Studio Strobe Light repair

[RadoK]

Hi,
I am trying to repair studio flash light. It does not trigger.
Main capacitors seems to be charged ok at fist look. 350+ volts measured at the flash tube connector. When turned off after charge, when I use standard 230V incandescent bulb it lights on for few seconds. Powered from capacitors.

I do not want to measure live parts because it can be quite dangerous. Therefore I am trying to think twice first, measure individual parts and so on.
I did measurement when PCBs were powered from bench supply. I mean low voltage part. I did not see any issue in triggering part.
Unsoldered SCR seems to be ok.

Measurements I did when the light was i mains:
- main transformer - several voltages for logic board and triggering part - ok
- SCR gate voltage - not sure

SCR gate voltage was measured by DMM. It was showing only 0.5V at max (checked both DC and AC). Maybe it is ok, not sure. I did not feel safe to use oscillocope for it.
The high voltage coil has 0.3ohm on primary side and 200+ohm on secondary. I did not want to play on the secondary side of the trasformer with any equipment.
Any suggestion how to safely check if ignition of flash works (high voltage pulse expected)?
Any suggestion what to check?

Attaching reverse engineered schematics of the triggering part.

Thanks.
Rado

[impulsite]

Hi, Rado!
What is the exact name of studio strobe? May be I have the circuir diagram...

Any suggestion how to safely check if ignition of flash works (high voltage pulse expected)?

Short connection anode and cathode of ignition thyristor "2P4N" should trigger flash if xenon tube, trigger coil and trigger capacitor "0,22uF" are OK.

[Someone]

Flash tubes can age and stop triggering, also the trigger transformer is usually highly stressed in the secondary insulation and can short out preventing the trigger from forming. As above the check is that a large current is being pulsed through the trigger primary.

[RadoK]

Hi Impulsite / Someone,
If I remember right it is Quantuum PT-200.

One thing I forgot to mention. They told me that it was not working. Then they bought new flash tube, but not original, stronger 300W. It did not help The tube difference may be reason why it is not triggering I guess. Or it may be because the strobe is faulty. Hard to say.
I wonder how more powerful flash tube behaves. Maybe it requires higher triggering pulse. Looking at the coil secondary capacitor rating (3kV), the high voltage pulse is probably not as high as usually mentioned for strobes (5-10kV).
I do not have original flash tube available to eliminate option that the tube is not suitable.

Ok, I will try to short thyristor's Anode and Cathode. Plus I will check the capacitors around the triggering coil and thyristor. Especially I do not like 0.5V max at the thyristor gate. 0.5V would be fine for PN junction between G and C but I was measuring voltage between G and ground. Thyristor has two diodes in series from Catode to ground. Forward voltage measured around 1.1V. So I expect it needs at least 1.5V at gate to trigger the flash.

Thanks for suggestions.

[impulsite]

When strobe is set on max power what are voltage on
- main capacitors  ?
- between xenon tube pins ?
- on  thyristor's anode relative GND ?

Can you make the detailed picture of PCB and connections between PCBs?

[RadoK]

Xenon tube pins voltage is 350V.
Main capacitors are at 120V.
I will measure voltage on thyristor tomorrow once I replace BTA16 triack I killed accidentally :-(

I am attaching reverse engineered scheme. You can find some voltages marked there.
There is one additional board that is not on the picture. It is control board. Main parts there are 7seg. display, buttons, sync jack and a microcontroller. Probably PIC16Fxxx. It has scratched label but according pin configuration it seems to be pic16. It controls user input, display and charging. There is feedback (divided voltage from main capacitors) going from board 2 back to the micro ADC. The PIC then controls charging through optocoupler and triack (the one I killed).

I do not understand why I measured only 120V at main capacitors while there was 350V at xenon tube. There are just two diodes in parallel between these two. Plus one connector where I believe its two wires are not directly related. One is for charge voltage feedback and the second goes to the thyristor that switches HV pulse.
I do not understand what the black wire from the main transformer is good for. Maybe this one causes the big difference between capacitors and xenon tube voltage.
One more note. When I switched the mains off I measured 350V at main capacitors.

Main transformer configuration is weird too. I drawn its windings according resistance I measured but maybe it looks different inside.

Wiring between the boards is quite messy. For start I am attaching overall view of the strobe internals.

[Someone]

They told me that it was not working. Then they bought new flash tube, but not original, stronger 300W. It did not help The tube difference may be reason why it is not triggering I guess.

Each different design of tube has quite specific trigger requirements and they are rarely interchangeable.

[RadoK]

Correction: the caps around the thyristor are 220n 630V. Not micro.

[OiD]

Hi RadoK,

What is the voltage at at the point between the 470K, 1M resitors and the 220nF cap? Should be pretty high, 120-350V.

If you can disconect the "positive" cable that goes to the main caps. Leave the "negative" connected. If you can, solder a relay across the SCR and use a battery to activate the relay (just touch the wires quickly agains the battery to fire the trigger transformer). Wear some rubber gloves just in case and always wear eye protection when working with HV+caps/flash tubes.

Now with the main capacitors disconected (and discharged) trigger the trigger in a dark area and check to see if the xenon tube flashes with a light blue/violet glow. Basically this will show if the trigger coil is actually doing something or not.

If the trigger coil works then try it again with the main capacitors connected. If it refuses to fire then the flash tube is provably a higher voltage model (higher trigger voltage and maybe 480V for example).

I've built, repaired and modified numerous strobes, so i'd start there. Only try this if you believe you can do this safely. HV caps are very dangerous.

[RadoK]

Thanks OiD for feedback.

Today I replaced the failed charging triack and checked the 220n and 1n capacitors. Caps look to be ok. Tomorrow I will measure the SCR anode voltage (this is the voltage you meant?) and try to short the SCR using a relay to see if triggering part works.

I am not sure if charging works correctly. Voltage on xenon is 350V but voltage on main caps is much lower. 120 - 190V. I do not fully understand where the remaining volts come from.
On the schematics I probably incorrectly marked where connector A and B contacts from the capacitors pcb go. The lower voltage from caps (120-190V) seems to be source for the SCR triggering part (I will double check wires). Therefore I expect voltage on SCR anode to be around 100V.
The second wire from the connector has 350V and it goes to voltage feedback part (voltage divider and then microcontroller ADC).
And then there is the black wire from the main transformer. Is this the source of additional voltage that increases xenon voltage to 350V? It would mean that they feed xenon partially from HV caps and partially from 230V mains. Is this common? Otherwise I do not understand how xenon can be at 350V while HV caps are under 200V.

[botcrusher]

Is there maybe a negative voltage on the other side of the bulb?

[RadoK]

Issue found and resolved! The strobe light works now. Even with stronger xenon tube.

Murphy's law again. Before, I was not measuring voltage directly at the thyristor anode but one resistor away where the main caps votage was entering the triggering board. Just because it was much easier accessible for measurement. I assumed that two thrids of the voltage should be at thyristor. Wrong.
Today I was following OiD's suggestion to use relay instead of the thyristor (to short it). Once I had cables soldered it was easy to measure real voltage at the thyristor. And it was 0V. The resistor going to the anode was open. No sign of demade but it was faulty. Quite surprising because it was half mega 1W resistor used to limit charge current for the triggering capacitor. Voltage 350V max. It is less than 1 mA charging current.

Thank you all for suggestions. I was ready to give up last weekend but your support kept me trying.
Rado

[OiD]

Glad you got it working  Strange for it to go open with no signs of damage... ah well, better open than a short!