Voltage Increase, failed component? - Page 1
Forum section: electronics / repair

author: ajp8868
author_rank: Regular Contributor
timestamp: July 26, 2024, 09:25:37 pm
content: Hi All,I'm currently repairing a DJ light which uses two voltages, a 12v and 28v supply.Currently, a couple of the steppers on the 28v side aren't working at all and I've been swapping chips for a few days to no avail.I've just noticed that the 28v side is being pulled up to ~31v. I've checked another unit and it doesn't do this, I've also checked with a known good supply to make sure it's not that either.So I'm a bit puzzled, could this be a shorted component? Any suggestions as to what would cause that?Picture attached of the board. The 28v is the brown wire and supplies the LED circuit, and two of the steppers controllers.Thanks!

author: fzabkar
author_rank: Super Contributor
timestamp: July 26, 2024, 11:28:40 pm
content: Where is the 28V supply being generated? Is it a raw supply, or is it regulated?

author: ajp8868
author_rank: Regular Contributor
timestamp: July 26, 2024, 11:59:59 pm
content: Direct from a dual smps, I believe it's regulated...Just for info, I tested the same supply with a working board and it doesn't do it. I also tested another supply with this faulty board and it does the same.

author: fzabkar
author_rank: Super Contributor
timestamp: July 27, 2024, 12:16:25 am
content: What is the component on the vertical heatsink?FWIW, I notice that one of the 4-pin connectors in the following photos is labelled24V, Gnd, Gnd, 12V.SHOWTEC SPCI126 PHANTOM 50 SPOT MAIN PCB:https://www.indiamart.com/proddetail/showtec-spci126-phantom-50-spot-main-pcb-2852762037848.htmlhttps://5.imimg.com/data5/SELLER/Default/2023/10/356541585/SZ/DP/QW/10222995/showtec-spci126-phantom-50-spot-main-pcb.jpghttps://5.imimg.com/data5/SELLER/Default/2023/10/356541509/RP/GS/XX/10222995/showtec-spci126-phantom-50-spot-main-pcb.jpghttps://5.imimg.com/data5/SELLER/Default/2023/10/356541492/IZ/PO/NW/10222995/showtec-spci126-phantom-50-spot-main-pcb.jpghttps://5.imimg.com/data5/SELLER/Default/2023/10/356541474/RN/TE/TB/10222995/showtec-spci126-phantom-50-spot-main-pcb.jpghttps://5.imimg.com/data5/SELLER/Default/2023/10/356541848/AW/RQ/SG/10222995/showtec-spci126-phantom-50-spot-main-pcb.jpghttps://5.imimg.com/data5/SELLER/Default/2023/10/356541781/HN/SY/UP/10222995/showtec-spci126-phantom-50-spot-main-pcb.jpghttps://5.imimg.com/data5/SELLER/Default/2023/10/356541666/QJ/QQ/AS/10222995/showtec-spci126-phantom-50-spot-main-pcb.jpgUser manual:https://www.farnell.com/datasheets/1518634.pdf

author: ajp8868
author_rank: Regular Contributor
timestamp: July 27, 2024, 08:18:11 am
content: Yes the boards labelled 24 but the PSU supplied in these runs 28 and 12v. It might be that these same boards are used for different lights.That's a 5v regulator and is connected to the 12v rail

author: fzabkar
author_rank: Super Contributor
timestamp: July 27, 2024, 04:51:37 pm
content: I'm trying to understand how these lights work. IIUC there are two steppers for moving the light in the horizontal and vertical planes. There are two additional steppers for rotating the Colorwheel and Gobo-wheel discs. Is that about right?I'm also wondering why this simple(?) device would need two Megawin 82FG216 micros. :-?You say that you have been swapping chips. Which chips have you swapped?

author: ajp8868
author_rank: Regular Contributor
timestamp: July 27, 2024, 05:06:25 pm
content: Ah ok, yep there's 4 steppers for colour, gobo, gobo rotate and prism, these have drivers connected to the 12v supply and 5v for the controllers.Then there are 2 steppers for pan/tilt which are connected to the 28v line.The LED is also connected to the 28v

author: fzabkar
author_rank: Super Contributor
timestamp: July 27, 2024, 05:21:45 pm
content: I would start by drawing up a block diagram which tells me which micro is responsible for a particular function. Does one control the colour/gobo wheels while the other controls the pan/tilt? What does the DAC do?AFAICT, the two 24-pin ICs under the long thin heatsinks are the pan and tilt stepper motor controllers/drivers. They appear to be driven via a 14-pin IC, and this IC would presumably be controlled by one of the micros. I would measure the inputs and outputs of this IC (what is it?).Datasheet for MG82FG216:https://semiconductors.es/semiconductors.php?id=829616Edit:ISTM that the micro nearest the crystal is a slave and controls all the steppers. The other micro appears to be the master and is responsible for controlling the display and processing the user inputs. It also drives the quad TLC7226 DAC (whose function I don't understand).https://www.ti.com/lit/gpn/TLC7226Just for reference ...HV9910B - PWM high-efficiency LED driver control IC:https://ww1.microchip.com/downloads/en/DeviceDoc/20005344A.pdf

author: ajp8868
author_rank: Regular Contributor
timestamp: July 28, 2024, 10:38:17 am
content: I believe that's correct that the micro close to the stepper drivers is responsible for all stepper control.It must be partially working because the colour, gobo and gobo rotate steppers are working fine.Do you think it's likely that the micro could be partially faulty? I've checked to make sure all pins are connected.Or with the slight voltage increase, could it be a capacitor or resistor issue on that rail?

author: ajp8868
author_rank: Regular Contributor
timestamp: July 28, 2024, 10:40:49 am
content: Quote from: fzabkar on July 27, 2024, 12:16:25 amBtw the unit is a chauvet intimidator 255 irc, not the showtec phantom version which has the same board, different firmware. The boards are available for replacement for £165 but I'd love to be able to repair the board if it's not the micro controller at fault

author: fzabkar
author_rank: Super Contributor
timestamp: July 28, 2024, 11:23:54 am
content: There appears to be a 14-pin IC between the micro and the two stepper controllers. What is that IC?

author: ajp8868
author_rank: Regular Contributor
timestamp: July 28, 2024, 11:27:47 am
content: It's an op-amp but can't remember the exact chip type/markings. I've swapped this chip from a working unit but it didn't change anything.

author: ajp8868
author_rank: Regular Contributor
timestamp: July 28, 2024, 03:38:59 pm
content: Quote from: fzabkar on July 27, 2024, 04:51:37 pmSorry I missed this question, so far I've swapped the TL7226 DAC and the Op-ampI've also swapped the LED driver, but actually I've removed it from this board since I shorted a pin and blew it up..

author: fzabkar
author_rank: Super Contributor
timestamp: July 29, 2024, 12:43:06 am
content: If you select PAN or TILT functions from the menu, does the light behave as if it is responding to the command even though it isn't moving? In other words, does the MCU think that the light is moving even though it isn't? If you remove one of the stepper controller ICs, does the MCU complain?How does the unit know when a stepper motor has reached end-of-travel? Does it simply count the steps, or is there some positional feedback? Limit switches? How many wires are there for each motor?

author: ajp8868
author_rank: Regular Contributor
timestamp: July 29, 2024, 07:02:09 am
content: There are no errors shown and it behaves as normal with the controllers or without.There is a limit switch on the pan and a limiting reed switch on the tilt. No positional feedback on these units

author: fzabkar
author_rank: Super Contributor
timestamp: July 29, 2024, 07:14:03 am
content: I think I would be breaking out the scope and looking for step pulses. Start with a working unit and then compare with yours.I'm wondering how your high supply voltage observation relates to this fault. Can it really be significant, or is it a red herring? The only explanation I can think of is that the supply is unregulated, and the fault is unloading it in some way. I suspect that if you examine the power supply circuit, you will find that the 12V rail is regulated while the 28V rail just finds its level via the turns ratio in the transformer.Is it possible that the light is stuck in a program where pan and tilt are disabled???

author: ajp8868
author_rank: Regular Contributor
timestamp: July 29, 2024, 09:04:51 am
content: Yeah I'm going to try and get hold of a scope. I've never used one for repairs before but I think it's about time! Where should I be probing? Direct from the micro controller?I tried probing the oscillator on a working unit with my DMM on frequency mode, but it made the light act crazy. Is there a way to do that properly?It could be a red herring, but that supply doesn't do it on a working board..It's a good thought but it doesn't have any functions where the pan/tilt could be disabled and even then it would at least perform it's startup routine.

author: shakalnokturn
author_rank: Super Contributor
timestamp: July 29, 2024, 09:54:37 am
content: Quote from: fzabkar on July 29, 2024, 07:14:03 amIf you have access to a thermal camera that would also be of great help for comparison.Quote from: fzabkar on July 29, 2024, 07:14:03 amI was thinking of this too. Excessive loading on the regulated rail would bring the unregulated one higher than intended, so there again look for anything running hot.

author: ajp8868
author_rank: Regular Contributor
timestamp: July 29, 2024, 02:26:43 pm
content: I've tried with electronics freeze spray which doesn't seem to show anything obvious sadly.I don't have access to a thermal camera unless I can rent one but even then it's quite expensive.I'll keep looking to see if there's anything I can feel getting hot. Do you think possibly a short on the 12v side? Everything seems to be working fine on that side.

author: themadhippy
author_rank: Super Contributor
timestamp: July 29, 2024, 02:49:29 pm
content: The most common fault on nodding buckets is broken wires in the arms,presume you've tested continuity whilst wiggling the wires

author: ajp8868
author_rank: Regular Contributor
timestamp: July 29, 2024, 02:56:29 pm
content: Swapped this board to a known working unit and it does the same, so it's 100% a board fault

author: ajp8868
author_rank: Regular Contributor
timestamp: July 29, 2024, 06:04:47 pm
content: Had another look and the lm7805 regulator seems to be getting very hot.. could this be a symptom of it failing and drawing too much current? Or is it more likely to be a failed component on the 5v rail?Bearing in mind the 5v side is working perfectly

author: fzabkar
author_rank: Super Contributor
timestamp: July 29, 2024, 06:31:12 pm
content: Quote from: ajp8868 on July 29, 2024, 06:04:47 pmBeing a linear regulator, there would be a 7V differential between input and output. A load current of 140mA would dissipate 1W in the regulator.

author: ajp8868
author_rank: Regular Contributor
timestamp: July 29, 2024, 06:49:03 pm
content: Quote from: fzabkar on July 29, 2024, 06:31:12 pmMeaning..?

author: ajp8868
author_rank: Regular Contributor
timestamp: July 29, 2024, 06:50:28 pm
content: I've tested the 28v side without the 12v side connected and it doesn't increase in voltage - so I'm assuming the 12v is possibly pulling more current than it should - I'll check with the DMM

author: shakalnokturn
author_rank: Super Contributor
timestamp: July 30, 2024, 02:51:41 pm
content: Quote from: ajp8868 on July 29, 2024, 06:04:47 pmIn my experience statistically (not exclusively) it's down stream from the regulator and should show as heat.If it is the regulator by itself it should be even easier.

author: fzabkar
author_rank: Super Contributor
timestamp: July 30, 2024, 04:07:06 pm
content: Quote from: ajp8868 on July 29, 2024, 06:49:03 pmI'm just saying that it doesn't take much current to heat up the regulator, so a warm regulator may be normal.

author: ajp8868
author_rank: Regular Contributor
timestamp: July 30, 2024, 09:39:28 pm
content: So, I thought I'd made great progress. I found a shorted smd capacitor!I couldn't find any 10uf caps on my donor boards so I went for a 5uf thinking it might not make much difference and might get it at least working.Slight success.. I got the pan/tilt working!However none of the other steppers are working now!!Could it be that cap..? I've circled it on the attached image - could 5uf make that much difference??

author: ajp8868
author_rank: Regular Contributor
timestamp: July 30, 2024, 10:48:07 pm
content: Quote from: fzabkar on July 30, 2024, 04:07:06 pmAh that makes sense, yeah they both get hot, this one just gets a tad hotter. It's working though so wasn't the fault.

author: ajp8868
author_rank: Regular Contributor
timestamp: July 30, 2024, 10:53:33 pm
content: So it seems that a 74HC04 has now blown which I believe is from me disconnecting a stepper driver while powered up... I'm not exactly sure of it's function but it definitely is controlling these steppers.Swapped one from a working board and it seems to have sorted it (for a brief moment until I pulled a stepper driver while powered on again, which is how I know that was the cause of it blowing)Ordered a couple of replacements and a new driver chip, so I'll keep you guys updated!

author: shakalnokturn
author_rank: Super Contributor
timestamp: July 30, 2024, 10:55:15 pm
content: Well it can't have been a dead short surely? If it was is there more than one linear regulator for the 5V supplies?I doubt 5μF is enough to make the difference but then again if the 5V regulator has been running hot for ages it may not be giving the optimal output any more.Ideally you'd measure DC voltage and ripple or scope at the capacitor pins (CPU decoupling I assume) .If in doubt you can always stack a second 5μ on top of the one you fitted.As for other steppers... Re-check wiring, I'm the first to neglect obvious things when rushing to test after replacing components.Are the other steppers controlled by the same CPU?

author: ajp8868
author_rank: Regular Contributor
timestamp: July 30, 2024, 11:48:15 pm
content: Actually you're right, it measured a very slight resistance so not fully dead short. But when measured out of circuit it had no capacitance (DMM) and my little ESR tester thought it was a resistor.What would I be looking for when measuring ripple at the caps? Sorry for noob question but I don't have a scope yet, bidding on one so finger crossed.Fairly certain it's not wiring as it broke as soon as I removed the stepper controller, while live, for the second time. This unit was fully working, I've swapped the board into it to rule out things like wiring, motors etc.

author: ajp8868
author_rank: Regular Contributor
timestamp: July 31, 2024, 08:17:34 pm
content: Just to update, light is now fully working!So I think the original issue was simply that capacitor, and I've just blown chips by unplugging steppers while powered up.Replaced one 74HC04 which fixed colour/gobo and then replaced the other which was causing issues with prism and gobo rotate.Thank you for all your help and suggestions. Another light saved from landfill!

author: shakalnokturn
author_rank: Super Contributor
timestamp: August 01, 2024, 12:05:41 am
content: As for the ripple question it's a bit of a tough one...When scoping a power rail you wouldn't want the min / max values too close to the absolute min / max supply values stated for correct operation of any device powered by that rail.For 5V or 12V rails on a linear regulator I'm usually satisfied with 10-15mV RMS or 100mV p-p but then again it depends where you measure.Such values won't be acceptable for a 1.8V rail though.It really depends on system requirements... You wouldn't tolerate the same amount on an instrumentation amplifier as on your stepper motor supply.