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PCB needs repairs

Discussion in 'Troubleshooting and Repair' started by partyanimallighting, Oct 31, 2012.

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  1. partyanimallighting

    partyanimallighting

    287
    3
    Oct 22, 2012
    Hello everyone,

    This PCB runs on 12V AC in from a transformer and the is an AC/DC converter to 18V DC and it has three motors that control a shutter, an iris and a color wheel (the terminals for these motors on on the right of the image). I repaired a similar PCB before which had a faulty relay and diac (located bottom left) but the other pcb has a different problem (I swapped out these parts from the repaired pcb and this pcb still did not function). When I power it up, I get 18V at the relay terminals and an audible click from the relay but the red LED does not light. My electronics knowledge is very basic but I really need to get this pcb repaired. A moderator and someone else was helping me before but I have not gotten a response for some time now as they're probably busy. Any ideas?
     

    Attached Files:

  2. QuantumCheese

    QuantumCheese

    73
    0
    Apr 27, 2012
    A schematic would seriously help here.

    edit: the 18V is just the rectified 12Vac. i would be looking for the state of the 5v rail and then wondering if the led is powered directly off that rail or from a pin on whatever flavour of uController that is
     
    Last edited: Nov 3, 2012
  3. QuantumCheese

    QuantumCheese

    73
    0
    Apr 27, 2012
    I commented on that 'PCB' already, in it's original thread and you never replied?

    As i said before, a schematic would be very helpful here - especially as that control device appears to have lost it's descriptive printing. the rest of the circuit 'looks' pretty straight forward.

    on the Right there are a pair of transistor arrays that are driven by what ever the big IC is. bottom there is a 555 so on one pin of that you ought to be able to find some pulses - google ne555 for a pin-out description.
    data sheets are readily available and have examples of most configurations so you can even work out what it's supposed to be doing too

    below the two large caps looks like a linear reg be it 7815/7812/7805 i can't read round corners need schematic!

    the led however, and i'm guessing here, appears to be connected to the input pin on the reg, so if it's not lit there is a fairly fundamental issue at the front end of the supply (or the LED's popped). though i could be wrong, it's not easy to tell with a picture of a populated double sided PCB what's connected to what.

    If you can get a schematic for this board i might be able to help a bit more.
    As this is off-topic for this thread and you already have a thread for this, please continue it there - I have posted this reply there also

    QC
     
  4. partyanimallighting

    partyanimallighting

    287
    3
    Oct 22, 2012
    Hi QC, a schematic is impossible as it's an old unit and the company won't be of any help with this. The big IC drives the two transistor arrays, which I am sure are the motor drivers for the color, shutter and iris motors. I swapped out the large IC, the transistor arrays and the 555 before with the working pcb and they are all functional on that pcb. The "linear regulator" (don't know what you mean here, totally ignorant to the jargon) is a 7805 and I also powered up the LED before and it's fine. Terminals J11 and J12 are for optical sensors for the color wheel and shutter wheel that are driven by the motors fed by the two transistor arrays. The optical sensors look like this:

    http://parts.elationlighting.com/Pr...Number=27-001-0575&ProductLine=POWER SPOT 250

    and the color and shutter wheels pass between the sensor and the revolutions are counted (not sure of the proper electronic jargon for this). Where do you think I should start? Do you think the problem lies near the linear reg? Or the 555? How do I start to check? Please remember I'm clueless as far as where to check and how to check so please be detailed in your response.
     
  5. QuantumCheese

    QuantumCheese

    73
    0
    Apr 27, 2012
    OK no schematic, but if you swapped out all the IC's and they worked then frankly theirs not much left on that board to fix!
    I don't mind helping you fix this but your going to have to at least try & learn a few things along the way - it'll all help for your fixing of future things i promise.

    A 'linear regulator' or sometimes called a 'three terminal regulator' is a basic building block of older power supplies. It's got 3 pins, IN, GND (or 0V if you prefer) and OUT - looking at it from the front and reading left to right (put "7805 datasheet" into google and you'll see what i mean). It's function is to turn in incoming DC voltage into a lower DC voltage , in this case 5V (that's what the 5 stands for in 7805), it then wastes the remainder in heat. other common variants are 7812 & 7815 - can you guess what their outputs are :)
    so in this circuit it takes the rectified 12V ac input (rectified by the bridge rectifier (black four legs top of board)) that's now 18V dc and turns it into 5V. The two large capacitors will sit either side of it so with your meter you should be able to read 18V across one of them and 5V across the other - anything else & the 7805 is dead.

    Briefly on the 555, it's a very common IC, been around since the arc - just download a data sheet and scan through it, you may not want understand how it works but it'll show you how to use it (and therefore fault find it). I don't think your Fluke 77 does frequency measurements so to truly know if the 555's OK you'll need an oscilloscope (or a meter that does Freq)

    Check out those measurements on the reg & let me know how you get on.
     
  6. partyanimallighting

    partyanimallighting

    287
    3
    Oct 22, 2012
    Hi QC, I fully understand it's a working experience for me but I have learned from experience that experience gained from actual practical repairs is better than anything read from a tutorial but both are necessary. So, the bridge rectifier (what I referred to before as an AC/DC converter) converts the 12V AC to 18V DC and the linear regulator breaks it down further to 5V (thus the last number on the reg will represent voltage out). The balance is dissipated in heat (hence the heat sinks or plates that are normally attached to these devices). Quick question...if I measure voltage in at the 7805 I should get between 12~18V DC (pin 1 and common) and I should get 5V DC (common and pin 2). Tell me this is correct and I'm not making a fool of myself here.
     
  7. partyanimallighting

    partyanimallighting

    287
    3
    Oct 22, 2012
    Following up further with my learning process here, the capacitors (both 35V 200uf) handle the input and output from the 7805? One handles 18V going in (before the 7805?) and the other 5V coming out (after the 7805?) I was wondering, what's the purpose of these polarized capacitors? Are they there to filter and "clean up" the voltage?
     
  8. QuantumCheese

    QuantumCheese

    73
    0
    Apr 27, 2012
    Yes, that's basically it. pin 1 is the input and pin 3 is the output.measuring between these pins and ground (the middle pin (and also the tab)) should give you the voltages your expecting, :)

    QC
     
  9. QuantumCheese

    QuantumCheese

    73
    0
    Apr 27, 2012
    Sorry, I missed you extra reply,

    the input cap smooths the rectification from the, err rectifier diodes (otherwise it would just be essentially dc but with a BIG wobble on it!). the second cap on the output smooths the output of the reg.
     
  10. partyanimallighting

    partyanimallighting

    287
    3
    Oct 22, 2012
    Hi QC, here's the update. I took the working pcb out of the unit and benched it side by side with the non-functional pcb. Here's all the readings I gathered. On the working pcb I'm getting the 18V and 5V at the 7805 as you said (that's why it works!!) and I'm getting 18V and another 18V on the two caps, not 18V and 5V as you thought. On the bad pcb I'm getting 0V on both legs of the 7805, 0V on one cap and 18V on the other. I then removed the "working" 7805 from the working pcb and replaced it on the bad board.......same result, the board still does not function. This tells me that it's the cap with the 0V reading that's the problem but I'm not the expert here. You guys are. What do you think?
     
  11. QuantumCheese

    QuantumCheese

    73
    0
    Apr 27, 2012
    hmm, i'm not really surprised about the caps, they are both 35V jobs.
    "On the bad pcb I'm getting 0V on both legs of the 7805" so the 7805 is not getting a supply.
    I'm not sure, as the components are covering a lot of the upper board tracks, but it looks a bit like the relay controls the 18V going to one of the caps. have you checked the function of the relay or just assumed "as it clicks it's working"?
     
  12. partyanimallighting

    partyanimallighting

    287
    3
    Oct 22, 2012
    Hi QC, I replaced the relay already with the one from the working pcb so it has to be another component. You said you're not surprised about the caps as they are 35V. Can you explain? Also, should I change out the caps?
     
  13. partyanimallighting

    partyanimallighting

    287
    3
    Oct 22, 2012
    Hi QC, I swapped out the caps and the problem still exists so the problem is somewhere else on the pcb. Any ideas?
     
  14. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,267
    Nov 28, 2011
    Hi partyanimallighting, I've been following this thread and I think it's time to put my two cents in.

    You've actually given plenty of information, with your photos of the top and bottom sides of the board. Here's what I've figured out about the design.

    As you and QuantumCheese have already figured out, your 12V AC is rectified by the bridge rectifier (between the big electrolytics and the edge of the board) and smoothed by one of the electrolytics (the one closest to the fan connector). You should measure about 18VDC across it with power applied.

    This voltage then passes through the contact of the relay (bottom left corner) and to the second electrolytic (the one closest to the big IC, which is a microcontroller), and to the two ULN2803 driver ICs on the right side, and to the three connectors for colour, shutter and iris.

    The relay is driven by the transistor in the bottom left corner, which is driven by the 555. I would normally expect this circuit to be a "watchdog" circuit. While the firmware inside the microcontroller is running correctly, it toggles (i.e. generates a continuous stream of pulses on) one of its pins, and these pulses are detected by the 555. While the pulses are present, the 555 keeps the transistor and the relay energised, so the outputs will work. If firmware fails, the pulses will (or should) stop, and the relay drops out. This is a safety mechanism to prevent damage to the equipment being controlled, if the microcontroller goes haywire (due to program corruption or a bug).

    In this case, I can't see the connection from the micro to the 555, so I can't be sure that the 555 is being used in this way. There are some signals from the microcontroller that go to connectors J11 and J12, which are close to the 555, and I would like to know what those connectors are for.

    As for the 5V power rail, it is derived from the first electrolytic by the 7805 regulator. The middle pin is 0V. The right hand pin (on the top side view in your first post) should have +18V from the first electrolytic, and the other end pin should have +5V on it. This +5V rail powers the microcontroller, and drives the LED.

    Since you say the LED isn't lighting up, perhaps the 5V rail is not present. This could be due to a failed 7805, but you say you've replaced that. It could also be caused by a short circuit on the 5V rail. There are several capacitors across the 5V rail, which could have failed. You need to find out for sure whether the 5V rail is present or not. If it isn't, then you could try replacing the two mustard-coloured capacitors that are on either side of the 7805.

    If the 5V rail is present but the LED isn't lighting, the fault is either the LED itself, or the current limiting resistor, which is connected to the LED with a track that's visible on the top side. It looks like the colour bands are green, brown, brown, which is 510 ohms, which is about right.

    I'd be interested to know what J11 and J12 are for, and the results of your testing now that you know more about how the circuit is supposed to work.
     
  15. partyanimallighting

    partyanimallighting

    287
    3
    Oct 22, 2012
    Hi Kris, I really appreciate any help I can get with this. Christmas is coming up and my wife wants this piece of equipment OUT (!!!) of the house! The good thing is, I have a working pcb for comparison so this helps a lot. What you think is a fan connector is actually a connector for DMX control. You're probably familiar with this type of control system used for controlling intelligent lighting for clubs and stages. The voltages at the electrolytics are both 18V DC so I don't think they function on either side of the linear regulator. The optical sensors for the color wheel and shutter are fed by J11 and J12. The cooling fan for the unit connects to the terminal to the left of the first electrolytic. I'm not finding any 5V reading at any point of the 7805 so I'll need some help to troubleshoot this problem. Hope this info helps....
     
    Last edited: Nov 14, 2012
  16. partyanimallighting

    partyanimallighting

    287
    3
    Oct 22, 2012
    Hi Kris, didn't mention that the 18V on the electrolytics in on the good pcb. I'm getting 18V only on one electrolytic on the defective pcb.
     
  17. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,267
    Nov 28, 2011
    OK. Can you confirm for me that you're measuring all your voltages with the black lead of your multimeter connected to the "0V" rail of the circuit board? That's the tab at the top of the 7805. You need to measure all your voltages with the black wire of the multimeter connected to that tab. Are you doing that?

    So you have 18V on one side of the 7805 and nothing on the other side, and you've replaced the 7805 so you know it's not faulty. That seems to indicate there's a short on the 5V rail. The 7805 should be getting hot, because its output is being shorted out.

    The next step is to check for a shorted 5V rail. Set your multimeter to resistance range, leave the black lead connected to the 7805 tab, and put the red lead to the left side pin of the 7805. If there's a short, the multimeter should read only a few ohms, or less.

    If you get that result, I would suspect the two decoupling capacitors on the 5V rail. These are the little mustard-coloured components on either side of the 7805. To start with, just remove them, and re-measure for a short.

    If you need to replace them, check the markings. They're probably marked "104". If so, replace them with "0.1 microfarad ceramic capacitors", rated for 10V or more (they'll probably be rated for 50V). These are very common components and should be available from Radio Shack or any local electronics shop.
     
  18. partyanimallighting

    partyanimallighting

    287
    3
    Oct 22, 2012
    Hi Kris, I was checking the voltage before from center pin to left and to right on the 7805 and I assumed that was correct. I re-checked as you advised and the results are the same: On the good pcb, 18V and 5V. On the bad pcb I'm getting a negative reading of -0.03V which I found strange. And the 7805 isn't getting hot. The only thing on the pcb that's a little warm to the touch is the relay. When I check the electrolytics, I'm getting 18V on each one on the good pcb and 18V on the outer one and 0V on the inner cap on the bad pcb. I next checked the resistance on the 7805 and I'm getting 22 MOhms at pin 1 and pin 3. It seems to me that the required voltage is not reaching the 7805 but you guys are the ones to guide me here. What's next?
     
  19. CocaCola

    CocaCola

    3,635
    5
    Apr 7, 2012
    Since all the ICs are socketed remove them all and test the 7805 again... If the short still percsist you have then narrowed it down to only a few components... If the short is gone you have narrowed it down to an IC...
     
  20. partyanimallighting

    partyanimallighting

    287
    3
    Oct 22, 2012
    OK Cocacola, here are these results. I tested with and without the NE555, live and powered down:

    With NE555 in socket (live):
    Voltage at 18V leg: 0.1V Resistance: 2.4KOhms
    Voltage at 5V leg: 0V Resistance: 180KOhms

    With NE555 in socket (powered down):
    Resistance @ 18V leg: 224KOhms
    Resistance @ 5V leg: 2.457KOhms

    Without NE555 in socket (live):
    Voltage at 18V leg: 0.35V Resistance: 340KOhms
    Voltage at 5V leg: 0V Resistance: 2.5KOhms

    Without NE555 in socket (powered down):
    Resistance @ 18V leg: 340KOhms
    Resistance @ 5V leg: 2.5KOhms

    I swapped out all the IC's before from the working PCB so I don't think that's not the problem anyway. What do you make of these readings?
     
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