Poorly Snap On 160 Pro / Cerbora 838 - lack of penetration.

[Nomad]

I recently collected a tidy Snap On 160 (rebadged Cerbora 838) with a known "low power" fault courtesy of a very nice gent @plewlandsbob

It works but the weld is very weak with a few very nice beads when it decides to work properly.
My next task is to find out why it keeps going into "very poorly" mode


Manuals and circuit diagrams are available which is handy https://welding.cebora.it/en/assistance/items-out-of-production?b_start:int=240

When I checked the voltages they are stable but a lot lower than expected.
Actual readings are 18v-29v but spec says they should be 29v-44v.
Tests were done with no load, pressure off the feed roller, I measured at the bridge rectifier output just to remove any problems due to bad joints on the earth cable etc.


I tried measuring the AC into the rectifier but I need to recheck.
Should voltages be about the same when I compare AC-in and DC-out ? (so if I have 35V AC it should show about 35V DC out) ?

The bridge rectifier looks to have 16 diodes (8 on the top plate, same on the lower plate).
They are solderer in sets of 4. Without cutting them out of the circuit, is there a way I can measure them individually to see if any have failed ?

If the rectifier turns out to be faulty, is it difficult to make a new rectifier using standard power diodes? 70HFR120 / 70HF120 seem very common but are they suitable?
A pack of 20 diodes ( 10 forward + 10 reverse) are a fraction of the cost of a new rectifier
Any suggestions for other - cheap & common - diodes if these are not suitable ?

Earlier threads with tests that I'll look at next time :
Faulty power selector switch https://www.mig-welding.co.uk/forum...ig-187-low-on-power.95599/page-2#post-1435395
Check and remake high voltage connections https://www.mig-welding.co.uk/forum/threads/snap-on-pro-mig-187-low-on-power.95599/#post-1429084
Tests for power loss https://www.mig-welding.co.uk/forum/threads/snap-on-pro-mig-160-power-fading.116789/

All suggestions will be welcome so fire away !

Many thanks.

 

[Nomad]

It's not sorted yet but progress is good.

I ran a flylead from direct from a mains socket to the transformer input to bypass the contactor and switch - no difference so the problem is upstream of the rectifier.

The switch checked ok with a meter but bypassing the switch helped.

The circuit diagram shows 6 power settings best described as Low 1-3 and High 1-3.
Low range is always poor, the highest setting (not needed any time soon) trips the workshop fuse/mcb,

The good news is I can now get a stable weld on settings High-1 and High-2
The nearer welds are todays test runs, the ones at the far end were yesterday - like night and day


Same on the back - today has good penetration, yesterday had traces.


I need to spend more time but for now I have a working welder and I think the problem is the switch or the transformer "Low" winding (or both)

 

[Monkeypeas]

Did you check the rectifier at all?, all of those welds look abnormal, or not ideal at least, these machines should be pretty smooth

I had one of them for a while and semi restored it after it was in a fire at one point and it was a pretty nice machine to use
Think i replaced the wire feed control pot and the contactor and generally tinkered with it.

As those threads you posted above says though though check any and all bolted connections, connections at the contactor etc, remove the rectifier and test it however you can, maybe heat it gently with a heatgun and see if anything changes, if any of it does what it shouldnt you dont really need to know which exact diode is bad at this point you just wanna know if any of them are