1981 330 A/BP DC issues

[marcbark]

I've just acquired a 330 A/BP and after getting the years of dust, dirt and grime removed, wiring it up and putting in the shop, I've begun playing with it for SMAW with some 6010, 6011, & 7014.

Having played around with electrons once or twice in life, I've found that on AC this is a dream machine. On DC, it's castrated, to say the least. I've studied the schematics and it appears that one of the following is at play:

-The power diodes are going bad
-The Stabilizer "Z" (large inductor) is burned up
-A bad connection somewhere in the DC only connections (switch).

In an attempt to alleviate the problem, I've cleaned the contacts in the switch, tugged on every connection and nothing seems amiss.

When I pulled the diodes and checked them, I get a 0.4 v drop one direction, and an open the other. If they failed this test it would tell me if they were completely bad, but I'm not sure that they're not starting to fail. When running with an AC clamp meter it appears that the current through one set of Diodes is much higher than the other, but still not as high as I would expect it to be. When running in AC mode I measure the current that I'd expect at the welding lead, so I'm quite sure that only the DC portions of the power circuit are affected.

Does anyone have guidance on what to look for in a failing rectifier? I'd like to know that the diodes are bad before I sink any more money into this.

[SquirmyPug]

So it just seems to have low amperage on the DC side? Did you separate all the diodes before testing them? Make sure the input jumpers are correct? Have you checked OCV for the AC and DC side? Which position is the knife switch in? There's not a lot to go bad in these old machines (same as my Airco)

[marcbark]

Thanks for following up.

I have the knife switch in the correct position (SMAW, or METALLIC), and the OCV voltage is correct for both AC and DC, but when I strike an arc with DC, the voltage is low, around 15-25 volts, jumping around a lot, while on AC, the voltage is stable around 31-32 volts.

As for the Diodes, I did pull them so that I was only measuring the individual component. I've also run the AC clamp meter on each diode, and found that they are not carrying the same amount in current, whether in AC or DC. In AC mode, each diode set is run in parallel, but DC puts once from each set in series.

[jst1t]

Probably just diodes or something simple. The transformers practically never go bad. On another note,, these machines like low hydrogen rods better than cellulose rods, but that's not your problem with this machines

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[marcbark]

I figured it should be simple, these aren't complex machines, just a lot of copper.

Everything I'm seeing keeps pointing me toward the diodes, but at $200 to replace all four, I want to be sure. I'm not a fan of using the parts cannon.

Thanks for your comment about the low H rods. I've never run any before, and without a DC machine previously, have never had a need to pick some up.

[SquirmyPug]

Thanks for following up.

I have the knife switch in the correct position (SMAW, or METALLIC), and the OCV voltage is correct for both AC and DC, but when I strike an arc with DC, the voltage is low, around 15-25 volts, jumping around a lot, while on AC, the voltage is stable around 31-32 volts.

As for the Diodes, I did pull them so that I was only measuring the individual component. I've also run the AC clamp meter on each diode, and found that they are not carrying the same amount in current, whether in AC or DC. In AC mode, each diode set is run in parallel, but DC puts once from each set in series.

I don't know tons about electronics, can diodes fail slowly? I thought it would be good or bad, nothing in between.

[marcbark]

I don't know tons about electronics, can diodes fail slowly? I thought it would be good or bad, nothing in between.

I thought the same thing, but what I'm seeing is different than a sudden failure. I'm going to try measuring the current through each diode tonight on both AC and DC. I think it should paint a better picture.

I did check the output of the main transformer, only changing from AC to each of the DC settings, and the DC output is around half of the AC (for reference, at the main transformer output, it will always be AC).

[SquirmyPug]

I thought the same thing, but what I'm seeing is different than a sudden failure. I'm going to try measuring the current through each diode tonight on both AC and DC. I think it should paint a better picture.

I did check the output of the main transformer, only changing from AC to each of the DC settings, and the DC output is around half of the AC (for reference, at the main transformer output, it will always be AC).

That's strange, I'm interested in seeing what you find. Have you checked to make sure you have 240v in? While under load? Could one leg of the input power having issues cause that? Just a random thing to check

[marcbark]

I checked the input voltage, and everything is as it should be, even under load. So that bit is fine.

Last night I put an AC clamp on the four diodes as well as the output of the main transformer, and on AC it got 160 amps at the transformer, D1 was 50 amps, D2 was 40 amps, D3 was 62 amps and D4 was 19 amps. This doesn't concern me too much, as I'm getting the OCV and amps the front dial says that I should have at the lead.

When changing to DC, no other changes, the output of the main transformer drops to 80 amps, and all four diodes are measuring 40 amps. I can't figure out why the output is cut is half. I've run the same experiment at higher amps, and it's the same. I don't see anything on the schematic that would cause the output to be cut it half when switching to DC.

At this point, I'm going to run it as-is, understanding that DC amps are half of what the front reports. I'm also going to install a DC ammeter to watch what is going on.

If anyone has any additional ideas on what I could look for, I'm open for suggestions!

[SquirmyPug]

Are you sure it welds like half power on DC? Could the meter be wrong? Only thing I could think of to try is burning a 5/32" 7018 with the coarse setting in the middle and fine adjustment at 100, Reverse polarity (electrode positive). That would give you an idea if it's making the amperage it should.

[jst1t]

You can also call Miller. I talked to a gentleman there that had been there a long while and knew a lot about this machine. He was very helpful to me. I had a broken wire wound resistor. That was back in 2019......that could be your problem..... but if it is working on AC, maybe not..

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[zapster]

I ran one for years but just for TIG.
100 amp breaker.
Never any issues.

...zap!

[zapster]

Member 7A749 knows these machines inside out.
I got the one I ran from him.

...zap!

[marcbark]

Thanks for your posts! I am also running this on a 100 amp breaker, with 150 amp panel service, using #2 AWG supply lines from the breaker to the machine.

I spent some time watching some videos on 6011 rod with DC and AC and it has me wondering if I'm making something from nothing (this would not be the first time). I've never run rods on DC, never been in person when they've been run, and consequently have no idea what it looks, sounds or feels like. I've found that for the same rod, a 1/8" Hobart 6011, it takes a bit higher setting on DCEP for the same penetration as what I get with AC. I've noticed that on AC the arc is a LOT more violent (is that a good way to put it?). Regardless, this thing does circles all day long around my old 225AC, and I'm going to run it as-is until it stops working.

Now if I can find the funds to get a TIG torch and accessories....

[SquirmyPug]

If you look at the coarse setting lever you will see that AC output is a bit higher than DC. Because of that you're going to need a higher fine amperage setting on DC. DC is usually smoother and less violent than AC and also has a bit lower OCV than AC.

[MinnesotaDave]

I would open the knife switch (tig position) and see if that helps.

[MinnesotaDave]

If you look at the coarse setting lever you will see that AC output is a bit higher than DC. Because of that you're going to need a higher fine amperage setting on DC.

This was my 1st thought as well. When I got mine I didn't notice that for a few minutes, caused a little "head scratching."