Mod-Your-151 How-To Guide

[BruceTS]

Never had issues with heat, other than working past the duty cycle. Don't think it'll be any more of a problem. if so I'll simply add another fan.

No need to cover the positive terminal since it's the same voltage that runs through the spool wire, unless I decide to add positive and negative blocks to have the ability to reverse polarity for flux wire. If I do I'll make a cover to go over that section.

The other lead coming off the cap will go to the block on the euro connector once that is installed. What I did was use the old wiring from the original torch to connect the cap.

The 2 connections I still need to make would be the 2 leads from the euro block, one will go to the new rectifier and the other to the PCB.

[Bluewelders]

Power to the wire feed should come from the welders choke, does yours not have a choke ?

[BruceTS]

The choke is on the negative side, you can see in the first pic the red wire running to the main relay.

[BruceTS]

original wiring diagram and the modified one

[Bluewelders]

Interesting, I haven't seen a welder with the contactor in the secondary circuit for quite a while.
.

[BruceTS]

Finished the euro conversion today, decided to mount the gas solenoid in the main compartment. Found a plastic bracket at the local ACE hardware store to make a support for the brass adaptor. there is some adjustability in the assembly so making it fit the HF unit should be just as easy.

[weld]

Does the HF MIG 151 have a contactor like the Astro Power Mig 130 does?

Why not power the feed motor off the new DC rectifier?

[Bluewelders]

The HF 151 has a contactor/relay.

[weld]

I just picked up an Astro Power Mig 130 that had a broken wire feed mechanism. I fixed it and it welds ok, but not as good as a Miller 120v machine. I am going to add the output capacitor.
The gun switch feeds power to the PCB which feeds power to the drive motor. I don't see why the extra DC rectifier is necessary. When the gun switch is open then no power is applied to the motor. Why is it posted that the drive motor runs until the new cap. is discharged? It looks like once the switch is open the cap. voltage isn't applied to the drive motor.


EDIT: One other question: Why not put the cap and resistor after the contactor towards the ground clamp? This would not run the drive motor after the switch is released with the original DC rectifier.

[weld]

What about putting a N.O. relay in series with the motor power wire. When the gun switch is closed the relay closes. This could be done instead of adding the extra DC rectifier, correct?

[transit]

EDIT: One other question: Why not put the cap and resistor after the contactor towards the ground clamp? This would not run the drive motor after the switch is released with the original DC rectifier.

Locating the cap on the NO contact would have the cap at 0 volts until the relay is energized. Ideally when the relay connects the cap in the circuit, the inrush current would be infinite. It would not be long before the relay points will burn out.

[BruceTS]

The gun switch feeds power to the PCB which feeds power to the drive motor. I don't see why the extra DC rectifier is necessary. When the gun switch is open then no power is applied to the motor. Why is it posted that the drive motor runs until the new cap. is discharged? It looks like once the switch is open the cap. voltage isn't applied to the drive motor.

The cap affects the feed motor speed, already tried...... had to add the second bridge to isolate the circuit.

[weld]

Locating the cap on the NO contact would have the cap at 0 volts until the relay is energized. Ideally when the relay connects the cap in the circuit, the inrush current would be infinite. It would not be long before the relay points will burn out.

Good point. It would arc the contacts.

[weld]

The cap affects the feed motor speed, already tried...... had to add the second bridge to isolate the circuit.

I found the MIG 151 cap mod. thread. Blue posted that the motor drive is SCR controlled. I don't know how the MIG 151 gun trigger controls the power to the motor. I was guessing that the trigger applied power to the PCB like the schematic shows a few posts earlier in this thread. It appears that the schematic in this thread doesn't apply to the MIG 151.

In this thread the schematic has the trigger feeding power to the PCB to drive the motor. Once the trigger is released then there would be no power to the PCB or motor. I don't see why they added the extra DC rectifier.

In the other MIG 151 cap mod. thread Blue said the MIG 151 uses an SCR controlled so I am guessing that the trigger doesn't cut power and that is why the motor runs afterwards.

Bruce,
The schematic you posted is the Astro Power Mig 130? I will have to take the PCB off and see how it controls the motor. I just don't see how the motor runs afterwards with the switch open and no power going to the PCB and motor.

[weld]

The cap affects the feed motor speed, already tried...... had to add the second bridge to isolate the circuit.

Sorry for so many posts, but something just dawned on me. In your original schematic it shows the motor getting power right from the original DC rectifier and motor ground controlled by the PCB.

My machine has PCB and motor power coming from the gun switch and the PCB and motor ground connected directly to the original DC rectifier ground.

EDIT: for some reason it tells me the upload failed when I try to add the schematic showing the motor tied to ground.

[Bluewelders]

On the 151
The gun trigger is powered from the PCB transformer to just run the contactor / relay.

The PCB takes the rectified 120hz power from a bridge diode and feeds it to an SCR to drive the motor.
The motor has a feedback line that is fed to a transistor on the PCB, the transistor holds the gate on the SCR from firing if the motor voltage is high enough, based on the pot setting and the back EMF of the motor.
Because it is an SCR circuit, the SCR will not switch off if there is enough charge carriers to maintain a holding current in the SCR.
So the voltage in an SCR circuit must go to zero for the SCR to switch off, a capacitor of any reasonable size will keep it from switching off.

[BruceTS]

I

In this thread the schematic has the trigger feeding power to the PCB to drive the motor. Once the trigger is released then there would be no power to the PCB or motor. I don't see why they added the extra DC rectifier.

In the other MIG 151 cap mod. thread Blue said the MIG 151 uses an SCR controlled so I am guessing that the trigger doesn't cut power and that is why the motor runs afterwards.

Bruce,
The schematic you posted is the Astro Power Mig 130? I will have to take the PCB off and see how it controls the motor. I just don't see how the motor runs afterwards with the switch open and no power going to the PCB and motor.

OK let me explain this a bit more in detail. I thought the same thing after seeing how the circuit functions, the motor does stop once the switch is released, but after doing the cap mod without the second rectifier with the trigger pulled, the feed motor ran very fast and the speed control did nothing to slow it down. The cap affects the feed motor controller, so the bridge was added and problem solved. The HF151 needs it to prevent over run after the switch is released, but the MIG130 needs it for a different reason, to Isolate the controller board.

[weld]

OK let me explain this a bit more in detail. I thought the same thing after seeing how the circuit functions, the motor does stop once the switch is released, but after doing the cap mod without the second rectifier with the trigger pulled, the feed motor ran very fast and the speed control did nothing to slow it down. The cap affects the feed motor controller, so the bridge was added and problem solved. The HF151 needs it to prevent over run after the switch is released, but the MIG130 needs it for a different reason, to Isolate the controller board.

Good, we are on the same page now for the Astro Power Mig 130. The schematic you posted has an error in both pages. It has the motor negative side going to DC rectifier +. It should be going to DC rectifier -. You will see that you have the motor + coming correctly from the PCB which gets + from the gun trigger. But the motor neg. is also going to +. It should be -.

I agree that the motor will run too fast. Once welding is under way there is voltage drop feedback causing the motor to slow down. With the cap. on there the voltage will be much higher causing the motor to be too fast. I was thinking of adding a LM7809 or LM7812 between the switch and the PCB. Maybe doing the isolated DC rectifier is just easier. Might need diodes still to drop the voltage to the PCB and motor a little.

[weld]

On the 151
The gun trigger is powered from the PCB transformer to just run the contactor / relay.

The PCB takes the rectified 120hz power from a bridge diode and feeds it to an SCR to drive the motor.
The motor has a feedback line that is fed to a transistor on the PCB, the transistor holds the gate on the SCR from firing if the motor voltage is high enough, based on the pot setting and the back EMF of the motor.
Because it is an SCR circuit, the SCR will not switch off if there is enough charge carriers to maintain a holding current in the SCR.
So the voltage in an SCR circuit must go to zero for the SCR to switch off, a capacitor of any reasonable size will keep it from switching off.

After reading your MIG 151 cap. mod. thread where you said the motor is SCR controlled I could see that the non-return-to-zero problem that the cap. introduced to the motor drive ckt. You just re-assured my thoughts with this post. Thank you.

EDIT: On a side note, I just joined this forum. It is great to see that there is other people out there carefully modifying these simple machines to make them work better.

[BruceTS]

Good, we are on the same page now for the Astro Power Mig 130. The schematic you posted has an error in both pages. It has the motor negative side going to DC rectifier +. It should be going to DC rectifier -. You will see that you have the motor + coming correctly from the PCB which gets + from the gun trigger. But the motor neg. is also going to +. It should be -.

actually the motor negative is going to the negative on the bridge, but the way the original diagram is drawn, appears to connect to the positive, maybe I should have redrawn it to show it passing past the positive connection. I didn't draw the original diagram, it was a download from manufacturers website. It threw me at first too, but I quickly figured it out.

[weld]

The original diagram I have from Astro is drawn different and shows the negative connection on the upper side of the motor more clearly. In the drawing in this post the neg connection would have to run under the transient surpressor to get to the negative bridge connection. It just seems odd were the trigger + and motor negative looks to tie into the bridge +.

Astro Power Mig 130 info: The reason you can use the second bridge rectifier is because you added a new gun with a switch having two wires. With the original gun the only way to add the cap. and have motor control is to add a voltage regulator to feed the PCB that is feed from the new higher weld voltage. The only other way is to make the trigger activate a relay that turns on the second bridge voltage to the PCB. That seems too complicated. I think the LM7812 to feed the PCB is better. The only possible problem is over-voltage to the LM7812. It is only rated at a max. input voltage of 35 volts which I think might be too low.
If the OC voltage is 20 Vac (RMS) then with the capacitor it will be 20*1.41 = 28 volts. The highest the LM7812 can handle without any margin is 24 Vac OC.

I also took a look at my Campbell Hausfeld 80 amp fluxcore machine. It is basically the same as the Harbor Freight MIG 100 box. It has a reactor, but no contactor. I have a 12v contactor that I am going to add to the input side of the transformer. I have a 40VA 120vac->24vac transformer, a reclaimed bridge from a PC power supply, and a LM7812 regulator. Together the parts can do the contactor function and feed the motor drive PCB. I figure I might as well order a capacitor & resistor for it and add that too. Probably will throw in an old PC power supply 12v fan as well.



Where did you download the schematic from? I looked at Astro's website, but couldn't find it. They emailed me a PDF of a scan of the original manual. The schematic I have is different in the layout, but the wiring is the same except for the thing I mentioned. I did verify the schematic they sent with the 130 machine I have.

[BruceTS]

the diagram came from another vendor, since the MIG130 was made under many labels, mostly in Europe.

Here is a sample bead I ran after completing the mods..... setting 3 .035 wire Argon/co2

[weld]

The weld looks good. All of the parts in my machine are labelled with Italy. It looks like the manufacture date is 1993. You also bring up another point about the power mig 130. When I was fixing the wire feed I noticed only 1 groove in the drive roll just like the picture you posted. At one point I tried using the drive mechanism from a campbell hausfeld box I had. It has two grooves for different wire. It worked great with the CH wire feeder, but I couldn't just throw out the Astro wire feeder. I fixed it and re-installed.

The only bad part is the Astro feeder drive roll has a slot for .023 - .030" wire (as per the manual). You mentioned using .035" wire, but won't this wear a bigger groove in the roll? Also, the drive roll with the groove is like no other that I have seen. I am guessing once my Astro drive roll is worn that I will have to replace the entire assembly with the CH one or one from an HF unit. They are really the same.

I also thought about making a jumper block for the Astro to do fluxcore (DCEN), but again I thought about the drive roll not handling fluxcore wire.

I am going to order a 100mf cap, 50 ohm-50watt resistor, 24vdc relay, LM7812/LM7809, 10amp bridge for the Astro. I figure at worse that I can use the bridge and relay if the LM78xx regulator doesn't work out using the original gun. I can run the relay coil using the gun trigger and the relay would feed power to the PCB from a new DC bridge if the LM78xx is a no-go.

EDIT: I emailed Astro for the manual and they sent it to me via PDF within 24 hours in case you are looking for it.
The Clarke 130EN I also picked up non-working is a lot like this Astro Power Mig 130. It has a contactor and reactor, but doesn't have the output capacitor.

[weld]

In terms of HF MIG 151 mods: I just noticed that the gas valve in it is a completely mechanical unit in the gun itself. How reliable are they in terms of closing and sealing tight? The Astro Power Mig has a valve in the gun like that but the electric switch is also integrated into it. I am converting over to a 120v gas valve (ebay special) to try and enhance reliability because the mechanical valve doesn't always close completely.

Anyone do the electric gas valve mod yet?

[weld]

HF technical support for model 93793 (MIG 171) quoted:
P/N 15 - wire feeder assembly $11.75
P/N 51 - torch assembly with cable $22.75