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08-12-2011, 09:05 AM   #16
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Join Date: Jul 2011
Location: NJ USA
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I have repaired a lot of flashes in the past, years ago, and know some fundamentals. Don't know a lot about the ttl features though. Flashes have an oscillator circuit that generates pulses from the DC voltage (batteries) that power the flash. The pulses are used to drive a step up transformer, usually a ferrite core transformer. This is the piece that produces the familiar buzzing noise when the flash charges. The output of the transformer is rectified (to convert the AC back to DC) and used to charge the storage capacitor - this capacitor is charged to around 300+ VDC - gives a nasty shock if you are careless, guess how I know! Once the capacitor charges, a neon light is used to show charge status. Neons work at around 100V and a voltage divider (3:1) is used to fire the neon at 300V. The flash bulb is a xenon lamp. When you trigger the flash, the capacitor discharges through the xenon tube. Once the amount of light needed for the exposure (ttl etc.) is attained, there is usually a secondary discharge circuit that is triggered to drain the remaining charge and thus stop the xenon tube discharge and hence cuts off the light.

Now based on the above workings: you hear it charging and then the charging noise stops - this tells me the capacitor and oscillator circuit are good and capacitor has attained charge, which is why the noise stops. If the capacitor was leaky (rare) then the charging sound would persist and burnout the driver circuit components. Please note that all capacitors will eventually leak charge but will take a long time (hours) to completely discharge. But you don't see the xenon fire after it charges - so its not being triggered? If that's the case then the capacitor should remain charged and you shouldn't hear anymore charging sound, but you do! This could indicate that the secondary discharge tube was fired and it discharges the capacitor. Most likely a triggering circuit problem. Should be a relatively easy fix. But here I am talking component level repair - because that's what I am used to.


Last edited by debmalya; 08-12-2011 at 09:11 AM.
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