Xenon Strobe repair - seeking troubleshooting advice

Hello,
I have an old xenon strobe (disco light) that had been stored for a very long time and when I tried to turn it on recently, it no longer worked. I first assumed it was the xenon tube bulb so I replaced it but it still does not work.

I opened it up (circuit was inside wood box) to try and measure voltage at different points to see if I could determine any current interruptions but as soon as I plugged it in, there was a point on the back of the circuit that lighted up (burning) and even some smoke came out. I immediately unplugged it.

Searching information on this I found this topic: https://www.aaroncake.net/Circuits/strobe2.asp , and it seems like my circuit is very similar. The burning came from one of the terminals on what seems to be the 4KV Trigger Transformer (see red arrow on attached photos).

I am no electronics expert (I do own and can operate a multimeter), and that is where I would greatly appreciate any support on this forum, is there a way that I can troubleshoot this circuit to get it working again? Any suggestions or ideas?

Thanks on advance for any comments.


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The burn up is where the high voltage from the transformer has jumped across to one of the tubes connections. First of all this is a dangerous circuit, you can kill your self if you touch the wrong point. Yhr first thing to do is to switch the beast off and disconnect it from the mains. get your self a sharp scapel and dig out the burnt area between the two PCB tracks. if you have a soldering iron, it would be best to remove the right hand track (to tube) completely. basically the transformer produces a pulse of 3-5 thousand volts and its jumped across the minute gap between the printed circuit tracks, setting fire to the board. I would expect the circuit could still work after this. Its a b*m design.
Frank

chuckey said:

The burn up is where the high voltage from the transformer has jumped across to one of the tubes connections. First of all this is a dangerous circuit, you can kill your self if you touch the wrong point. Yhr first thing to do is to switch the beast off and disconnect it from the mains. get your self a sharp scapel and dig out the burnt area between the two PCB tracks. if you have a soldering iron, it would be best to remove the right hand track (to tube) completely. basically the transformer produces a pulse of 3-5 thousand volts and its jumped across the minute gap between the printed circuit tracks, setting fire to the board. I would expect the circuit could still work after this. Its a b*m design.
Frank

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--- Thank you for the response and for the warning on the danger of the voltage, I was not aware of this. It does seem like there was arcing as the burn is right between the 2 points. However, the strange thing is that this circuit had been working before without any problems so why would it suddenly burn like this at this point? ...would the fact that I plugged it in without the Xenon Tube Bulb in place have anything to do? That's the only difference in circumstances that I can think of that was present when this occurred. I had never tried plugging the circuit in without the Bulb.

By just scraping out the burnt area, how will that prevent the voltage from jumping across again?
I do have a soldering iron, by removing the "right hand track", can you please be more specific as to what this is and if you are suggesting to remove it completely without placing it back?
Thank you.

Yes unplugging the tube would allow a much higher voltage output from the pulse transformer, hence the flash over. The burnt (black) bits of the printed circuit board are conductive, so need to be removed physicaly, i.e. just scrape them away. What I was thinking about was if you cut the right hand track, say at two points 10-20mm away from the burn up, then remove this piece of track by heating it up with a soldering iron and peeling the copper off. Then solder a bridging wire to rejoin the tracks, but making sure that it is looped well clear of the burnt bit. So now there is a large clear space around the burnt up piece of board.
Frank

Ok, I scraped away the carbon area between the lines and even put some nail polish on it (I read somewhere that this was a good insulator - ???). I placed the xenon tube back in and powered up. The arcing burn is no longer there! ...however, no strobe flash.

So, I got rid of that issue (thanks again for the advice!), now I just need to get the circuit to flash that tube again.

I read somewhere that the electrolytics could be dead due to being inactively stored for such a long time, correct? If this is the case, would you recommend that I try replacing these?

The current electrolytic caps on the circuit read: 22 uF, 250 WVDC

As replacement for the electrolytic capacitors, I found these online from a local store:

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The specs are:
- Radial
- Aluminum
- 22 uF, 250 Volts
- 13 x 29 mm.

I assume that these will do the job, right? Or, is there anything else that I should consider when looking for the replacement?

Would you suggest that I go ahead and replace this or is there anything that I could test before replacing parts?

Thanks!

I don't expect that the electrolytic capacitors are damaged. But the trigger transformer secondary shorted the about 300V across the capacitor during arcing and might be burnt.

You can check the DC voltage at the tube electrodes (outer connector pins). If it's 300 to 340 V (with 120 VAC input), the power resistor, rectifiers and capacitors are O.K.

Regarding circuit repair, provided you did manage to completely cut the burned PCB material, the potential arc gap should be sealed with epoxy resin or acid-free silicone. Hot-melt glue can work as well. The next arcing candidates demanding for better insulation are the connector pins, by the way.

Another option is to completely strip-off the trace with insufficient clearance and replace it by an insulated wire.

One guide to fault finding is the neon, L2. This should flicker and the rate of flicker should be changed by the speed control. It would be best to view it in the dark (or shadow). if the neon has not got 90+ volts across it, it won't work as an oscillator. Every time the neon strikes it puts a pulse of current into the SCR gate. This then should switch on the SCR, which puts the full 300V across the trigger transformer. This is transformed up to 3-5KV, which triggers the tube. The tube then conducts and discharges the capacitor C2, which causes the HT voltage to fall to zero momentarily. The whole cycle then starts again.
Frank

Thanks FvM

FvM said:

You can check the DC voltage at the tube electrodes (outer connector pins). If it's 300 to 340 V (with 120 VAC input), the power resistor, rectifiers and capacitors are O.K.

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Can you please explain which ones are these tube electrodes? Will I be OK to check this while the circuit is powered? My understanding is that I should not get my hands near the circuit when it is powered due to the extremely high voltage.

FvM said:

The next arcing candidates demanding for better insulation are the connector pins, by the way.

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Which are these connector pins?

Thank you.

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Thanks Frank

chuckey said:

One guide to fault finding is the neon, L2. This should flicker and the rate of flicker should be changed by the speed control. It would be best to view it in the dark (or shadow). if the neon has not got 90+ volts across it, it won't work as an oscillator.

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I powered the circuit in the dark and the neon is not flashing at all. I tried measuring the voltage at the terminals of the neon but as soon as I place the first test lead on one of the terminals, a spark comes out at the place where the arcing burn took place. I quickly remove the test lead but when I try to place it back on the neon terminal, the same spark comes out... very strange! any idea why this could be happening? why would the multimeter lead trigger this spark?
Thank you.

This is good and bad. Its good because it would seem that you have lots of voltage around the circuit, so it is likely that the input diodes and capacitors are OK. Its bad because because the main tube did not flash. When you say you connected your meter to the neon, which end? and where was the other lead going to?.Looking at the picture again, the tube seems to be plugged in, Is it making contact properly?, has some one plugged the tube in back to front? - it could make a difference.
Frank

I guess you don't see the neon tube light because it's triggering the thyristor after a few microseconds.

The observed spark suggests that the trigger circuit is working correctly. If you plug in a suitable xenon tube, it should work. Please consider that without a tube in place, the trigger voltage is probably higher and more likely causing flash-over.

It's O.K. to be cautious with mains connected circuits. A possible way to strictly avoid the vicinity of the circuit is to connect the meter by clamping, plugging or soldering the measurement leads to the circuit nodes of interest and power it afterwards.

chuckey said:

When you say you connected your meter to the neon, which end? and where was the other lead going to?

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I actually meant the small neon tube on the circuit, not the xenon tube that does the strobe flashing, is this how it was understood? Which end? I am not sure how to describe which end. Does this make a difference?

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FvM said:

If you plug in a suitable xenon tube, it should work. Please consider that without a tube in place, the trigger voltage is probably higher and more likely causing flash-over.

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I did plug in a brand-new xenon tube, which I assume should be good, but still no strobe flash.

The small neon :- one end is connected to the SCR Q1, the other C3.
Frank