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reading AC voltage in a schematic

Discussion in 'General Electronics Discussion' started by pishta, Nov 23, 2016.

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  1. pishta


    Nov 23, 2016
    I have a 3 tap AC transformer in a vintage cassette deck that calls out various stepped down AC voltages. 110ac in, 3.2, 30 and 20ac out. the schematic shows 30 and 0 on both "ends" of the 30Vac tap. how do I measure this? Is it as simple as probing across points 9 and 10 with a DMM's black and red probe? Or do I use black to chassis ground to probe 9 and 10 with the red probe to get 0 and 30? I dont think that way would detect 0v on point 9...My issue is Im showing +18 and +12 on these insted of 30 and 0 and its throwing the rest of the board voltages out of spec. They go straight to a bridge rectifer. Also why does it call out more than 30v (36v) at the first NPN TR2 transistor collector? Does the rectifier make 30Vac to 30vdc? When I lift 9 off the PCB, I get the correct +30Vac on pin 10 again...? Something on the board is backfeeding voltage on my 0 leg? Confused.....I can post entire power supply schematic if you want to see it. This is about 25% of it.
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Incredibly: yes. Set the multimeter to AC volts range, at 30 V expected it's probably the 200 V range on your meter (depending on the make of the meter).
    measured against which potential? When measuring between these two points it is impossible to see +8 and +12 simultaneously, therefore you measure against some other potential, possibly completely unrelated and the measurement makes no sense.

    The common potential (ground, 0V) within the cassette deck is most likely the thick line which is connected e.g. to the common cathodes of C6, C7, C19 etc. Any potentials to the right of the rectifiers (Di SVBIO-100) are referred to this common ground.
  3. dorke


    Jun 20, 2015
    From your questions it is clear that you are a novice.

    First thing for you to understand is what is the difference between AC and DC voltages.
    Then you should learn how to measure AC and DC with a DMM.

    Notice that AC is measured in RMS(for simple non true RMS DMMs only correct for sinusoidal low frequency voltages!).
    Look up the term RMS for sinusoidal voltage it will help you understand(at least partially) your question:
    "why does it call out more than 30v (36v) at the first NPN"

    Look up the term "bridge rectifier" and "un-regualted power supply" as well.

    Like @Harald Kapp said you should measure the transformer voltages in AC range ,not DC like you did (+ is only meaningful for DC voltages).

    The secondary wingdings of the transformer are isolated from each other.
    you should measure the AC in pairs,probes on(polarity dost matter) :
    7-8 for 20V AC
    9-10 for 30V AC
    11-12 for 3.2V AC

    DC voltages should be measured in respect to DC ground (the pink line in the pic),
    black probe of DMM in DC voltage mode.

    Last edited: Nov 23, 2016
    Harald Kapp likes this.
  4. pishta


    Nov 23, 2016
    Fabulous, and yes, I am a complete novice to this side, but mechanically I think I am competent. I measured across 9 and 10 and sure enough I got 27V , I just didnt know the proper way to measure them. ie, RMS or using chassis ground (not). If I measured the AC off 9 and off 10 seperately using chassis ground, that is where I erred as I read 14 on each leg...I guess I was just used to DC probing like that. Thanks! I will post again with silly novice questions I am sure. This schematic is out of a service manual that shows all the proper voltages off the transistors so Ill post a few more discoveries. The unit works, it just has some anomalies in the record voltages.
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