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  1. KrisBlueNZ

    Harman/Kardon HK6500. Loudness not working when button is pressed

    R557 and R558 will be fine. C532 and C530 won't be damaged but you can replace them if you want, to keep everything looking the same. Yes, Q520 is a PNP and Q519 is an NPN. These are the two that are bolted to the heatsink, right? They are called "TO-220" packages. There will be some kind of...
  2. KrisBlueNZ

    What is the actual purpose of a Capacitor?

    Yeah that's a reasonable summary Gryd3. You can look at a capacitor from two points of view: the time domain, and the frequency domain. The frequency domain means that you look at the behaviour of a capacitor in response to different frequencies. This tends to be quite mathematical and I don't...
  3. KrisBlueNZ

    Harman/Kardon HK6500. Loudness not working when button is pressed

    OK, so the rest of post #57 applies. You need to replace, at a minimum, Q519, D503, C529 and C531, but I recommend also replacing Q520 and D504.
  4. KrisBlueNZ

    Timing Circuit? Counter?

    OK I'm working on a design but it won't be ready until tomorrow. In the meantime maybe Gryd3 would like to suggest something.
  5. KrisBlueNZ

    Timing Circuit? Counter?

    So there is actually another threshold somewhere between 0.7V and 0V. The timer should be enabled only when the voltage is between 0.7V and 1.3V. Less than 0.7V or more than 1.3V should disable it. Right? And you want both of those voltages to be fairly accurate? Would 0.55~0.75V and 1.2~1.5V be...
  6. KrisBlueNZ

    Harman/Kardon HK6500. Loudness not working when button is pressed

    Q519 emitter should not be 39V. Check the voltage on D503 at the anode - the end opposite the end with the band around it. It should be 0V. If it's not 0V there is a problem between that point and CN401 - possibly CN401 is not properly mated with JL501 on the other board. I'll continue, assuming...
  7. KrisBlueNZ

    Do I understand OpAmps wrong?

    This is called "high-side current sensing". There are several techniques, and many ICs available for this purpose, depending on your requirements and your budget. Start with Digi-Key's selection guide at...
  8. KrisBlueNZ

    Harman/Kardon HK6500. Loudness not working when button is pressed

    OK, now we need to work forwards from there. 1. Check the voltages on the emitters of Q519 and Q520. This is the rightmost pin when you're looking at the marking side of the transistor with the leads pointing downwards into the PCB. Be careful not to short the pin to the heatsink or to the...
  9. KrisBlueNZ

    Timing Circuit? Counter?

    Yes, it can definitely be done. I'm just considering the most appropriate way to do it. Thanks for the clear definition of behaviour. I had realised that you want the three second timer to reset if the voltage goes back over the threshold, but it's good to see it spelled out like that. How...
  10. KrisBlueNZ

    Timing Circuit? Counter?

    Hi Ernest and welcome to Electronics Point :-) You've listed four different voltages there, but there only seems to be one decision threshold - somewhere between 1.3V and 3.6V. If it's greater than that threshold, the voltage is OK, and if it's less, the voltage is out of specification, and...
  11. KrisBlueNZ

    Harman/Kardon HK6500. Loudness not working when button is pressed

    Yes, but connect your multimeter across one of them and make sure the voltage doesn't exceed about 2V. If it does, turn the thing off, because the resistor is going to overheat. And keep an eye on the other resistor (the one the multimeter isn't connected to) to make sure it doesn't overheat...
  12. KrisBlueNZ

    Harman/Kardon HK6500. Loudness not working when button is pressed

    Yes. They're about the same size so I assume they're about the same wattage.
  13. KrisBlueNZ

    Find Fault in my built amplifier

    So both output transistors (the TIP142 and the TIP147) have collector-emitter voltages of nearly 0V? They are connected in series between the +45V rail and the -45V rail. Their collector-emitter voltages should add up to 90V. You must have connected at least one thing wrong. I actually want to...
  14. KrisBlueNZ

    Stranded Hookup Wire and Logic Circuits?

    Maybe with stranded wire, the electrons get confused and can't figure out which strand to go down. Maybe that's how fuzzy logic was discovered. Seriously, I don't know of any electrical reason not to use stranded wire with logic circuits either. Solid wire is used for links in prototyping...
  15. KrisBlueNZ

    Need ideas for my blimp project

    Smartphones have GPS and a magnetic compass built in, and if it's a temporary requirement, you could borrow one, or get an old cast-off or spare smartphone. The blimp will be self-orienting vertically, so a compass might be enough to determine orientation, unless there are strong local magnetic...
  16. KrisBlueNZ

    Harman/Kardon HK6500. Loudness not working when button is pressed

    Stack them. That's why I suggeted the 39 ohm ones. Start with the top one, bend both leads at 90 degrees to make a C shape, then solder the other three across, between those leads. Keep the resistors close to each other so there's plenty of lead length left, and mount the whole thing raised up...
  17. KrisBlueNZ

    LED circuit, resistors!

    Yes, just like the diagram in post 5 except (a) there is the same number of LEDs in each string, i.e. 4, and there are four strings, and (b) the LEDs need to be the right way round - anode (the triangle side) towards the positive supply.
  18. KrisBlueNZ

    Harman/Kardon HK6500. Loudness not working when button is pressed

    The power rating is not high enough, but you can make a 2W 10Ω resistor from four 10Ω 0.5W resistors, or from four 39Ω 0.5W resistors, like this:
  19. KrisBlueNZ

    LED circuit, resistors!

    I guess you didn't read the tutorial Harald linked you to. You need to group your LEDs into series strings, each with one series resistor, and put those strings in parallel. If each LED has a nominal forward voltage of 3.2V, that works out to 4 LEDs per series string for a total of (3.2 × 4V)...
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