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Discussion in 'General Electronics Discussion' started by bill999, Jan 8, 2018.

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  1. bill999


    Nov 10, 2011
    Hi All,

    I am repairing a 44 year old Tape deck and have found some suspicious transistors. I am trying to find equivalent replacements, but am not having much success (mainly due to my lack of knowledge).

    There seems to be a ton of properties of transistors and I am wanting to understand which ones MUST be a match, which ones are not important and which ones can have a higher or lower rating than what I am after.

    These are some of the properties I have found :

    Material Used
    Order of C/B/E pins
    Collector-Base Voltage
    Collector-Emitter Voltage
    Emitter-Base Voltage
    Collector Current
    Base Current
    Collector Power Dissipation
    Junction Temperature
    Storage Temperature
    Collector Cut-off Current
    Emitter Cut-off Current
    DC Current Gain
    Base-Emitter On Voltage
    Collector-Emitter Saturation Voltage
    Current Gain Bandwidth Product
    Output Capacitance
    Noise Level

    I am trying to find replacements for 2SC1222, 2SA640, 2SC945 and 2SC1247A

    Thanks for any help anyone can offer.

  2. dorke


    Jun 20, 2015
    There is zero chance you can find an equivalent that will match 100% on all parameters.
    The most important ones are Type,Breakdown voltages,max currents,Wattage,Beta,and Ft.

    A suitable replacement will depend a lot on the circuit the original was in.
    Most times Trs aren't "stretched" to their limit,that allows for a lot of "slack".

    Can you provide a circuit diagram or a model and make number?

    Why do you think the TRs are defective in the first place?
  3. AnalogKid


    Jun 10, 2015
    For *signal* transistors (as opposed to power transistors):

    Material Used - SAME
    Polarity - SAME
    Order of C/B/E pins - not important, but pay attention!
    Collector-Base Voltage - ignore
    Collector-Emitter Voltage - same or larger
    Emitter-Base Voltage - ignore
    Collector Current - same or larger
    Base Current - ignore
    Collector Power Dissipation - ignore
    Junction Temperature - ignore
    Storage Temperature - ignore
    Collector Cut-off Current - ignore
    Emitter Cut-off Current - ignore
    DC Current Gain - same or larger
    Base-Emitter On Voltage - ignore
    Collector-Emitter Saturation Voltage - ignore
    Current Gain Bandwidth Product - close is good enough
    Output Capacitance - ignore
    Noise Level - same or better

    Keep in mind that the original transistors were not the perfect parts for the job. They were selected at least in part because of cost, or because they already were in the company inventory from other projects, or were parts the designer was already familiar with (maybe his first transistor (seriously)), etc.

  4. bill999


    Nov 10, 2011
    Thanks for replying.
    The board I am trying to repair is an amplifier board from an Akai GX400D. The board part number is TW-5032
    I don't have a good quality circuit diagram available to hand, but could probably find one somewhere.

    The problem I have is that I have lost sound from the left channel. The amplifier unit of the tape deck has 2 boards for playback (left channel and right channel). if I swap the boards, the lost channel follows the board so I know its the board. The reason I believe the transistors are defective is simply that I have tested them and get a bad result. I am testing them in place, which is not always a good idea, but if I test the same components on the known good board, I get a good result. To me, that does suggest they are defective.

    I will continue to look for replacements based on the important properties you have mentioned.
  5. bill999


    Nov 10, 2011
    That looks like some great info there. Thank you. I hope I can find some suitable replacements based on that info.
  6. AnalogKid


    Jun 10, 2015
    Grand old deck. Viet Nam vets brought Akai back with them; they blew away anything in the US consumer market at that time. Cross field bias, specs out the wazoo.

    Nothing wrong with your technique. Without a schematic there will be a lot of trial and error, but as long as you don't hurt the pcb traces swapping parts you should be ok. What test equipment do you have?

    hevans1944 likes this.
  7. 73's de Edd

    73's de Edd

    Aug 21, 2015
    Sir bill999 . . . . .

    I have good enough documentation here, to work with easily.
    That is not being too much of an antiquated unit at all.

    It's using "common as dirt " . . . . most honnable . . . Japonie . . . " silicron tlansistors ".

    2SC1222------2SA640------2SC1247A and 2SC945.

    Now, of TR1-----TR7 which are you picking fault with ?
    Plus . . . you also have some 13 odd electrolytic capacitors being on this board, but fortunately, their degenerative failure should result in a diminutive performance decline instead of creating a dead unit, which I surmise is what you have on this left channel.

    AND do you minimally . . . . . have a DVM and knowledge to use it in its diode test mode to test a transistors 2 junctions ?
    With there being a set of two of everything, you can compare evaluative tests between like numbered units and test the sole '1247 as a '945 in like comparisons.

    Do you have a set of headphones to use for monitoring ?

    Fill me in . . . . .



    73's de Edd

    Last edited by a moderator: Jan 8, 2018
  8. Cannonball


    May 6, 2017
    Check voltage on the transistors that you suspect are bad and compare these voltages with the transistors on the good board.

    If they are different, take them out of the circuit and test them. Replace if they are bad.

    There is a way to test them in circuit but you have to be real careful not to short something else out. Monitor the collector voltage and short the emitter to the base. This should turn the transistor off. Replace if bad.

    Transistors don't wear out unless they are abused. This may save you some time.
  9. AnalogKid


    Jun 10, 2015
  10. Audioguru


    Sep 24, 2016
    Bill999, you do not say where you are and if you will use Japanese, European or American replacement transistors.
    Look at the pinouts on their datasheets to see that they are different in those different worlds.
  11. bill999


    Nov 10, 2011
    Yes, its a wonderful deck :) I've had it for a while - done a complete restoration about a year ago. Just needs a little TLC every now and then.
    As far as test equipment, I am limited to just a multimeter. I am ok with trial and error, thats the way I normally get stuff done - and I am very careful when soldering and desoldering to not damage anything.
  12. bill999


    Nov 10, 2011
    Hi 73's de Edd,
    Thanks for replying and taking an interest in my problem.

    The transistors I have fault with are TR6 and TR7. I am pretty sure they have failed as they do not check out with the multimeter. I am an amateur, but do have the knowledge as to how to test components such as these. I am also testing the same components on the left and right playback boards as I know one is a good board, and then comparing the results.

    When I did the original 'bringing the machine back from the dead' I replaced all of the caps on both of my playback boards, so I am happy that these are good.

    I do have headphones and when using them, I get perfect sound from both channels.

    With the testing I have done and the comparison in results between the good and bad board, I am pretty sure that those 2 transistors are the problem. I Just need to find suitable replacements and try them. If they are at fault, I will most likely replace the other transistors on both playback boards with new ones.
  13. bill999


    Nov 10, 2011
    I am in the UK. I have not not looked at how transistors differ between regions, but will take a look. Thanks.
  14. kellys_eye


    Jun 25, 2010
    An old electronics magazine used to have a basic transistor use methodology that defined them as TUN or TUP, Transistor-Universal-N(PN) or P(NP).

    They specified basic minimums for Vce, Ic, Ptot and gain etc and they showed many circuit designs where the specc'd transistor was simply TUN (or TUP). Most enthusiasts would have a typical choice somewhere in their scrap box.

    As a result I've always kept the BC547/557 (or 548/558) pairs (NPN and PNP respectively) which are easily available in the UK as well as many other places. Years ago I'd have used BC108 for the common NPN version. Times change.

    The American equivalent might be the 2N3906 and 2N3907 (?)

    Either way I reckon you'd manage with the BC547/557 for T6 and T7 respectively. Try a place called ESR Electronics (situated in Cullercoats) as suppliers. Very reliable, very cheap, very helpful.
  15. 73's de Edd

    73's de Edd

    Aug 21, 2015
    Sir bill999 . . . .

    If being TR6 and 7, that's being about half way down your daisy chained audio flow path, from origin at far left to the output into the headphones at far right.

    Do you have any electronics " junque " that we can scavenge for its transistors, as just about ANY common small plastic case, small signal NPN and PNP transistor is absolutely going to suffice as a sub part in testing.

    ONLY need to know the part ID to confirm if the lead layouts of BCE or EBC are the same as the original, or if having the need to swing one lead around to make the same basing.

    That might require mini wire extensions soldered on in case the sub part has short leads.
    If any questions on potential found transistors to be used, pass on #'s and we will research compatibility.
    BUT those transistors are absolutely no problem to get a la USA..

    Aside . . .
    The American equivalent might be the 2N3906 and 2N3907 (?)
    Highest echelon older technologists would probably be having / using:
    2N2222 NPN

    Preferring to NOT pull parts from your good working R channel board.

    73's de Edd
    Last edited by a moderator: Jan 11, 2018
  16. Alby


    Jan 9, 2018
    73's de Edd likes this.
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