Hi partyanimallighting,
OK, I've looked at the schematics and the design is pretty straightforward (although the schematics are not very clearly drawn). I will describe the general design, starting with the universal power supply schematic.
AC mains voltage enters at the top left, on "HOT" and "NEU" and is filtered by T1 and the capacitors, and appears across CC1.
The voltage feeds down to T3, which provides two low-voltage rails for the digital board. Pins S5 and S6 feed a bridge rectifier that feeds VR2 which regulates the +5V rail. There's also a feed from C11 called VOLT S01 which is used by the MCU in a clever way (I think) to detect the mains voltage.
T3 pins S7 and S8 feed half-wave rectifier D10, which supplies VR1 which generates the +12V rail, and also D9, which provides a mains-frequency signal into U4 on the logic board. (This voltage is generated across D8.) I suspect the MCU uses this for mains frequency detection, although I can't see how this knowledge could be used by the MCU.
These supply rails are all referenced to "GND" (indicated with the standard earth symbol near C13). This may or may not be connected to mains earth, which is shown with a different symbol, which I believe indicates a connection to the metal chassis, if there is one...
Going back to the direct AC mains voltage across CC1, this voltage is rectified by D4 (the large stud diode) and smoothed by C2 (with bleeder resistor R8) to provide the main supply rail for the strobe light. It also feeds a voltage doubler comprised of C1, D2 and D3, which supplies the firing circuit which is based around Q1.
Current via D1 and R3 and R2 charges C3 up to about +120V (limited by D5). When U1 receives an activation signal (current pulse) from the logic board, it feeds the voltage developed across R5 into the gate of Q1, a small SCR, making Q1 fire and conduct. This causes C3 to discharge rapidly into pulse transformer T2, whose secondary generates a very steep pulse of over 1kV peak which is connected to the trigger electrode on the Xenon flashtube. The pulse triggers conduction of the Xenon in the flashtube, which then converts the energy that is stored in C3 into a burst of bright light, discharging C3, which then recharges through R3 and R2.
The amount of power dissipated by these resistors depends on the voltage across them, which I believe will be significantly affected by the mains voltage, and their actual resistance (15K+15K or 22K+24K).
The digital board runs from the 5V and 12V rails generated on the power board. These come from T3 and (according to the schematic) are isolated from the mains. The 0V rail MAY be connected to mains earth; I'd like to know if it is - can you do a continuity check?
In this post I'm going to ask you to do a few things and answer a few questions; you may want to start a list
Hi Guys, if anybody's out there, I got unit # 2 repaired. Resistors R2 and R3 (22K/2W and24K/2W) were open and the pcb was a little scorched also. I replaced them, tested and the strobe works fine.
Me again. The LED was blown. Replaced it and the unit's fine. Someone please advise me as to the scorched parts on the pcb. I re-soldered everything back pretty good but please let me know if there's any cause for concern here.
I can't see a clear scenario where the resistors might overheat if nothing else was wrong. Has any component got bright solder on it, indicating that it was replaced, or at least resoldered, recently? I'm especially thinking of Q1, because if it failed, R2 and R3 could burn up.
It's interesting that this power supply claims to be "universal", but the voltage applied across the Xenon flash tube differs at different mains voltages.
I also noticed that R2 and R3 are shown on the schematic as both 15K, not 22K and 24K. I think there could be other variations from that schematic in the component values and types present in the unit(s) you have there. Can you find all the components in the high-voltage direct-mains-connected section and check that resistors and capacitors, especially R1, C1 and C3, are marked the same as on the schematic? And can you check that D1~4 are connected as shown in the schematic? That could help me be more confident that the schematic matches what you're working with.
I'm also interested in SW1 which is marked as a "THERMSW" (thermal switch I suppose). What can you tell me about it? Can you find it and take a photo of it? Also some photos of both sides with higher resolution would help, and you might want temporarily remove the flash tube socket for an "above" photo. You can email them to me if they won't fit here.
So your mains supply is 110VAC 60 Hz.
What voltages and other information are marked on the nameplate on the product you are using?
Do you know whether there are two types of the xenon bulbs, that are designed to work at different voltages? From my understanding of the circuit, the volage that this power supply delivers to the strobe lamp should be twice as high at 230VAC compared to 110VAC; in other words, it's not universal.
This might explain the problem with the unit that's flashing dimly. It may be designed to run on 230V not 110V. This is just a possibility that you can investigate.
Is it possible that the unit that had R2 and R3 burnt up had been used at a very high flash rate for a long time?
To all concerned, I am very very careful when I'm testing this pcb. QC made the dangers of working on this type of pcb very clear to me and I am very careful, switching off and unplugging before I go anywhere near it. I basically treat it like an ex-wife. You don't really want to have anything to do with it, but you have to!! I'll post some pix next.
Yes, many traces on the power supply board carry high voltages and are directly connected to the mains, so they can kill if you make any kind of contact with them.
As well as your good idea of securing the board firmly in an insulated jig, and working with one hand in your back pocket, you can improve your chances of not being electrocuted while measuring a live board (which is potentially very dangerous and you must only do it if you wish to accept that risk) is to solder short insulated wires with loops in the ends, onto the critical tracks (nodes) in the design. Never push a multimeter probe onto, or towards, the board, or anything live. If it slips, your hand can easily touch something live, and that can kill you.
At the moment I'm not asking you to make any measurements.
Also I'm confused by which unit we're talking about. Can you summarise each one in terms of what was wrong with it, what you're replaced and why, and what its current status is.