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Thyristor controlled flash gun

Discussion in 'Troubleshooting and Repair' started by ThePhoenix, Oct 6, 2017.

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  1. ThePhoenix

    ThePhoenix

    8
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    Jan 1, 2016
    Hi all

    I have a "Sunpak Auto Zoom 5000 Thyristor Flash", which I have owned literally man-and-boy; I bought it in 1977 with hard saved pocket money and after months of research. In those 40 years it has performed flawlessly with the only issues the need to replace the NiCds from time to time. I know I should dump it, but it's an old friend and I don't want to see it go to land fill just yet if I can fix it.

    When working correctly the power output is is ether automatically controlled by an optical sensor system, or manually selected by the user: 1/1; 1/2; 1/4; 1/8 .....1/128 fractions of full power. In the middle of a session of using it last week the power control quietly failed (no magic smoke, etc) and it now only functions at full power.

    On taking it apart I can see no obvious burnt or damaged components. I guess the thyristor (in the title of the model name) is used to cut the voltage to the flash tube once the required amount light has been delivered so the fault is associated directly with this component. I am guessing that the thyristor is a pretty robust device and if it had failed maybe there would be no output at all? My initial thought from experience of ageing electronics is that a capacitor in a timing circuit may have failed, but as you can tell I am far from being any expert.

    My test and repair kit comprises a soldering iron and digital multimeter. Does anyone have experience of such flash guns? Am I likely to be able to repair my trusty pal (with assistance), or should I just be packing him off to the great flash graveyard?

    PS I am aware of the high voltages present in such devices and think I have identified the section to steer clear of, but advice on staying shock-free probably wouldn't go a miss either. :)

    Thanks in advance.
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    9,668
    2,019
    Nov 17, 2011
    A bunch of well lit and sharp photos is a good starting point for remote inspection.
    Since, as I understand you, neither the automatic nor the manual control are effective, it is likely that a main component of the turn-off circuit in the flash unit is broken. The thyristor really could be the culprot - or the circuit controlling the thyristor.
     
  3. hevans1944

    hevans1944 Hop - AC8NS

    4,379
    2,046
    Jun 21, 2012
    Well, probably you won't be able to find replacement parts without a schematic diagram and/or a BOM (Bill of Materials, aka parts list). Before packing it off to the "great flash graveyard," consider purchasing a used replacement at a reasonable cost. You can then use the replacement for A-B comparisons while you troubleshoot your original flash unit.

    Be aware that "thyristor" is a buzz-word used by the photography industry to describe any electronic photo-flash that uses a variable light-output scheme to control exposure, whether it actually uses a thyristor to accomplish this task or not. Here is a link to an article that describes the history and evolution of such devices. It took years to develop the technology that allows a flash tube discharge to be interrupted without completely discharging the energy storage capacitor, but that didn't stop the development and marketing of earlier units because the advantage of automatic exposure trumped the disadvantage of long re-charge times.

    Your flash is old (I won't say ancient) so it is important to identify the technology used to interrupt the discharge of the energy storage capacitor through the flash tube. It has nothing to do with timing capacitors per se! The amount of light reflected from the object being illuminated by the flash is measured by a photocell and that signal (perhaps integrated by a capacitor) determines when the flash is terminated. All this takes place in a time period on the order of a few microseconds, from when the flash is triggered to when the circuit measuring the reflected light issues a signal that terminates the flash tube discharge. Not likely you will be able to troubleshoot this circuit with just a multimeter, even with expert assistance.
     
  4. ThePhoenix

    ThePhoenix

    8
    1
    Jan 1, 2016
    Thank you for your comments so far; Harald, I have uploaded a photograph of the circuit that I think is doing the controlling and Hevans1944, it seems like this is indeed controlled by a thyristor, as the CR02AM pictured is described as a thyristor to control, amongst other things, "a strobe flasher". I think in the first instance i will attempt to to replace this, unless someone can see something in the photograph which may also be a likely culprit?

    Just for more info, as I have it, the leftmost face down TO-92 component is marked A641 F8Y and the one next to it A641 F8X

    Hevans1944, your suggestion of buying another for comparison is a good one, but if I were to spend reasonable money, I would be wanting to buy a more modern TTL flash as a replacement. The article you provided was interesting and I thank you for taking the time to pen such a comprehensive reply.
     

    Attached Files:

  5. ThePhoenix

    ThePhoenix

    8
    1
    Jan 1, 2016
    Well I have just replaced the thyristor and still it only fires on full power. Any last thoughts from anyone of things I might try before I drop it in the bin?

    Thank you
     
  6. 50x70

    50x70

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    Mar 5, 2020
    Bonjour Phoenix. Did you finally get a solution? Thanks.
     
  7. ThePhoenix

    ThePhoenix

    8
    1
    Jan 1, 2016
    Nope, I gave up soon after and bought a different replacement.

    You have the same issue, I guess?
     
  8. 50x70

    50x70

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    0
    Mar 5, 2020
    Yes, same ancientness (1978) and same problem exactly! "Power" or "Auto" setting = 1/2 of Full Power. Mesured with flashmeter and whatever the setting (%) or watever the distance. Do you think CV12E9B (thyristor also? / high + left side at your view) ) might be the responsable? Cordially from France, 50x70.
     
  9. Harald Kapp

    Harald Kapp Moderator Moderator

    9,668
    2,019
    Nov 17, 2011
    As the thyristor seems to operate, it is likely not the culprit. The effect may simply be attributable to wear of the flash tube (see e.g. here) after a long service life. Check the tube for indications of wear. A worn out tube may require more energy to flash. The tube usually can be replaced if you're fit in soldering.
     
  10. 73's de Edd

    73's de Edd

    2,805
    1,176
    Aug 21, 2015
    Sir ThePhoenix . . . . .


    My . . . my . . . my . . .that PC board materiel type and carbon composition resistors certainly relate to a '70's date of construction.

    OBSERVATIONS . . . .

    Bottom center I am able to get the best view of 1 of 3 NE-2 lamps used in the unit, probably, with the 3 being voltage threshold detectors (60-70'ish volts) in series, with some nearby resistors in a voltage divider series configuration.
    The one unit to the left appears to be the only one that I can see its designated assignment of " N
    eon Lamp 2 " with the other two being NL1 and 3 .
    Also, the #2 unit seems like that it might have a white electroluminescent strip on its viewable half.
    The top one is not showing up well enough to be visually evaluated.
    If they have darkened the clear glass envelope with a grey to black discoloration / ion burns, that could be indicative of heavy or extended current usage and could have effected their striking voltage ( voltage referencing effectiveness.) Don't confuse . . .sometimes seen . . internal silvering . . . .with being ion burns.
    Another usable factoid, is that those high resistance COMPOSITION carbon resistors . . . . with a yellow-green or blue third band . . . have a proclivity to drift upwards in ohmmic value over time.
    In that unit, I see the bottom center Neon lamp has 3 nearby candidates that surely will be associated with its circuitry . A 1 meg, 820K and 680K.
    Then move vertically to the side of the SCR, where there is being another 1 meg unit.
    Finally, at the top right corner there is a 330K , and there looks to be a 150K OR 1.5 meg to its right, the two pink/red wires are blocking its full viewing.
    With those being visible, my next testing step would be to do a quick heating of one lead of each resistor to be able to have its solder blob melt and release the lead , so that the tip of a toothpick could push that then freed lead thru the board, and thereby be out of circuit connection.
    Then you go along with ohmmeter and test the resistance of each one, out of circuit . . . .due to one lead floating free . . . to confirm their resistance to their marking, all seem to be of 10 or 5 % tolerances.

    What your photo did not show, is ALL of the board. One needs to see ALL of it, since the right half seems to have the DC to DC converter circuit that ups 3VDC to 300+ volts for the flash storage capacitor. There may be some electrolytic caps involved within that circuitry that have deteriorated /declined in capacitance, such that the PS section, NOW, may no longer be developing its full OOOOOMPH of its original, day one purchase condition . . . 40 yrs ago ? .

    Thaaaaaaaaassit . . . . .

    73's de Edd . . . . .


    I was waiting in the next cubicle over and I overheard a woman say to my / our dentist . . .
    "I don't know which is worse having a tooth pulled or having a Baby."
    Our dentist then said . . . . " Well hurry up and make up your mind, I'm gonna have to adjust the chair accordingly ! "
     
    Last edited: Mar 18, 2020
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