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  1. (*steve*)

    Lab Volt Model 790A will not power up

    Before you do anything, set your voltmeter to its highest voltage range (1000VDC if you have it) and measure across those capacitors. Make sure the voltage is under 20V or so. If you've left the scope a couple of days since you last tried to power it up, it is *probably* (not certainly) safe...
  2. (*steve*)

    My brand new oscilloscope

    Interestingly, this article suggests zener diodes on their own (ok, in series with a silicon diode) are an option. It also has quite a bit to say about longevity and reasons for selenium rectifier failure. Regardless, I think that characterizing the rectifier would be an interesting exercise...
  3. (*steve*)

    My brand new oscilloscope

    Various parts arrived today. Would you believe these are 4kV diodes? They are the same size as 1N4001's. The image shows the RS part number so you can look it up. I think I might just use two of these in series to replace the selenium diodes (that were rated for 0.5 mA). Now... A series...
  4. (*steve*)

    Lab Volt Model 790A will not power up

    In in the middle of working on an oscilloscope myself (actually 3 of them, but I'm too embarrassed to post about all the at once :-)). I assume you just want to make it go again, and are not too concerned about keeping it looking original. I note that you've found a suspicious looking...
  5. (*steve*)

    Lab Volt Model 790A will not power up

    I can't find any details on that specific model, but let me make some observations: It may be rebadged. See if you can find evidence internally of the original model and manufacturer. If it has a CRT (which also seems likely) then beware that there are dangerous voltages inside. These...
  6. (*steve*)

    Woo-Woo Land telephone giggles

    Wow. Just wow.
  7. (*steve*)

    Smoothing a charging current for a phone?

    Yes. But you have to make sure your MPPT controller is suitable. Most are a buck configuration -- they assume that the solar panel voltage will always exceed the battery voltage.
  8. (*steve*)

    My brand new oscilloscope

    I'm leaning toward preventative replacement. Originally I was going to replace ALL the electrolytics (and selenium rectifiers), but I wonder if it's justified replacing the low value, low voltage caps. There is an oily deposit on the case near the mains transformer. A this stage I don't know...
  9. (*steve*)

    Smoothing a charging current for a phone?

    Have I mentioned an MPPT charger yet? And you need to charge the battery directly. Charging a power bank via the USB charging port has the same inherent problems as directly charging the phone. Something like this or this.
  10. (*steve*)

    My brand new oscilloscope

    And here it is inside. I'm planning to replace the electrolytic caps and the selenium rectifiers. I'm also a bit concerned about those high voltage ceramic capacitors near the selenium rectifiers. they look OK from above, but take a look at the end of them: These are 0.01μF 4kV caps...
  11. (*steve*)

    Why Load Resistance is used in Crystal oscillator ?

    In the example circuit shown there, it provides a DC path that will ensure oscillation. The inverter shown there is typically a buffered inverter that internally has three inverting stages. If you study CMOS design, providing a feedback path like this to an odd number of inverting gates (>1)...
  12. (*steve*)

    24VAC Step-up Transformer

    Placing the led in series with a SSR may be an even better option.
  13. (*steve*)

    Blinking LED’s

    Firstly, determine the maximum current through the LEDs. Let's say you find they have a Vf of around 4V and you want 20mA through them. Ignoring the voltage drop across the transistor, you need a (12 - 4)/0.02 Ω resistor in series with each LED (that's 400Ω -- use the closest standard value...
  14. (*steve*)

    Blinking LED’s

    There are several problems: The forward voltage of the LEDs when added together is probably higher than the battery voltage. You have nothing to limit the current to the LEDs (and also through the transistor) Placing LEDs in parallel without individual current limiting resistors is a bad thing...
  15. (*steve*)

    Seeking a voice changer with mic and amplifier in order to communicate with hearing impaired

    Here is an interesting option.
  16. (*steve*)

    help with homework

    When an op-amp is operating normally with negative feedback, what can you say about the two inputs? If you can't answer this, go back and revise your knowledge of op-amps. Then go and revise zener diodes. What that does can be answered by one of the implications of the gain of an op-amp (you...
  17. (*steve*)

    help with homework

    The first question is answered easily if you have a basic understanding of op-amps. What have you figured out so far?
  18. (*steve*)

    garage door opener

    We'll, it depends on how it was miswired. But it is certainly a possibility. It may not be easy to determine what has been damaged. A good clear image of the board and a description of what wires were connected incorrectly (preferably exactly how they were miswired) may assist us in...
  19. (*steve*)

    Small low-power light sensor

    A problem with your circuit is that all the current is sourced from the output of the inverter. The two Vcc connections should go to the power supply and the inverter should simply control the enable pin. That doesn't explain the fault though. I would start by seeing what is happening at the...
  20. (*steve*)

    Retrofit house with LEDs

    Either buck or boost will do that. A linear constant current source will exhibit a constant current.
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