Need Help with Amp

Discussion in 'General Electronics Chat' started by dexterdoo, Jan 31, 2014.

  1. dexterdoo

    Thread Starter New Member

    Jan 31, 2014
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    I have a turntable head-amp that I've had for about 30 years and it recently died. I've attached a copy of the schematic diagram for it and the thing is so simple 'til I cannot believe I haven't been able to repair it??? I've checked the parts and found nothing bad and when the unit receives power from the 9 volt battery, all of the transistors seem to have the correct bias voltage on them, but still there's no audio out (checked with my O-scope)! I don't get it??? I'm wondering if someone could point me in the right direction in terms of repairing this amp? Thanks!


     
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    If I have DC (the bias) but no AC (the music) I'd start looking at the caps. 30 years is several cap lifetimes, and one or more may now be open.

    Shorted caps have the same voltage on both leads, but so do caps not blocking DC so it takes some skull sweat.

    Open caps are a breeze to check: just put another cap in parallel and see if musical excellence returns.
     
  3. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    You sure that schematic is complete? I don't see how it can work as a turntable amp.
     
    #12 likes this.
  4. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    I don't see how it can work as ANY kind of amp! There is no load resistor on the transistors at all! At first it looks like a push pull grounded base configuration....but the collectors are shorted together!

    Weird.

    Eric
     
  5. Dave_UYZ

    New Member

    Jan 16, 2014
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    Lets face it, it's mostly a filter of known slope, rather than a straight Amp (look at the strange values of R). It's an Emitter Follower of sorts, so there's not a lot of gain in it.

    I agree with the "check the caps" suggestion.
     
  6. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Collector are short only for AC signal. And If you drawn the small signal equivalent circuit You will see two common base amplifier connect in parallel.
    Also notice that the 9V battery work as a floating power supply.
    http://users.ece.gatech.edu/~mleach/headamp/
     
  7. crutschow

    Expert

    Mar 14, 2008
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    Yes it is a common-base push-pull amp as KL7AJ and Jony noted, likely to amplify the small output from a moving-coil cartridge. It looks a little strange since it depends upon the floating 9V battery power supply for proper operation. I'm still scratching my head a little trying to follow how it works. R7 is the AC output load for the two collectors.

    I simulated an AC gain of about 43 with R7 = 3.9kΩ.


    The only frequency roll-off caps are C1 and C2 which give a corner frequency of about 3MHz. All the others are just filter or coupling caps.

    As others have noted, I expect one of the (likely electrolytic) caps has gone bad in your amp. Try following the signal from the input to the output with your oscilloscope.
     
    Last edited: Jan 31, 2014
  8. dexterdoo

    Thread Starter New Member

    Jan 31, 2014
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    Yes, the amp is for my moving coil cartridge and I'm pretty sure that is the correct schematic. Obviously it's only showing one of the channels. I suspected the caps also. I checked every one with my DVM (don't have an actual capacitor checker) and didn't notice anything suspicious although I may've missed it? I'll go back and check them again. The DVM should be ok to test them, right?

    What a cool forum! Found it out of desperation after spending waaay more time on this project then I intended to. I'll feel like a dork if I have to find someone to repair this for me but I'm gonna give it one more shot. I'll post back and thanks again guys! :)
     
  9. crutschow

    Expert

    Mar 14, 2008
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    The DVM will check that the cap is not shorted but won't check if it's open.

    Are both channels bad?
     
  10. #12

    Expert

    Nov 30, 2010
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    +1 on the head scratching!
    Never seen such an animal before.
    If anyone wants to elaborate on how it works, I'll be watching.
     
  11. dexterdoo

    Thread Starter New Member

    Jan 31, 2014
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    @ crutschow
    Yes, voltage all over the place but nothing from either channel. Won't checking out of the PCB for opens work?

    @ #12
    Not so sure about this schmatic now because there are actually (10) 220uf caps on top of the board and (9) on the bottom which doesn't exactly square with the diagram even if you double the amount to allow for the other channel. The "final Inspection" tag inside the unit is dated 7-21-80! That being the case I think I will change all 19 of the caps and see where that gets me. I've already ordered them.
     
  12. #12

    Expert

    Nov 30, 2010
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    Probably a smart thing to do. Forget the head scratching and shotgun it. :D
     
  13. dexterdoo

    Thread Starter New Member

    Jan 31, 2014
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    I agree ... LOL!!! :D
     
  14. Jony130

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    Here you can read from original designer how this circuit work
    http://users.ece.gatech.edu/~mleach/headamp/
    And simply draw a small signal model and you will see how this circuit work.
    So you never see a amplifier with a floating power supply?
    http://en.wikipedia.org/wiki/Circlotron
    See also QSC amps,Peavey amps and Yamaha A-S1000; A-S2000; A-S3000
     
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  15. #12

    Expert

    Nov 30, 2010
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    Yes. We did some a few weeks ago with a model number something like 700A or M700. I just can't see how to drive the emitters of a push pull stage to get a voltage gain.

    I learn something almost every day on this site.

    Edit: OK. That helped. Thanks for the link.
     
    Last edited: Feb 1, 2014
  16. crutschow

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    Mar 14, 2008
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    In a common base amplifier the AC collector current is essentially the same as the emitter current generated by the input source voltage (differing only by the base current). The voltage gain is derived from the collector resistance load being much larger than the equivalent emitter resistance.

    In the subject amp, when one transistor emitter/collector current increases from the signal voltage, the other emitter/collector current decreases. The two AC currents are opposite polarity thus they add at the output load (through the output caps C9 through C12), since the collector resistance of each transistor is much higher than the output impedance. This summed current flowing through the output load R7 generates the voltage gain. The gain value is thus proportional to the value of R7.
     
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  17. Gones

    New Member

    Feb 1, 2014
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    What is Amp. what does it mean. maybe it is related with electronic device. if this is then i wanna told you on thing that is e cigarette. this is also a electronic device. hope we can discuss with this later.
     
  18. redplaya

    Member

    Jan 26, 2014
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    I agree with everything CRUTSCHOW said. The industry standard for lifetime on electrolytic was 10yrs where I was, so with that said who knows what complex impedance they are now. After that check the BJTs but it unlikely anything's wrong. After that, throw it away.
     
  19. crutschow

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    The is off-topic and hijacking of this thread. Please start a new thread with your questions.
     
    Last edited: Feb 1, 2014
  20. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    What hell is Gones talking about?
    I was reading through and that guy threw me off the topic.

    To the OP. Try replacing all the electrolytic. That is what I would do first
     
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