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  • More JC-120 woes

    A while back, I started a thread about replacing the driver transistors for my JC-120. It was a very fruitful thread, and I learned a lot about SS amplifiers. Unfortunately, some of the information I posted about my particular amp was not accurate. I thought it was based on the '79 schematic that is floating around, but it is not. It is also not like the '82 schematic. I think it is somewhere in between.

    The board numbers are AP-113 (141-113), AP114 (141-114), and PS-56 (146-056). If anybody has schematics for this amplifier, that would be awesome. It is not any of the three common JC-120 schematics often found.

    I made some progress perhaps but the symptoms confound me.

    Anyway, I finally got the amp back to my house, and fired it up. A 330 ohm resistor at the collector of the PNP driver was burning up (if you have the '82 schematic, this part of the circuit is identical as far as I can tell). I discovered that the 1/4w 10ohm fused resistor from the collector of the PNP to the base of its corresponding power transistor was burned out, so I replaced it to see if that would do anything good. The amp did produce sound for a while, mainly a loud hum that sounded like it was maybe 120Hz or so. The (fresh) 330 ohm resistor wasn't even breaking a sweat though and I didn't see any smoke or smell anything bad from anything, so I considered that progress.

    I also noticed that when power cycling, if I strummed the guitar and turned off the amp, for a split second the hum would disappear and I could hear the guitar clear as day, as everything was discharging - indicating to me that on some level there is a complete signal path.

    But after power cycling a few times, I heard a crackle and the fuses popped. This happened as I was pressing down on the switch a little harder than usual, so I'm not sure if this was coincidental or I was making a more (or less) complete connection with the switch which caused the overload.

    Anyway, I'm stumped and I don't want to break it more than I have. I'm pretty intimidated by this very complex, high-power amp at the moment, but I want to get it fixed.

    Oh, also as far as I can tell the power transistors all work, but they are not TO-3s, they are a package called SIP-flange. The transistors are 2SD736A and they are quite obsolete. If I do have to replace one or more of these, are there any modern replacements which have a compatible package?

  • #2
    These are the specs I could find for the 2SD736A (I think these are the 2SA736AB, at least the package says "D736A" above "OE3" and "B" in a circle, with a symbol that looks a bit like a crosshair to the left):

    2SD736AB
    Si NPN Power BJT
    Hitachi Semiconductor
    V(BR)CEO (V)=140
    I(C) Abs.(A) Collector Current=12
    Absolute Max. Power Diss. (W)=100
    I(CBO) Max. (A)=1.0m
    h(FE) Min. Static Current Gain=60
    h(FE) Max. Current gain.=120
    @I(C) (A) (Test Condition)=5.0
    @V(CE) (V) (Test Condition)=5.0
    f(T) Min. (Hz) Transition Freq=25M
    t(f) Max. (s) Fall time.=400n
    Status=Discontinued
    Package=SIP-Flange
    Military=N

    Also I've attached the '82 schematic, don't take that as the gospel as my circuit is different in places, but the output stage is very similar.
    Attached Files

    Comment


    • #3
      Yes, it is a complex amp compared to a Fender Champ, but if you break it down into individual sections it's not that difficult to figure out.

      There are two power amps correct? Are they both having problems or is it just one of them? Have you checked all of the output and driver transistors? How about the power supply is it working and stable?

      Until you find out the cause for the blown fuses, you will not be able to continue troubleshooting. Do you have a light bulb limiter?

      Comment


      • #4
        "Oh, also as far as I can tell the power transistors all work"
        Check them with a meter or better still a transistor tester.
        See this thread http://music-electronics-forum.com/t10098/

        Ye olde batwing Hitachi.
        Equiv = Sanken 2SC2921 The mounting holes line up with the "batwing" Hitachi.
        You will only need longer mounting bolts as they are thicker.
        Attached Files

        Comment


        • #5
          Originally posted by 52 Bill View Post
          Yes, it is a complex amp compared to a Fender Champ, but if you break it down into individual sections it's not that difficult to figure out.

          There are two power amps correct? Are they both having problems or is it just one of them? Have you checked all of the output and driver transistors? How about the power supply is it working and stable?

          Until you find out the cause for the blown fuses, you will not be able to continue troubleshooting. Do you have a light bulb limiter?
          There are two power amps. I'm working on the one on the board that is not on the same board as the power caps and effects - Channel 1. It's hard to say whether the other one is having problems, because I pulled the drivers from that channel, probably so I would have some known good drivers to test channel 2. It did work before, as I recall, but that was like 3 years ago.

          I have checked all of the output and driver transistors in channel 1, they are all good.

          You're right, I should make a light bulb limiter. I don't have much experience with amps so I miss these things.

          Comment


          • #6
            Originally posted by oc disorder View Post
            "Oh, also as far as I can tell the power transistors all work"
            Check them with a meter or better still a transistor tester.
            See this thread http://music-electronics-forum.com/t10098/

            Ye olde batwing Hitachi.
            Equiv = Sanken 2SC2921 The mounting holes line up with the "batwing" Hitachi.
            You will only need longer mounting bolts as they are thicker.
            Alright! It's good to know that there is a sub available for these. I did test the 2SD736As with my meter, I don't have a transistor tester handy so I just checked for a low resistance from base to emitter and from base to collector, and a high resistance when flipped (and a high resistance in either polarity from emitter to collector).

            Comment

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