Negative readings on voltage tests?

Discussion in 'General Electronics Chat' started by superduper, Feb 21, 2015.

  1. superduper

    Thread Starter Active Member

    Dec 5, 2010
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    Sometimes, in the course of repairing small portable radios, etc, I perform voltage checks and compare to the values specified in service manuals.

    On more than 1 occassion, I observed negative values. Typically, this is seen on transistors. Since ground probe is at ground (-), and these are DC powered devices, I don't understand how the readings can be negative.

    For example, specs say voltages for B of Q1 should be 0.00. However, I might read -0.6v.

    So asking if someone can explain to me the reason for this. Meter is good, its' a fluke 177 and I have obtained the same readings using my fluke 189-II as well so it's not something to do with the meter.

    Thanks for any response or advice.
     
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    With nothing else to go by I would have to offer you had your probe tips swapped.

    Next time you see this stop back and post again with the details fresh at hand.
     
  3. superduper

    Thread Starter Active Member

    Dec 5, 2010
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    volt_measurements.jpg

    For example, here is a recent repair I did. It's for a tuner and there were numerous printed resistor and trace failures. It's all fixed now but the neg reading persists on that one transistor. I'm 100% positive that the probe was not reversed because the negative probe was clipped to the negative feed connector and only the positive probe was used to take readings, jumping from one spot to the next. Only Q4 shows the negative reading. Everything else was positive and once again, I only wielded the positive probe. Since the radio is fixed and working well, I'm not too concerned about it but this isn't the first time I've seen this before so I'm just curious why I get readings like that so I'd understand and not freak out next time. Weird.
     
  4. WBahn

    Moderator

    Mar 31, 2012
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    We can't tell too much without some kind of schematic.
     
  5. Aerb

    New Member

    Jul 9, 2014
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    If the transistor you mentioned is a pnp, then it is not surprising to see a negative reading on the multimeter, since base-emitter junction voltage (Veb(on)) should be negative in order for the transistor to work. To be more clear, in order for a pnp transistor to work in desired mode, base voltage should be 0,6 volts lower than the emitter voltage. Which suggests that, in contrast to npn transistors, current flows from emitter to base.
     
  6. MrChips

    Moderator

    Oct 2, 2009
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    Think of how a charge pump works. It is quite possible that if you have a alternating signal, some PN junctions and capacitors you can end up with a negative voltage.

    upload_2015-2-21_21-12-31.jpeg
     
  7. superduper

    Thread Starter Active Member

    Dec 5, 2010
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    schem.gif

    Ok, I've attached the schematic for the circuit in question. Q4 in this case, is an NPN transistor. Resistor R19 had failed open. Black meter probe was clipped to ground at CP1. Red meter probe was used to take all mesurements. When all was said/done, many resistors and traces had failed open, or values changed to something much higher (these traces and resistors were all printed on the top/component side with what looks like carbon compound, like you would find in a potentiometer). All measurements were taken before repair and replacement of the failed traces and resistors. Base/Q4 is supposed to be 0v. It read -0.6v. I could not understand how I could obtain a neg reading with black probe at the ground supply terminal. If black probe was affixed someplace else, I could understand ground potential might have been higher than that observed at Q4, but in this case, it seemed to me that where I had it clipped should've been as close to 0 as possible.

    Perhaps that part of the circuit is behaving a bit like what MrChips described?
     
  8. WBahn

    Moderator

    Mar 31, 2012
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    It's hard to say what is going on. The fact that there were many failed resistors and traces indicates that this circuit board has seen some severe stress of some kind. I notice that the signal that seems to be driving the base of Q4 is the "meter" output of IC2. What is the voltage at that pin? If it is negative, then you might isolate that pin and see if it is giving you a negative voltage or if the voltage at the base of Q4 is still negative even with IC2 removed from that net.
     
  9. superduper

    Thread Starter Active Member

    Dec 5, 2010
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    The radio has already been reassembled and it's a rather large one so I'm not particularly interested in disassembling it again to take readings, much less taking that 18pin IC out of circuit, although I suppose that's the least difficult part of it since I did put it on a socket to facilitate much of the repair and resistance measurements. As for the board being stressed, actually almost all of the radios I work on are 35-40 years old and the more complicated ones use double sided boards but the top traces are not copper, rather they appear to be printed traces with a conductive carbon material. Looks like that's how the early double sided boards were manufactured. Same with all the top side resistors attached to those topside traces, carbon printed, not discrete. The only stresses that are evident is environmental stress from corossion and degradation of those traces from that corossion. None of the discrete components showed any damage, nor did any of the copper traces on the bottom foil side.

    Yes, pin 15 of IC2 is for RF strength meter, and it read 0v, as it was supposed to. However, at the time of initial testing, the trace that connects that pin to the resistor that feeds Q4 b was OC.

    I guess the reason for my question is that I just did not expect to ever see any negative readings on a pure DC circuit with the black probe of a voltmeter planted on the negative ground rail, and as it's not the first time I've seen this phenomenon, I was simply wondering what possible reasons or conditions could account for readings like that, and MrChips did state one such condition and C29 is connected to the resistor feeding Q4b, but I've no idea if it's presence in this circuit would account for that behaviour.
     
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