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TIP120 Transistors Appear Charred

Discussion in 'Troubleshooting and Repair' started by vestaviascott, Jan 15, 2019.

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  1. vestaviascott


    Jul 28, 2013
    Transistor equivalents are TIP 102 (not 120)
    From appearance are these 2 transistors toast? They do appear to have continuity unless I'm measuring wrong.

    transistors-burned.jpg transistors-burned2.jpg

    309 left+ to right- = 11.9
    309 left+ to middle- = 11.9

    307 left+ to right- = 5.8
    307 left+ to middle- = 4.1

    305 left+ to right- = 7.6
    305 left+ to middle- = 1.2

    303 left+ to right- = 9.3
    303 left+ to middle- = 9.0

    301 left+ to right- = 5.8
    301 left+ to middle- = 4.8
    Last edited: Jan 15, 2019
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    They look fine from what I can tell (but it's hard to see to much detail on a phone).

    What are you measuring (is it voltage)? Do you have a diode test range on your meter? Use the diode test range with the power off and the filter capacitors in the power supply discharged.

    You should read the values both ways, and now the reading in your meter when the process are not touching anything. As an example, your meter may read 2.8 when the process are not attached to anything, and one pair of pins on a transistor might read 0.602 in one direction and 2.8 in the other.
    Last edited: Jan 15, 2019
  3. 73's de Edd

    73's de Edd

    Aug 21, 2015
    Sir vestaviascott . . . . .

    What gives ?

    I am seeing (5) TO-220 cased power transistors and their smaller companion TO-92 drivers.
    I see no heat sink compound residue on the faces of their heat sink tabs . . .unlesss you have cleaned it off.
    I see that flat spaced apart metal heat sink ? strip . . . . but no extra connective heat sink mass between it and the sink tabs of the individual transistors.
    Or do the units just run naked, without additional heat sinking ?
    What are the transistors loads . . . and if being inductive loads . . . the back EMF quench diodes must be mounted apart from this board, right at the inductors.

    The 3 RCA units ABSOLUTELY seem to be bonafide RCA units, such as I have see them thru the decades with their markings , they even have the RCA EIA identifier of 274 stamp pressed within their heat sink tabs :

    The 393-1 is being the transistors drawing number, stock identifier number or a custom requested ID for the buyer.
    Or if being Magnavox related . . .(Blue PCB) . . . only the last 3 digits and dash number of their typical 6-7 digit full part number .

    The 7739 . . . .7735 . . .and 7733 are being manufactiring line date codes of the 39th 35th and 33rd week of 1977 year.

    And now . . . .for your two little odd ducks.
    Is there a possibility of the BLACK on the heat sink tabs being a flat black paint mist ?
    Or does it seem to be a chemically, time formed black oxide ?
    In times, have seen some select few transistors that had a silver flash coating to them and would oxidize to a dark black with atmospheric exposure / introduction of acidic, alkalinic or sulphuric contents.

    Do they seem to have untampered original factory soldering on their 3 terminals, such as the 3 RCA's probably still do.

    If NOT, I certainly do not remember that LOGO that they are using . . . . in that 1970's time frame era.

    If replacements it's almost like they might be decades later bogus unit replacements.

    Its like one hunts for this transistor and finds some opportunistic supplier that ascertains if it is NPN or PNP and standard or darlington and then the breakdown voltage rating and current guesstimate and then pulls a like generic unit and takes it to the back room to mamma-san with her rubber stamp dies, tweezers and their holder and then creates the new part numbering ID with white ink on the stamp pad.
    The real kicker is being the replicating and re use use of the current time " red herring " 77xx series code date on their unit .

    Also your Vf readings supplied for the testing of units are WAAAAAY off base IF being in diode test mode and having tried BOTH of the meter lead polarities possibillities .
    .500 - - - - -.700 . . . . . .or 500 700 mv on your meter would be the suspected Vf voltage range, expected to be read for a silicon transistor.
    But you DID seem to be testing from base to the collector and then base to the emitter.
    If you do happen to be testing a darlington type, the base to emitter reading will almost be twice the value of a standard transistor.

    If I look up that ONLY 393-1 identifier in a cross referencing . . . . . . every supplier wants to make it a NPN in a much smaller T0-5 metal case.

    Soooooooo fill us in and . . . . .what do that board do ? . . . and in what brand of equipment ?

    73's de Edd . . . . . .
    . . . . . . . . . . .

    Avoid parking tickets by leaving your wipers turned to 'fast wipe' whenever you have to leave your car parked illegally.
    Last edited: Jan 16, 2019
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