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Rickenbacker TR35B....low distorted output

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  • #1

    Rickenbacker TR35B....low distorted output

    This amp was apparently sitting in the garage for several years after it stopped working. It would power on now, but no sound. Signal out of the effects send into another amp was normal, tone controls all working fine. I found a couple of burned resistors and shorted transistors on the power amp board. Still no sound. I replaced the 4500uF caps and a couple of other electrolytics. I have sound now, but it's not very loud and it's distorted. I twiddled the DC balance and Bias pots to dial in the sound better, but it's still weak. All the resistors are in spec, diodes check out ok, and I also replaced the 0.1uF coupling caps.

    One interesting voltage reading...on Q13, the NPN output transistor, there's -36V on the emitter and -20V on the base, while on Q9 (PNP), there's 36V on both emitter and base. I replaced both Q13 and Q9...no change. There is 135mVDC on the speaker connection. Anyway, thanks for your input on solving this....

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  • #2
    To have those kind of readings on an NPN transistor, the B-E junction must be open. The B-E junction is just like a diode, when forward biased the max. voltage it can have across it is around .7VDC.
    Could it be a PNP there?
    Originally posted by Enzo
    I have a sign in my shop that says, "Never think up reasons not to check something."


    ​

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    • #3
      q13 is an NPN

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      • #4
        Then it must be bad. Those voltage readings are impossible on a good NPN.
        Originally posted by Enzo
        I have a sign in my shop that says, "Never think up reasons not to check something."


        ​

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        • #5
          Shouldn't some of the -36v on the emitter ending up at the base via the 470r resistor? On the PNP side, emitter and base each have 36V and have the same 470R resistor as on the NPN side.

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          • #6
            Yes, the NPN should be giving you roughly equal but opposite readings or the PNP (Q9). So you should have around -36V at the base of Q13. The most positive it can go is about -35.4V. The b-e junction of an NPN is like a diode, emitter is the cathode, base is the anode. When it is forward biased the anode can only ever be about .6 or .7V more positive than the cathode.
            But you are measuring -20V at the base and -35V at the emitter. Are you measuring right at the leads of the transistor? What is the Q13 part number and are you sure it is installed according to the schematic? (base to 470ohm/Q12 junction, emitter to -36V line)
            Originally posted by Enzo
            I have a sign in my shop that says, "Never think up reasons not to check something."


            ​

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            • #7
              ok....I measured the negative dc on the base of npn q13 while I turned the dc balance pot...I now have 37vdc on both emitter and base....but I still have low distorted output. sounds splatty like choked down blocking distortion.

              btw, q13 is a 2n5991 equivalent (nte182), and it is installed correctly. it meters normally when out of the circuit

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              • #8
                Q8 was bad...replaced it and amp is fixed

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