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Meter recommendation

Discussion in 'Troubleshooting and Repair' started by Sternwheeler, Mar 18, 2018.

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  1. Sternwheeler

    Sternwheeler

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    Mar 18, 2018
    Hello everyone! I’m an electronics noob. I’m following the service manual to repair an old Akai reel-to-reel tape deck. I need to check bias voltage (supposed to be around 105V, AC, 95 kHz) as I have a weak recording channel. The manual says to use a vacuum tube voltmeter. Is there a modern equivalent, or should I hit eBay? Also, given I’m working on a 50, perhaps 60-year-old piece of equipment, will RMS be relevant? I have no idea what was available in meters back then.

    Since I will need a frequency counter, I guess, to check the frequency, wouldn’t an oscilloscope handle both of these tasks for me at the same time? Then I only have to buy one tool. Perhaps it measures voltage differently?
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    The recommendation for a vacuum tube meter is from a time when a typical instrument used to have a comparatively low inner resistance which would have burdened the bias voltage circuit and thus would have led to false readings. A modern multimeter with >= 1 MΩ inner resistance is well suitable. For about 40 € you can get hold of a decent multimeter which also offers frequency measurement, e.g. UNI-T UT139C True RMS Digital Multimeter or similar.

    Yes and no. Yes, an oscilloscope will allow you to measure both amplitude and frequency of the signal. Plus you'll be able to see the waveform. The accuracy of an oscilloscope is, however, usually much less than the accuracy of a good multimeter. At least in the price range of affordable ones.
     
  3. 73's de Edd

    73's de Edd

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    Aug 21, 2015
    Sir Sternwheeler . . . .
    You say . . . the magic and defining words

    as I have a weak recording channel.

    That osc is used on BOTH channels, and only in record mode , therefore no differentiation would be attributed to only one channel.
    Need the AKAI model number for any in depth analysis.
    You will probably end up inter-swapping channel connections to analyze in that manner.

    73's de Edd
    .....
     
  4. Sternwheeler

    Sternwheeler

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    Mar 18, 2018
    Mr. Kapp,

    Thank you for the recommendation! I’ve viewed a couple reviews and some YouTube videos concerning the unit and it certainly received a lot of praise. One question, though: According to one reviewer, the ability to measure AC voltage begins to roll off after 1000 Hz. Has this been your experience?

     
  5. Sternwheeler

    Sternwheeler

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    Mar 18, 2018
    73's de Edd,

    A simple review of the schematic would have shown me, and a little thought would have told me, that there would be only one oscillator. Thank you for setting me straight. Your recommendation to begin switching connections is very elegant and I wonder how long I would have poked around the inside of the machine before that occurred to me.

    If you do not mind, allow me to detail the symptoms I’ve noticed so far. The unit is an X-200D found in a local thrift shop. The right channel is weak and somewhat distorted upon recording. I tested this by recording a 1000Hz sine wave on another deck. I used this tape to calibrate the output levels of the Akai, and play back is strong. I then recorded the same 1000Hz source at 0dB, on the Akai and played the recording back into Audacity software from the other deck. When zoomed out from the view of the soundwaves, the right channel was markedly weaker with erratic peaks and troughs of the wave.

    When PLAYing in reverse mode, the right channel is weak from any source, even-recorded tapes.

    Unit’s voltage is set correctly, the frequency is set correctly, and I am using the line inputs and outputs instead of the DIN socket.

    I apologize for the length of this post. Thank you for your help.

     
  6. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    I do not have any experience with tis meter. Sorry.
     
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