7b blew up on assignment. Any thoughts?

Discussion in 'Lighting Equipment' started by toby|1, Sep 25, 2011.

  1. So, on Saturday I flew to Minnesota for an assignment, and brought along my trusty (and aging) 7b. In the middle of the shoot, the strobe fires, and there is an extremely loud popping sound that caused some amount of panic to my assistant (aka the voice activated light stand). Not quite gun shot loud, but loud enough that several people came out of offices from along the hallway we were shooting in to see if everything was ok.
    At first I thought I blew the bulb, but I shot a few more frames, and it seemed to work fine. I then noticed that there was now a strange, and much less loud, popping sound coming from the pack itself (not unlike the sound the bulb makes when it is on full power). After 5 or 10 more frames, i also heard a sorth of frankenstein'esq / short circuity kind of sound, at which point I figured that this was not a problem that was going to fix itself, and turned the thing off.
    So, it is the weekend, and I haven't had a chance to call mac yet, but has anything like this happened to anyone else? I've googled about a bit, and haven't really been able to find any similar stories.
    Further, does anyone have any experience with associated repair costs?
  2. My guess is that because you got several shots out of your strobe after the initial noise, the capacitor and active electronics in your pack are probably still OK. My guess is that you had an arc-over on the surface of some insulator in your strobe, and with each successive arc-over, the carbonized track on that insulator got worse and worse to the point where you were getting the sound you described as "frankenstein'esq". I'm going to presume this was sort of a sizzling sound.
    If this is what happened, and the arc-over didn't damage anything else, repair of this type of problem usually is not difficult at all for a knowledgeable person, but would be at the level life-threatening-dangerous if you don't know what you are doing.
    Tom M
    PS - What make of strobe?
  3. I have worked on an old pack. The conductors between capacitors are aluminum bars. The connections degrade over time, and I observed significant pitting and some of the clamping bolts were frozen into the capacitors. I was able to disassemble and rebuild the pack. Yours is probably suffering from some similar problems.
    I like Tom's theory of an arc. Another possibility is that a jumper wire vaporized and your pack is not putting out full power any more.
  4. You probably blue one of the energy storage capacitors in the pack, a very common occurrence.
    Capacitors have the highest failure rate of any electronic component, by far.
    This would likely result in reduced power, proportional to the number of caps in the pack.
    Depending on the physical damage in the pack you could get leakage and sparking.
    Definitely need to send it to a shop for repair.
    - Leigh
  5. Leigh, I just don't buy the blown cap theory. The reason is that when caps fail, after the failure they usually are shorted (ie, either a dead short, or much lower leakage resistance), not open, and certainly not open under normal operating voltages. Most power supplies for photo flash units (both hot shoe and studio) have their caps in parallel, not in series, so if one cap was shorted, you would get nothing out of the bank.
    The only possible exception to this that I can think of would be if someone designed the power supply like a Marx bank (ie, multiple caps charged in parallel and discharged in series - http://en.wikipedia.org/wiki/Marx_generator ). This is commonly done in large plasma physics experiments that require tens of kV, and higher, but, because of the extra complexity, it's unnecessary and rare for systems such as we are talking about which only need to put out a few hundred volts.
    In a Marx bank, if an individual cap fails, upon triggering, the bank won't erect to the full voltage, but it can still put out a HV pulse. OTOH, if you connect the Marx bank to a system and attempt to draw current from it, since the full output current goes through each cap, that usually completely destroys the bad cap on that next shot.
    FYI, typical circuits for photo flashes can be seen here:
    A discussion of caps in parallel in studio sized strobes is here:
    Tom M
    PS - Toby, by any chance was it humid when the problem happened. Surface tracking (aka, arcs) is vastly more common in humid conditions.
  6. Hey guys,
    Thanks for your thoughts on the matter. To answer your question, it is a profoto 7b, and I was using the older style PB batteries.
    I'm not sure (I looked for a schematic, but couldn't find one) how they caps are set up in paralle or in series. I was sort of wondering that myself.
    I suppose there is nothing to do but see what the repair man says, as I am not thrilled at the idea of having a charged capacitor discharge onto my soldering iron.

    Finally, as for humidity; I don't believe it was excessively humid, probably about 60-70 percent I would guess.
    Anywho, thanks again, wish me luck with the repair folks.
  7. Hi Tom,
    It's difficult to generalize about catastrophic (explosive) capacitor failures.
    Computer-type caps have a thin metal strip connecting the studs to the internal foil. They can open in a couple of ways:
    1) the initial explosive release pushes the plastic end cap far enough to break the connections, or
    2) the capacitor is in fact shorted internally and the link fuses open.
    In either case you may hear arcing after the failure if high voltage is present and the cap is shorted.
    When I was going to night school I worked full time as a repair tech in a service company that did warranty repairs on Lambda and Sorenson power supplies. Some of the Sorenson units were so big we moved them around with a fork lift.
    More recently I was a warranty service station for Novatron strobes.
    Over the years I've seen a number of failed caps.
    - Leigh
  8. Hi Leigh -
    You absolutely right that caps are probably the #1 point of failure in these systems and also about the difficulty of trying to generalize / diagnose problems at a distance.
    As you correctly point out, if a cap fails, you can often still hear arcing once your charge it up past a certain voltage. However, in my experience, because this usually occurs at a voltage much below the usual operating voltage, you might hear a series of pops as the power supply keeps trying to recharge the caps (not reported by the OP), and there's usually so little charge stored in the caps at that point that you'll have none or little left to fire the actual flash tube. In contrast, the OP stated that he was able to take another 5 or 10 frames, apparently with reasonable light output.
    In addition, the OP did not report any smell. As you know, a chemical smell often accompanies cap failures, but the smell from arc-over / tracking problems is less obvious (...often, more ozone-like), but he reported neither.
    So, my bet is for tracking / arc-over and the cap may still be OK. I might bet a hamburger on my guess, but, the truth be told, I wouldn't bet a steak.
    Tom M
    PS - It sounds like we worked in similar areas. I worked for decades designing and building very high voltage (50 - 200 kV) pulse power systems for gas discharge lasers and plasma physics experiments.
  9. Hi Tom,
    I think you missed my point about the cap opening.
    Even if the bulk of the device is shorted, an ooen connecting link to one terminal will cause an open circuit, and allow the remainder of the supply to function normally.
    In this scenario there's a reduction in available pulse power, and transient failures may be observed.
    - Leigh
    p.s. I never designed stuff at that level. After I graduated college I went into designing stuff at the other end of the power spectrum... data comm gear that works at the milliwatt level. I'm not a fan of crispy critters. ;-)
  10. Hi Leigh - No, I think I understood what you meant, but, in my experience (albeit at much higher voltages), the length of that open link is small enough (and there is often enough carbonization, and possibly ooze from the dead cap) that it often arcs right over and doesn't really protect anything.
    Tom M
    PS - Unfortunately, the only crispy critter I came close to having to deal with involved my technician. Another tech told him that the few megajoule 150 kV Marx bank they were working on was down and crowbared. It was down but without the shorting stick in place, so the caps partially repolarized themselves without the power supply being on. I wasn't there at the time, but apparently, he was flung across the room and his heart stopped for a minute or so. Fortunately, he recovered. He tells the story of waking up hearing people talking about him being dead but he couldn't move yet. :-( We fired the other tech for not following safety procedures.
  11. Hi Tom,
    Yeah, he's lucky he walked. Too many people in that situation have left in a rubber bag.
    I only got hit once, by the HV supply in a radar display in a friend's basement. It was supposed to be off, and the control panel was dark, but the HV was still live. Fortunately I just got tossed across the room, and lost the use of my right arm for a few days. Nothing permanent.
    - Leigh
  12. Hey guys,
    So i took the pack to the shop (which is a profoto authorized repair center: Silvino's Pro Flash RX, in Hollywood) and the owner (whom I might add was literally ensconced in walls of broken profoto packs) said that after 5 or 6 years the capacitors on Profoto 7b packs tended to go out (presumably the popping sound), and that if you continued to shoot with them after that, even for a few frames, it tended to "fry the control board", as well as damage other circuit boards in the unit. He didn't give me a specific estimate, but suggested that the repair cost could easily be in the multiple thousands of dollars. My question is, why does Profoto make their boxes so that if the capacitor blows (which seems to be relatively easy to fix) you are then able to fry the rest of the unit by taking a few more frames (before you are even sure there is a problem)...
    Ugh. Standby for my next post, which will be titled "The Estimate" (and you know, wish me luck...)
  13. We'll see what the final estimate is.
    If one cap fails it's prudent to replace all of them. I'm not familiar with your unit, but I would guess there are four to eight caps. A representative part (computer grade 1000 mfd @ 500 volts) costs about $110 from Mouser. I don't know what his mark-up is, but you should figure at least 50% if he has to order them, or 100% if he has them in stock.
    If Profoto supplies capacitor banks in matched sets (a good idea), the price might be even higher.
    If there are assemblies that need to be replaced you can easily add several hundred dollars each.
    - Leigh
  14. Toby - sorry to hear the news.
    Leigh - I'm glad I only bet a hamburger, not a steak. ;-)
    Tom M
  15. Hi Tom,
    That's OK. I'll collect next time I'm in town.
    Be glad you didn't bet a sushi dinner.
    When I go to a sushi bar I'm accompanied by a Brinks truck and a dozen armed guards. ;-)
    - Leigh
  16. I had this happen many years back with a Comet PMT 1200 (very similar to the Pro 7B) when the capacitors blew it
    cracked the thick Lexan case.

    Fortunately it was a rental unit. They thought I'd dropped it until the opened the pack and what I heard was that the
    tech said it looked like a bomb had gone off inside, so I was off the hook for the repairs.

    If you have insurance on your gear does it cover catastrophic equipment failure?
  17. Yes, so I got it back from the shop a while back, the previous poster had it on the nose. The capacitors blew (hence the popping sound). Apparently, I was extremely lucky because often when the CAPs go, the explosion / whatever often takes out some other stuff with it ("I think he said the 'logic board', and maybe the 'control board'?). In my case however, the boards were ok, and only a few of the caps went out. It was I think about 300 dollars for repair. The scariest part is that the repair man didn't replace all the caps, just the blown ones. Further, the repair person just sort of eyeballed the non-blown caps to see if they were ok (as opposed to some sort of test involving futuristic electronics equipment), so now I am a bit nervous that the non-replaced caps will go out at an inoportune time, but still, beats having to buy a new one.
    Word to the wise though, if you pop a CAP while shooting, immediately stop shooting and turn the box off (why Profoto wouldn't make it impossible to shoot with a blown cap is beyond me), if the other parts are destroyed, it seems likely you'll have to purchase an entirely new unit.
    As for insurance and catastrophic equipment failure. I am not sure actually. I have pretty amazing insurance through the Hartford on my equipment, and I know if it were lost / stolen I would be ok, but I have no idea about just broken...
  18. I would have recommended replacing all the caps, although that's an expensive option.
    The remaining original caps might work fine for a long time... or they might not.
    - Leigh

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