SB800 Flash too bright after replacement bulb

ducky2468

Hi guys,

I recently purchased an SB800 with an intermittent firing fault. Seeing as the unit would charge the capacitor successfully every time I figured it was probably just the bulb.

I ordered a new replacement bulb from our favourite place, Ebay. It eventually turned up and I've literally just fitted the bulb now.

Good news, the flash fires every time now.

Bad news, for some reason the flash now fires too bright even when it's set to 1/128. Every fire drains the capacitor completely. It's as if it's running at 1/1 power.

Has anyone else ever had a similar problem after replacing the bulb?

Before you all have a go at me, I'm in communication with Nikon spares to get a new genuine bulb ;-)

michael66

This sounds more like faulty circuitry than a bulb.

ducky2468

It was functioning perfectly before I replaced the bulb. lets see what the genuine bulb does....

Ade

Just curious, does the flash behave the same in TTL-mode driven by the camera (i.e., mounted on the hotshot)? Or does it only fire full power when in manual mode?

ducky2468

Hi ade, it happen both on and off the camera, using TTL or manual. It's so strange?

Ade

Hmm yeah… if it's also happening in TTL, maybe it's less likely that it's solely a bulb issue.

Thinking out loud here. Inside the SB-800 there is a circuit called IGBT that acts like very fast switch (relay) to turn on / off the flash tube. In TTL mode, the camera controls the IGBT circuit. If the bulb is brighter than normal, then the TTL will simply command the IGBT circuit to shut off early.

For some reason the circuit is not shutting off. Sometimes an IGBT is triggered by + voltage to turn on, and - voltage to turn off. If the - voltage supply wire is loose, that might cause the circuit to remain on. Also if there's a short somewhere, then the trigger voltage might stay positive and keep the circuit on or prevent it from shutting off fast enough.

Anyway, I'm totally just guessing. When you get the new bulb from Nikon, make sure to check that all the wires are secure -- nothing is loose and there's nothing that can be shorting. You mentioned that the flash had an "intermittent firing fault" before the bulb removal, so maybe something in the circuit was already failing.

Vipmediastar_JZ

@ADE you always have great explanations

Ade

Though not necessary the correct explanations, LOL.

Even if I'm off, I hope some might still appreciate to learn that in speedlights the flash output is controlled by varying the duration of the flash (by precisely turning the flash tube on and off using an IGBT circuit) -- while most studio flashes work differently, controlling flash output by varying the brightness of the flash tube (by increasing or reducing the supply voltage).

This difference might help diagnose issues, and might even help take better pictures, e.g., if you want to "freeze" action then a speedlight with fast IGBT switching might be preferable over studio flashes.

ducky2468

Hi Ade,

Thanks thats a really good explanation. I'll double check my work to make sure nothing is shorting out or any lose wires.

I'm pretty confident that the intermittent firing fault was down to the old bulb. It was still the original bulb and looking at it, it was black spotted and looking ropey. It actually fell apart when I tried to remove it.

I'll keep you all posted.

NeilCHicks

The fact that you were operating near the xenon tube's reflector suggests your problem is far more likely linked to the black fiber-optic cable which runs from the reflector housing, to the top pcb in the main body (the pcb that sits on top of the battery compartment).

It's this cable which transmits a proportion of the flash output to a photodiode whch, in turn, feeds into the metering cicuit. If it's become kinked or dislodged, the gun continues to fire up to its maximum output, still waiting for the right amount of light to arrive... which it never sees!