Camera flash repair

[frenzalrhomb]

Hiya, first post. I have a camera flash. A nissin Di622 to be exact. I have now spent nearly 12 hours trying to diagnose it. Everything appears to work but the main capacitor is not charging. It registers 6VDC. On one of the boards there are two transistors mounted on either side of the boards switching the 6volts onto a big ish transformer. I have compared the faulty unit with my other that works fine. I can't measure anything that is different. Just that the faulty one never looses the 6 volt i/p to the transformer. The good one switches it off when charged. The main capacitor measures 1400uF as it should, and should have a 330v feed.

Does anyone have experience of diagnosing this. I don't mind admitting I am in over my head here.

Thanks in advance for my advice.

 

[frenzalrhomb]

A key thing I don't understand is where/how my battery voltage is inverted I didn't think a transformer could work with DC?

 

[alec_t]

What did you use to do the measuring?
I trust you took great care when testing. The main cap can retain charge even when the flash is switched off. The cap voltage when charged is enough to KILL you.

I didn't think a transformer could work with DC?

It doesn't. The circuit comprises an oscillator which generates an AC drive for the transformer.

 

[frenzalrhomb]

Thanks Alex, I used a fluke 77 & a cheapo Aidetek vc99 for capacitance. I discharged the capacitor with a 2K resistor before I began my fumblings however later on discovered that it was not charging.
On the good flash I have 6 volts onto a board and 330VDC leaving it and on the bad one it's 6volts all the way.

 

[frenzalrhomb]

image by frenzalrhomb, on Flickr

This is the back side of the board where I believe the problem is the top wire runs up to the main capacitor and should have 300VDC on it. This only has 6 VDC. To my thinking this proves the circuit to be intact but maybe oscillator not providing AC to the transformer? I think that the transformer is bottom right (above) shown clearer in this picture where it is bottom left (below) :

image by frenzalrhomb, on Flickr

The large black thing with a lens at the top is an IR receiver for off camer triggering.

 

[alec_t]

Can't see those images. You can post images directly on this site by using the 'Upload a file' button. But please avoid posting large image files, to make life easier for those using portable device browsers or having capped/poor broadband.
Without a schematic it will be next to impossible to locate a fault.

 

[frenzalrhomb]

Alec thanks for your responses and i'd like to publicly apologise for getting your name wrong above. I read your'e signature and "Alec's rules" transferred itself in my mind to Alex. Very sorry mate.
Images fixed, I had hoped that someone here could say if you've six volts DC on the input to the transformer then change this thingy here. I,ve continuity checked the transformer and compared each pin to the other with a working unit and its the same. I can not find the schematic and so it's time to re-assemble and put this one in the cupboard for spares. I'd hoped in the beginning to find a wire off somewhere or a dry joint, just got sucked into it and thought it was possible.

 

[tvtech]

^^^ I am sure that Alec does not stress too much about that

Stay cool frenz....I have a Canon external flash unit giving me crap. My own one.

I will probably get round to sorting it out when I feel like it.....and as the warnings...be very wary of the Main Cap....battery power can shock when you least expect it.....because it's not supposed to happen....

Everybody (me included) have been fooled by these damn things and been belted.....always forget that no Mains is involved...therefore safe to handle ...

Treat with respect

Regards,
tvtech

 

[alec_t]

My money would be on Q107 and/or Q108 being the transformer drivers and the most likely part(s) to have failed. DC tests on those in circuit with a DMM might not detect a problem, but an oscilloscope could detect whether or not they are driving the transformer correctly. All this is guesswork, of course, without the schematic. Again, TAKE DUE CARE.

 

[zohaa]

Hey, your name sounds familiar!

Had tried unsuccessfully to download that file before.

As you noted, the schematic is fuzzy, but let's see!

Now go to bed!

 

[rumpfy]

Alecs post # 3 is quite right. These units are relatively simple in principle.
What can go wrong is that the main capacitor develops a high leakage current and this loads the oscillator circuit and the capacitor cant be charged due to the oscillator not having enough grunt. Capacitors can develope a high leakage if they are not used for a long period.
I suggest you confirm the capacitor is OK by taking it out of the unit and try to charge it with an external power supply of around 300 volt in series with a current limiting resistor of around 20 Kohm. If the capacitor is OK the capacitor voltage will rise to the value of the DC supply. You can measure the charging current and when the capacitor is charged you will be measuring the leakage current. If the capacitor is OK this leakage current should be less than around 10 microamp at capacitor voltage of 300 volt.
If the capacitor is faulty, then you can try to reform the dielectric. This is done by maintaining a low current around 1 to 10 milliamp, for an extended period and over time the current will gradually fall. Increase the supply voltage gradually over time until the leakage current falls to the microamp level with the 300 volt supplied to the capacitor. It could take a week or more to reform the capacitor.
If the capacitor is OK, then test the charging voltage of the flash unit by substituting a 0.1 microfarad capacitor for the main capacitor. You should then get 300 volt across the capacitor. If not, then the oscillator is faulty.