Cebora Pocket 130 Mig no power

[GSE]

I have an old Cebora 130 Pocket Mig, which I got new over 30 years ago of Snap On. I've decided to get it up and running again after about 10 years of no use, feeds wire ok, gas valve and torch Ok, cleaned the earth clamp etc. It does weld but has no power what so ever the weld just sit on top of 1 mm steel even on full power. If I measure the Open Circuit voltage between the torch and earth clamp and I get 19,22,24 and 28 DC volts on the 4 setting. But when I weld it drops to around 11 volts when on full power across the torch and earth and 30 amps measured using a current clamp on the earth cable. Checked the AC side of the rectifier and can see the AC voltage change with the 4 position of the 2 rocker switches on both inputs to the rectifier, Checked each of the diodes individually, and they all work correctly with the multi meter. Measured voltage drop across the contactor, torch main lead and earth lead and no significant drop. I have bypassed the main contactor too but no change. Any thoughts what else to check and or I am missing something else.
Graham

 

[a111r]

Welcome Graham,
You've done a good lot of testing along on the right lines.
I think I'd now be looking for some bad bolted joints or corroded high resistance things in the welding voltage path.

 

[GSE]

Yes there must be a "dry joint" so to speak somewhere, nothing obvious although the connections look a bit corroded externally they are clean when I've take them apart, well the ones I have taken apart. Pretty confident the coils are intact and working, although I'm no transformer expert and just wondering maybe if the rectifier is not putting out the full voltage under load

 

[a111r]

Possibly it is that. I've had one or two rectifiers whose diodes passed a DVM test at 9 volts but then fail to work properly under the higher welding voltage, after a few minutes.
Did you clean up and check the rocker switches? They can do weird things to the voltage to the primary winding.

 

[GSE]

I've not actually stripped the switches to clean the contacts internally, not sure if you can without damaging them? the external tabs are nice and clean

 

[a111r]

Neither have I, just good cleaned up tags.
Those used on Clarke and similar machines are lightweight.

 

[Hutcho]

If it has big contactor relay in it make sure it’s in good order - they tend to rust

 

[GSE]

I have bypassed the contactor by connecting the earth cables together giving a permanent live torch for diagnostic purposes, but this did not change anything

 

[a111r]

Post up some photos?

 

[eddie49]

You may be able to track down the bad connection/high resistance which is dropping voltage by using a load resistor.

It will only be a 5 Amp load, but a couple of 12v 60W car headlamp bulbs in series across the torch tip and the work clamp may draw enough current to show you where the problem lies. With the load in place, measure the secondary AC, and the DC across the rectifier output. Compare that with the DC directly across the two bulbs. If there is a large difference, then with the meter -ve lead at the rectifier, probe all the connection points along the positive route. If no joy, put the +ve on the rectifier and check along the negative path ( which will include the drop across the inductor ).

If this procedure does not produce any results, then as a111r suggested in post #4, the poor welding performance may be due to a bad connection on the primary side. You need to be able to draw 1kW to 2kW on that side to get 50 or 100 Amps out of the torch. If necessary, a hard-wired 13A fused cable directly across the mid-range transformer primary tapping will bypass any doubtful switches.

 

[GSE]

I will do as you suggest Eddie, I have pretty much have done a voltage drop test along the DC path after the rectifier by getting my lad to work the welder while I checked various points for a voltage drop, but your method with the load resistor is a better way of doing it so I'll do it again. I actually haven't done much testing on the AC side other than just checking for voltages un loaded. It's going to be a few days before I'll get chance to have another look at the welder. Thanks to everyone for their input and hopefully I'll be able to get some photo's of some decent welds to post

 

[eddie49]

Car bulbs may put enough of a load on the welder output to show something. For a serious test you need a Load Bank, such as this one made by Tec-Arc:
https://www.wellyweld.com/products/calibrator-1000i-ac/dc-73466.aspx
For a slightly cheaper version, Google "hillbilly load bank":
https://www.facebook.com/5mweldingc...put-297-amps-from-a-stocks-m/563264813823854/
Two electrodes in a large bucket of very salty water! Watch out for the chlorine...!!

 

[a111r]

Sounds like you'll get there, from your work done already and the implied inherent ability.
I'm a mechanical engineer and I often revive them.