what're these four tiny capacitors DO for me inside my ol' Dialarc HF?

I just bought an old miller dialarc HF "whiteface" vintage. while getting ready to test it, and cleaning it out, I noticed some of the
leads on the tiny capacitors in the main rectifier bridge are loose, kind of like how quarter-inch round rod would rattle around inside a piece of half-inch water pipe. they're "probably" bad, because of that alone, I'd guess, but, umm, what exactly is the electrical function of those four tiny things supposed to BE? like "what're they doing for me?"
here'a an extreme closeup of one of the 'loose leads' capacitors, next to one of the four "big honkin" diodes in the bridge.
http://www.image-upload.net/files/5385/capacitor_closeup.jpg
here's a page from the owners manual, where they're descibed...
http://www.image-upload.net/files/5385/parts_blowup.jpg
and here's the parts schematic, for the same machine.
http://www.image-upload.net/files/5385/welder%20schematic.jpg
electrically, I'm a "real slow learner", so, thanks for your help, guys :-)
toolie
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I think that their purpose is suppressing high voltage from HF arc starting subsystem, so that it would not damage the rectifiers.
i

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dave wrote:

they're probably to provide a DC path to ground for AC spikes.
Jim
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I was in RF before retiring.
1. Notice the #10 or such wire that the cap is attached.
The large diameter has lower inductance and higher DC and AC current. 2. Since the cap is in series (and yes legs wiggle mean it is dead) Only AC current flows. The value of the cap and the inductance of the total lead set determines what frequency it is shunting. The inductor Z when it turns on and turns off produces spikes. Switching hot does as well. 3. Notice the ones in parallel to the rectifiers (diodes). They keep RF current from inside the diode which might arc on the internal die and kill the unit. Down in the bottom right is a RF high voltage arc with a set of arc points that are used to limit the output RF arc. If these eat back then the voltage increases and can cause more problems. But normally, this is a source of RF noise that can harm transistors and diodes. 4. The module below the bridge is a filter. Likely an RC but with the 'V51' that tends to indicate an AC item. So resistor for better control. 5. The resistor R2 is a minimum load.
Replace the caps - keep their leads short. Higher voltage is just fine. Get the same value as in the spec - that is the 'frequency' it shorts...
Hope that helps.
Martin
Martin H. Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net TSRA, Endowed; NRA LOH & Patron Member, Golden Eagle, Patriot's Medal. NRA Second Amendment Task Force Charter Founder IHMSA and NRA Metallic Silhouette maker & member. http://lufkinced.com /
Jim Chandler wrote:

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    Hmm ... that looks to me (I still have the image saved locally) like about 22 ga wire, with insulating sleeves slipped on over it. Looks like the silicone rubber in fabric type of insulating sleeve -- something which is unlikely to go brittle over years of exposure to UV and weather.
    And if that is the case, then wiggle is perfectly normal.

    Yes -- it would, *if* the wire actually had a larger diameter. I don't think that it does. It should be possible to slide the end of the sleeve down a little away from the capacitor body to verify this.
    Enjoy,         DoN.
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The "water pipe" as you describe it, is insulation, either polyester coated fabric or shrink tube. If the cap leads are intact on both ends, I'll bet they're ok.

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dave wrote:

Those are standard ceramic caps. They have bare wire leads run inside insulating sleeves. As long as the wire leads are still attached to the caps they should be OK. Now if the wires pull out easy then they are bad.
From the schematic, they look like they are simple filter caps.
--
Steve W.
Near Cooperstown, New York
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dave wrote:

They probably do a couple of things. As Iggy mentioned, they will help keep the HF out of the diodes. The other thing is that when diodes start and stop conducting in a rectifier, they do so very abruptly, which creates high frequency noise. This noise can cause interference problems with nearby radio and TV sets (though nothing like the HF will) and it can also shorten the life of the diodes. These caps will short that noise to ground.
By the way, the Dialarc HF is a nice machine! I had one for many years. It only got replaced when I started doing more aluminum and small stuff. Be sure to follow the directions for cleaning and setting the spark gap points. Ernie L says that the caps in the HF circuit need replacing occassionally also.
BobH
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Bob, these are SCRs, for sure they stop conducting at zero crossing only. As far as starting to conduct, are you positive that they do it very abruptly? I thought that it was not the case, but maybe I am wrong.

These are great, indestructible welders...
i
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wrote:

those DIODES are DIODES, not SCRs. SCRs have 3 leads, Anode, Cathode & Gate.
john G.
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Dave wrote:

If they're conventional SCRs, yes. If they're GTO (gate turn-off) SCRs, they can be squinched off while conducting.
LLoyd
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SCRs are like switches, and conduction starts abruptly upon triggering, with a risetime in the microseconds. This abrupt rise can and does cause interference to nearby radio receivers.
War story from the late 1960s, when I was in school: In the Geophysics Department there were a number of furnaces used to heat samples of powdered rock in platinum capsules. Accurate temperature control was needed, and so one of the professors built SCR temperature controllers for the furnaces. (SCRs were new then.) It was soon discovered that all the furnaces were varying together, without regard for the temperatures of the furnaces in question. Huh? What was happening was that the RF noise pulse from the first SCR to fire was travelling over the AC power lines to all the other SCRs and firing them prematurely. The solution was to add a simple LC filter at the input of each controller.
Joe Gwinn
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http://www.datasheetarchive.com/pdf-datasheets/Datasheets-19/DSA-369352.pdf
My guess is that the part above might be similar to what is in the welder. I like the explanations of the caps used for filtering (especially seeing the choke in the circuit). Another way of explanation might be that the caps help keep a small forward voltage across the rectifier to keep it from cutting out. (See voltage vs. amp curve) The welder might work without the caps, but it could have a noisy erratic arc and you might have more problems with sticking electrodes when you got too close using SMAW.
<Now Ill return to my seat in the peanut gallery!>
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Ignoramus6985 wrote:

They are diodes. The Dialarc used a magamp for current control, no fancy electronics.
After thinking about the circuit operation more, I don't think that there is a large snap on or off of those diodes since they will only switch around the AC line zero crossing. The voltage will only be a couple of diode drops away from ground when they switch, so their abrupt turn on is a pretty low amplitude event. It will still be fast but only a couple of volts.
Bob
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    Nope -- they are *not* SCRs -- just plain rectifiers. No third terminal for the gate input of an SCR -- and just the schematic symbol for a plain (if heavy) diode.
    Enjoy,         DoN.
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