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  1. KrisBlueNZ

    HP 1741A Oscilloscope

    So if you replace the 68k resistor with two 68 ohm resistors in series, would you expect the clicking and flashing to reappear? I don't think the actual value of the resistor should make much difference... unless there's a short in the high-voltage transformer. I should have suggested this...
  2. KrisBlueNZ

    Report a line of sensors

    OK, wires will not be a problem if you include I/O expansion along with the LDRs (or whatever you end up using). A parallel-input serial-output shift register such as the 74HC165 or CD4021 will read eight input states (from eight LDRs) and send them out serially on a single line, in response to...
  3. KrisBlueNZ

    24VAC power supply question about VA spec

    It should be obvious that the "<4VA" marking means that the unit DRAWS 4 VA or less, not that the power supply must be RATED for 4 VA or less; as you say, Bob, that interpretation makes no sense, so you use the interpretation that does make sense. To try to answer the original question: the...
  4. KrisBlueNZ

    Bush Tv (CRT) having horizontal lines on the top half of TV

    On CRT TVs, vertical deflection problems including retrace lines are often caused by failing electrolytics in the frame output (vertical deflection) circuit. Have a look at Sam Goldwasser's excellent Repair FAQ on TV sets: http://www.repairfaq.org/samnew/tvfaq.htm It's seven years old now, but...
  5. KrisBlueNZ

    Understanding an oscilloscope schematic

    R1 and R4 are there to add a 1V offset to the output of the op-amp so the signal going into the MCU is centred around 1V, the half-scale voltage for the ADCs. The op-amps are DC-coupled buffers with total gains of 0.2. When the input is floating or grounded, their outputs will be at 0V. The ADC...
  6. KrisBlueNZ

    Relay configure trouble

    Yes. If you have a changeover relay, with the common contact connected to +12V, and loads connected from the NC and NO contacts to 0V, you can make the relay latch by powering the coil from the NO contact. To close the relay, you need a pushbutton from +12V to the top of the coil, i.e. in...
  7. KrisBlueNZ

    Rewiring 7-band EQ

    If you're working at audio frequencies, you don't need a flash oscilloscope. I'd go for a second hand dual trace analogue scope with 20 MHz bandwidth. I wouldn't pay more than about USD 150 for it. It's a bonus if probes are included, as these are not cheap. Avoid Chinese brands. I like Hitachi...
  8. KrisBlueNZ

    Switching PSU problems

    OK, good! Jamicon seems to be Taiwanese. I have no idea whether they're any good or not. Personally I would replace them with the real thing, but you may want to wait until they start to fail before you do that, I suppose. Did you check the ESR and ripple current ratings on the replacements...
  9. KrisBlueNZ

    Report a line of sensors

    Wow, that's a bit weird. It's like deja vu all over again! I guess we do become more predictable as we get older, but that's too much!
  10. KrisBlueNZ

    HP 1741A Oscilloscope

    Hmm, OK. Are you saying that the neons aren't flashing any more? Are you still getting the clicking sound? If so, is any sparking visible anywhere? Re the wire from pin 14 of the CRT, does it look like it was originally connected to that point or is there fresh solder? I've run out of...
  11. KrisBlueNZ

    LED Flasher help.

    Despite the lack of interest, I will continue my description of this circuit. Please use the following schematic. I've added a few more markings since the previous version, and renamed some reference point letters. SCHMITT TRIGGER OSCILLATOR Here's where a Schmitt trigger really shows its...
  12. KrisBlueNZ

    HP 1741A Oscilloscope

    It looks like there are three neons connected in series at the top right of the schematic, named OS1~3. They connect between the first grid and the cathode of the CRT. And the resistor that had been replaced connects between the cathode and one side of the heater. I wonder why it was replaced...
  13. KrisBlueNZ

    Switching PSU problems

    Yes, from your description I would suspect noisy supply rails too. You should be able to see the noise if you connect an oscilloscope across each rail in turn. The switching frequency will probably be in the range 30 kHz (for older power supplies) up to perhaps 100 kHz, so look for that in the...
  14. KrisBlueNZ

    HP 1741A Oscilloscope

    Does the service manual include a schematic diagram of the high-voltage power supply section? Even if it's older than the unit you have, it should be relevant. Can you post it? Also, can you mark it up to indicate the resistor that was the wrong value, and the neon(s) that are flashing in time...
  15. KrisBlueNZ

    Report a line of sensors

    LOL Chris :-) You know what they say about great minds... Or it could just be that we've all said the same thing so many times that we are all narrowing in on the best way to say it :-)
  16. KrisBlueNZ

    Report a line of sensors

    So the object will be placed on a platform and you want to measure its approximate size in one dimension? Will the object always be positioned squarely on the platform? Will the object always be at least as wide at the bottom as it is at the top, with no overhanging parts? Do you plan to put...
  17. KrisBlueNZ

    Speaker MoBo... Short circuit during (Failed) repair

    You can't really test ICs in a general way with a multimeter. They contain a lot of circuitry. Trace the tracks from the electrolytic to the nearest IC. Upload a photo of it. If it's a common one, and not mounted flat against the board (surface-mounted), you might be able to replace it. With...
  18. KrisBlueNZ

    Hunting specific, possibly rare tact switch, please help. DJ CD Player

    Hi mitchiemasha and welcome to Electronics Point :-) Go to http://www.digikey.com/product-search/en/switches/snap-action-limit-switches/1114207?stock=1 In the "Circuit" field, select "SPST-NO" In "Switch Function", select "OFF-Mom" In "Mounting Type", select "Surface Mount, Right Angle" Click...
  19. KrisBlueNZ

    Sunbeam Heated Mattress Pad Won't Stay ON

    Hi Erin and welcome to Electronics Point :-) It COULD be that the controller has detected a fault - probably an open-circuit - in the heating pad and is refusing to start because it knows there's a problem. Can you disconnect the controller from the pad? If so, try swapping the controllers...
  20. KrisBlueNZ

    Speaker MoBo... Short circuit during (Failed) repair

    As Dave says, the black cylinders look like inductors. Most of them will be in the power supply section of the unit. If you shorted one to the electrolytic, I'd say it's most likely you have damaged a device that's connected to that electrolytic; probably an IC. Try to trace the tracks from the...
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