Search results

Search Forums New Posts
  1. KrisBlueNZ

    PCB needs repairs

    An easier way to fix the board would be to just run an insulated wire from point D to point E on the underside. That way you don't have to remove the relay. It's not as tidy but would at least confirm that that's where the problem is.
  2. KrisBlueNZ

    PCB needs repairs

    The problem is between measurement points D and E. They should be connected together by a wide track on the top layer, so you should be seeing 17.7V on point E, but there's nothing. My guess is that the top right relay hole has lost its through-plating and the relay pin isn't making contact...
  3. KrisBlueNZ

    How does this circuit work?

    Thanks for that Doug. The mains waveform is detected by optocoupler U1. Input is at the bottom (on your drawing and on the photo) and output is on the top left pin, which is pulled up to the 5V VCC rail (which comes from the 5V regulator) by R2. This pin will normally sit at about half of VCC...
  4. KrisBlueNZ

    This one's a bugger, I think?

    Hi larhinds and welcome to the forums :-) The strange behaviour you describe regarding the volume control position could be due to noise coupled to the input of the stage that follows the volume control (or generated within the op-amp of that stage). The volume control is a variable voltage...
  5. KrisBlueNZ

    How does this circuit work?

    Doug, I think the mains zero-crossing detection is done by optocoupler U1, which is driven directly from the transformer secondary. Its output is on the track that goes to R2. This signal then goes off to the 74C14 but I can't follow it very well on the photo. Also, it seems to connect to...
  6. KrisBlueNZ

    PCB needs repairs

    Hi QuantumCheese, I believe he's already swapped the relay between another board, so we can assume it's OK. The relay is controlled by a transistor driven from pin 3 of the 555. I thought the 555 was a watchdog for the micro, but it seems that the 5V rail won't appear until the relay closes, so...
  7. KrisBlueNZ

    How does this circuit work?

    Thanks for the pics Doug, I'll have a look at them soon. Re the micro turning off the motor, I think it controls the motor through an electromechanical (standard) relay as well as the Crydom solid state relay. The electromechanical relay is used for on/off control, and the solid state relay is...
  8. KrisBlueNZ

    PCB needs repairs

    Hi again, sorry for the slow reply. Ideally I would like to see some good clear photos of the top and bottom side of the board with the large components removed. Then I think you should refit all the components, INCLUDING the 555 but NOT the micro and NOT the ULN2803s, and measure the voltages...
  9. KrisBlueNZ

    Led sequencer/scanner

    Cool! You won't need the protection components then. If you want to increase the number of LEDs, you don't need more transistors. Just parallel up more strings. Or you can increase the number of LEDs in each string, but you'll need to change the current-limiting resistors. As a rule of thumb...
  10. KrisBlueNZ

    PCB needs repairs

    Cool, no problem. I've just made a breakthrough on a SMPS from a flat screen TV that I'm trying to fix for my partner's brother. I've found a couple of zeners in the suppression circuit on the primary that are shorted. About time! I've been working on the stinking thing for hours! Now I just...
  11. KrisBlueNZ

    PCB needs repairs

    I've just noticed something. Look at the pictures in post 44. Start from the left hand electrolytic positive. Underneath, a track runs down to two terminals on the relay. On the top, a track runs from the bottom right relay pin over to the cathode of the back EMF diode. On the underside, a thin...
  12. KrisBlueNZ

    PCB needs repairs

    CC, I think he said it was marked 7805. Also, the left pin, which should be the output, DOES connect to the two MLCC decouplers and to pin 40 of the micro (via a ferrite bead), so I think it is a normal 7805. To the OP, I have some more questions. What do you plug into the boards when you test...
  13. KrisBlueNZ

    PCB needs repairs

    ... And guess what, it doesn't! So how does the +18V get from the left terminal of the left electrolytic to the input of the 7805? Apart from the bridge rectifier, the only place it goes is to the relay, where it comes through from the bottom side to the top side on the bottom right pin of the...
  14. KrisBlueNZ

    PCB needs repairs

    Good idea CocaCola. I have assumed that the positive (left) terminal of the left electrolytic is connected to the input of the 7805 by a fat track on the top side. I can see some of the track but not all of it.
  15. KrisBlueNZ

    PCB needs repairs

    OK. Those readings are normal; the mustard-coloured capacitors are OK.
  16. KrisBlueNZ

    PCB needs repairs

    You say you're getting "continuity on one" of the mustard-coloured capacitors? You mean continuity from one side to the other? In that case it's faulty. Capacitors should measure open circuit (except for larger electrolytics, which take a while to charge up, but eventually should read open...
  17. KrisBlueNZ

    Led sequencer/scanner

    D'oh! I knew that! I must have a screw coming loose. Thanks for pointing that out. The resistor value calculations are the same as before.
  18. KrisBlueNZ

    PCB needs repairs

    Yes, you've got the idea. You can't really just mount the capacitor above the PCB because you would need to get the soldering iron and solder in underneath it. Probably the simplest fix is this: scratch an area of the solder-resist coating off the track on the top side near the hole, tin the...
  19. KrisBlueNZ

    PCB needs repairs

    Yes, the problem is that there's no 5V coming out of the 7805. If there is 18V going into the 7805, this means that either the 7805 is faulty/damaged, or its output is shorted out, i.e. there's a short across the 5V rail. Also in an earlier post, you said that the voltage on the 18V leg of the...
  20. KrisBlueNZ

    How does this circuit work?

    Can you post the underside as well? And those pics are a bit out of focus and there's a reflection on the west one so that I can't see the markings on U5. I also noticed two resistors that seem to have burnt up, marked R13 and R14. What's the story with those?
Top