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Output Transitor Circuit Inconsistencies

Discussion in 'Audio' started by p1truman, Jul 30, 2018.

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  1. p1truman


    Jul 30, 2018

    I am trying to fault find a problem with an amplifier/mixer that was instantly blowing fuses when powered up.

    The unit is the IMG Stageline PX-400

    I managed to obtain some schematics and found that one of the main output transistors (TR217 MJ15004) on the audio left side of the amp was was short circuit.
    I have replaced the output transistor along with the bias transistor (TR216 2SB1186A) however when testing the resistance across the base and emitter of the output transistors I am seeing some variance in readings.

    I have attached a schematic of the left side of the audio amp circuit in question and was wondering whether someone could tell me what sort of resistance I could expect between the two points detailed on the schematic.

    The resistance I am seeing across the two points (detailed in red) is 12.3 ohms, which I believe is a summing of the two resistors (R236 and R238). However when I test resistances across the other output transistors across base and emitter I am seeing 10.1 ohms (detailed in green)

    On the audio right hand side of the amp I am seeing resistances of 10.1 ohms across both output transistors.

    As I say would be grateful if someone could take a quick peep and let me know what resistances I should be expecting at these two points.

    To me it seems as though 12.3 ohms is the value I should be expecting but seems strange that R238 and R237 could be short circuited on both left and right sides of the amp

    Thanks in advance

    Look forward to hearing your views

    PX-400 Right.jpg Closer view for component values.jpg
  2. p1truman


    Jul 30, 2018
    These resistances are being measure whilst the circuit is cold (no power)
  3. BobK


    Jan 5, 2010
    Try reversing the leads of the ohmmeter and make the measurements both ways. The measurement includes a semiconductor junction which will conduct with the leads in one direction, altering the measurement.

  4. p1truman


    Jul 30, 2018
    Thank for the response Bob

    Tried reversing the leads, same result

    The variance in resistance I am seeing (12.3 ohms and 10.1 ohms) seems to be too closely related to the resistor values of R235 (10 ohms) and R237 (2.2 ohms). Almost as though R237 (2.2 ohms) is being shorted.

    Assuming that output transistors M15003 and M15004 are out of circuit. What resistance should be expected at the two points mark on the schematic?
  5. kellys_eye


    Jun 25, 2010
    That's not the way it's done.... with the transistors out of circuit you can do a static test on them to determine if they are dud on any junction (you can do this in-circuit on most occasions).

    The other method is to measure the voltages at the base, emitter, collector and determine the transistor condition from that.

    In that type of amplifier the point 'B' should be at zero volts (near enough) with no input signal. If it is +ve (towards the +50V rail) then the top transistor is 'on' or shorted. Similarly if it is -ve (towards the -50V rail) then the bottom transistor is suspect.

    You can put a resistive load in the supply leads to limit the current in the event of short-circuit faults and do the relevant voltage tests from then on.
  6. p1truman


    Jul 30, 2018
    kellys_eye - Thank you for your response

    The output transistors have already been removed and the junctions tested. It was found that M15004 from the right audio channel was short circuit and has therefore been replaced

    Since I have right and left channels of the amplifier I have a second side to compare and cross reference resistances.

    All resistances between the other 3 M15004 and M15003 output transistors are reading 10.1 ohms however the one that I have replaced is reading 12.3 ohms

    Looking at the schematic I just wanted to try and understand which resistance is the expected resistance, 10.1 ohms or 12.3 ohms

    Thanks again
  7. davenn

    davenn Moderator

    Sep 5, 2009

    if that is between a couple of the various transistor pins out of circuit, they are all faulty
  8. p1truman


    Jul 30, 2018
    Thanks for your response davenn!

    The stated resistances of 10.1 and 12.3 ohms are resistances IN circuit

    I'm pretty sure that the output transistors are all ok

    I just want to know what the expected resistance should measure of the surrounding circuits, imagining that the output transistors are out of circuit. It should be assumed that multi meter probes are placed on the coloured dots marked on the schematic. Measurement 1 - Red dots, measurement 2 - Green dots

    Last edited: Jul 31, 2018
  9. kellys_eye


    Jun 25, 2010
    Your methodology is weird..... 'no-one' does that to establish whether a circuit is ok or not...... however, the measurement points you indicate are basically two resistors in series; the 10Ω and the 0.22Ω resistors. The readings you get will be 'ok' if you accept 10% tolerances although the 12.3 ohm reading is a little high - it won't make any difference to the basic circuit operation with the transistors back in circuit - everything else being ok of course......

    What do you expect to gain or learn from these two readings?
    davenn likes this.
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