get continuity one way and blocking the opposite on each diode.
With seperate diodes you get one end of the diode with a thread and nut to attach to the heatsink, and the other end with a wire lead out, maybe a tag for wire attachment, doesn't matter which.
The important thing to know is that two diodes will have their heatsink ends positively orientated, and the other two diodes will have their heat sink ends negatively orientated.
You have to know how a bridge rectifier looks when seperate diodes are on a heatsink, because there are connections to each of the top ends that are connected together and to input and output.
If'n you get no continuity on both sides of any diode that one is dead, needs replacing etc.
You should get continuity one way and no continuity the other on each diode, but only for each pair of diodes, two go one way, two go the other electrically speaking.
If'n you have problems understanding the set-up I'll make a sketch and post it.
Be warned, diodes in welders pass heavy amperage, and are in the secondary part of the welder transformer.:eek:
You may be lucky and find that the diodes are not making clean contact with the heatsink (which is connected to the earth output lead connection), so clean and check connectivity.
After the diodes and before the electrode output connection there is a heavy copper coil (about 10 turns of thick copper rod) with ferrite in the core, this is an inductor to smooth the lumpy DC after rectification and it's pretty bullet proof, so no problems there.
AC welding is the cheapest way to make a welder, and DC is a pure luxury.
With DC you get very little spatter and a smooth arc characteristic, AC crackles and sparks and throws out spatter.
If'n it welds OK on AC but not on DC, it's the diodes, especially if'n you only get some current coming through, no current indicates the switch is dead.
Hope this helps.