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Thread: Useful Troubleshooting tools question

  1. #1
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    Useful Troubleshooting tools question

    Hi there, I'd read a post from Tom Phillips about handy additions to a scope and had some questions. I've no idea where the original post is as I'd copied and pasted to notepad. Here it is though. My questions are
    #1 when adding the DC blocking cap inbetween scope CH1 output Do I put a cap on each speaker lead ( positive and Negative)what value and rating should I look at?
    #2 The other is making a probe for detecting Hum what would be an approprate coil to make?

    Thanks for any suggestions,
    nosaj


    First you need to determine if your scope has the signal output feature. On my scope the Channel 1 output signal is available via a BNC connector on the rear panel of the scope. The exact amplitude characteristic does vary among scopes that have the feature. By “normalized signal” I mean that the output is proportional to the size of the signal presented on the scope screen. The specification for my Tek 465 scope is that the Ch 1 output signal will be 50mV for each vertical division on the scope screen. For example, the channel 1 output signal will be the same for each of the following conditions:

    Vertical amp set to 5 mV per division, Observing a 25 Vpp signal using a 1X probe, Signal fills 5 vertical divisions of the scope screen, Channel output signal is 50mV x 5 = 250mVpp
    Vertical amp set to 20mV per division, Observing a 100 mVpp signal using 1X probe, Signal fills 5 vertical divisions of the scope screen, Channel output signal is 50mV x 5 = 250mVpp
    Vertical amp set to 1 V per division, Observing a 50Vpp signal using 10X probe, Signal fills 5 vertical divisions of the scope screen, Channel output signal in 50mV x 5 = 250mVpp


    The signal at the vertical channel output is the same amplitude in all the above cases. The channel 1 output circuit in my scope includes a back to back diode arrangement to clamp the maximum output to ~2.4Vpp. That seems like a safe max value to drive an amplifier. That clamping circuit doesn't kick in until the observed signal is much larger than the scope face. Therefore, consider it a fail-safe feature and if you adjust the scope's sensitivity to keep the signal within the scope face then I'd say that you are well within the safe range of any reasonable sound card. I use an amplified computer speaker as mentioned by dmeek in post #6. They are readily available as free cast offs or from thrift stores for really low prices. I only use the one speaker that has the volume and power switch built in. Even though I use the signal tracer function with the scope input set to AC coupling I do use a DC blocking cap between the scope and the speaker. Then, even if I inadvertently set the coupling to DC or I choose to do that on purpose, I don’t have to worry about any adverse effects of a DC offset in the signal driving the monitor amp. In addition I don't worry about leaving the monitor speaker connected to the scope even when it is not in use and is switched off.

    It is nice to see and hear the signal at the same time with this setup. If the scope vertical input is always adjusted to keep the signal filling a reasonable portion of the scope screen then the volume variation from the monitor speaker is minimal. By using the computer speaker’s volume control, overall listening volume can easily be adjusted to your personal preference.

    If you make a probe with an appropriate coil at the tip that is similar to a small single coil guitar pickup. Then you can use that probe with the scope and amplified speaker to sniff out hum and buzz producing fields.

    With this setup you have the detector portion of a signal tracer with the added visual presentation from the scope. All the standard troubleshooting techniques of traditional signal tracers apply as discussed in the vintage literature.

    Fun fun fun,
    Tom

  2. #2
    Old Timer Tom Phillips's Avatar
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    Hi Nosaj,
    That came from post #15 of the discussion at http://music-electronics-forum.com/t41004/
    My reply to Q#1: You only need one blocking cap. Put it in series with the positive (hot) lead.
    My reply to Q#2: You can use a cheap (or leftover) single coil pickup as a hum probe. I also had read about making a probe in the attached article. I have not actually made a dedicated hum probe. I just was just writing about the possibilities. Edit: BTW the reason I have not made or used a "hum probe" is because, although it may indicate areas of high vs. low hum causing fields, it does not tell you how to actually reduce the hum in your equipment. The how to fix info is covered in the various literature and discussions about electronic design and construction practices. Also, the hum probe is not a calibrated device so it isn't useful to quantitatively evaluate the effects of layout changes. Still seems like it would be an interesting & fun tool to have on the bench.
    Hum Sniffer From Radio Electronics Oct 1964.pdf
    Cheers,
    Tom

    Last edited by Tom Phillips; 07-18-2017 at 12:05 AM. Reason: Added comment

  3. #3
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    Quote Originally Posted by Tom Phillips View Post
    Hi Nosaj,
    That came from post #15 of the discussion at http://music-electronics-forum.com/t41004/
    My reply to Q#1: You only need one blocking cap. Put it in series with the positive (hot) lead.
    My reply to Q#2: You can use a cheap (or leftover) single coil pickup as a hum probe. I also had read about making a probe in the attached article. I have not actually made a dedicated hum probe. I just was just writing about the possibilities.
    Hum Sniffer From Radio Electronics Oct 1964.pdf
    Cheers,
    Tom

    Thanks Tom. Does anyone else have any other neat tools to add?

    Planning on making an octopus this week.

    nosaj

  4. #4
    Old Timer Tom Phillips's Avatar
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    Quote Originally Posted by nosaj View Post
    ...Does anyone else have any other neat tools to add?...nosaj
    I think the thread at Favourite DIY test gear covered the subject well and I'd encourage people to add to that thread if they have additional neat tool ideas.
    Cheers,
    Tom

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