Sanui 3000A Volume Control and Output

Discussion in 'Exclusively Sansui' started by rickygallagh, Apr 19, 2018.

  1. rickygallagh

    rickygallagh New Member

    Messages:
    37
    Have a friends 3000A that he wanted me to check out.

    Cleaned and dexoit'd the controls.

    I had one of these before and knew there was a mod required to save speakers.

    This one I believe does not have the mod. ( Need to look again)

    Upon powering up, it worked and sounded good. I checked the DC volts at the speakers and it was 4 volts.

    Pretty sure that is not right, also setting the volume control at zero, still results in sound.

    Any advice would be appreciated, I think another post talked about someone developing a mod board but don't know where to get it.

    Think I should tell my friend to send it to someone who knows more.
     

     

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  2. dr*audio

    dr*audio Fish fingers and custard!

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    It should not have any dc voltage on the speaker terminals if it has the mod. Part of the mod is to put capacitors in series with the speakers to block dc voltage.
     
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  3. rickygallagh

    rickygallagh New Member

    Messages:
    37

    Checked speaker outputs again. This time on tape mode instead of tuner mode. Was less than 1 volt. Should not have any d c volts? Told him he should look for some expert tech.
     
  4. dr*audio

    dr*audio Fish fingers and custard!

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    The best way to check the dc on the output is to connect the speaker outputs to load resistors, because if it does have the capacitors on the outputs they can hold a charge if the outputs are not loaded, and you will measure some voltage there. You could also use speakers you don't care about to do this.
     
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  5. tom3

    tom3 Super Member

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    1,901
    Location:
    So. Ohio
    I have a 3000a that has sound with the volume control down also. Cleaned the control really well, made it worse. Bought a replacement control but looks like a real bear to replace it. Just living with it at the moment. Might do a search on here, I posted pictures of modified and as built 3000a receivers for reference. Those Sansuis are pretty impressive - when right.
     
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  6. McGowdog

    McGowdog Active Member

    Messages:
    135
    Location:
    Pueblo, Colorado
    I have a Sansui 3000a which has about 5 volts on the left channel. The right channel, not that problem.

    When I first powered the rcvr up, right channel sounded ok and left channel badly distorted. So got inside the unit, deoxed it, looked around, saw a bank of four fuses. The - side of left channel is blown.

    Something going on with amp? Preamp? Don't know, have service manual on order.

    Replaced fuse and ohmed some things out, no obvious signs of damage, so plugged back in. No real signal from either channel besides slight crackling, still have 5 or so volts dc coming from left speaker, so hooking outputs up to 250 watt 8 ohm dummy loads.

    Heavy sob btw.

    Would like to fix this beauty looking rcvr someday soon. Will continue to dig for more info.

    Will keep you updated as I go along.

    20180507_211238.jpg
     
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  7. tom3

    tom3 Super Member

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    1,901
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    That's a nice looking Sansui, wood case is pretty rare on those. Well worth the trouble to get it up and running.
     
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  8. dr*audio

    dr*audio Fish fingers and custard!

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    18,648
    Location:
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    You should not be hooking up speaker loads when you have a DC offset on the output! That will draw all kinds of current and could blow the output transistors if they are not already blown. In this receiver there are 2 bias adjustment pots per channel and they affect the DC offset as well. You could easily have 5V on the output if they are off. Follow the procedure in the service manual and adjust them. You have to go back and forth between them a few times, checking the bias current and the output voltage. Then tweak one of them so the output voltage is 0V.
     
  9. McGowdog

    McGowdog Active Member

    Messages:
    135
    Location:
    Pueblo, Colorado
    Thank you. I will keep dummy loads hooked up and/or do what the service manual says. Don't think I blew output transistors yet. Sure don't want cascade failures either.

    Waiting for the manual and have my vtvm ready.
     
  10. dr*audio

    dr*audio Fish fingers and custard!

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    Location:
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    Don't hook up any kind of a load!
     
  11. kevzep

    kevzep Its all about the Music Subscriber

    Waiting for the Service manual? Just download it now.
    https://www.hifiengine.com/manual_library/sansui/3000a.shtml

    As the Dr says, do not hook that thing up to any load at all until you have it stable.
    You'll run into all kinds of trouble otherwise.
    I actually do not like these receivers, very early solid state designs, difficult to get them running right...and even then they are not exactly that stable...
     
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  12. McGowdog

    McGowdog Active Member

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    Ok, no load. I thought going open circuit on an output could be worse that short circuit with some amps. Also some require shorted inputs.

    Thought the Dynaco st120 was that way.

    Anyway, just got back from vacation and have pdf file for the Sansui waiting for me in my mailbox.

    Thx for the link btw.

    I, on the other hand, am fascinated by old tube gear and older solid state.

    New gear makes me puke, but hey that's just me.

    This is a Sansui 3000a thread, is it not? Or am I just smoking crack again?
     
  13. kevzep

    kevzep Its all about the Music Subscriber

    Solid state amplifiers run happily into an infinity/no load, right up full output swing. Solid State amplifiers only get sensitive and fail when driven into very low impedance loads, lower than what they are designed for as they then pull too much current through the output transistors which makes them fail.

    You are getting mixed up with tube amps which need to be connected to a load to swing any kind of output, otherwise you can damage the output transformer.
     
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  14. tom3

    tom3 Super Member

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  15. McGowdog

    McGowdog Active Member

    Messages:
    135
    Location:
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    Good to know I can go open circuit on the outputs unless the manual tells me to do otherwise.

    20180518_130409.jpg
     
  16. kevzep

    kevzep Its all about the Music Subscriber

    Do not connect any loads to this amplifier until you have it stable, I strongly advise this.
    I have worked on a few of these amplifiers, and they are now on my black list, I wont work on them now, they are too unstable, too time consuming to resolve. I can fix them but it is not in the price range of what my customers are prepared to outlay, but you are doing this yourself, you might get lucky and get a good result since time is no object.
    I have to think about these things differently.

    A tip, pull those fuses first, turn the amplifier on first, then connect your meters (if you don't and connect the meter first, the surge whilst voltages are stabilising can blow the fuses in your meter). Ideally you want to use 4 meters so you can monitor them all at the same time as adjusting one will affect the other.
    Its a fine balancing act adjusting quiescent current on these amplifiers.
     
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  17. dr*audio

    dr*audio Fish fingers and custard!

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    Actually, on my 3000A, I replaced the power amp completely. I had an eval board for UPC1298 driver ICs (now no longer available) and I built up a new amp and added a protection circuit and speaker relay. It sounds great and is reliable. I used MJ21193 and MJ21194 output transistors so the thing is bullet proof. There are many output amplifier boards and "modules" available for DIY. Parts Express sells some nice ones. It would be easy to convert this receiver. The 3000A is a wonderful receiver otherwise with a very sensitive tuner and separate power supplies for each channel, very unusual at the time it was made.
     
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  18. McGowdog

    McGowdog Active Member

    Messages:
    135
    Location:
    Pueblo, Colorado
    Okay Kev. I'm with ya. I'm buyin what you're sellin. I've gotta try something different.

    I set up my loads, pulled fuses and got about 80 mA on the first fuse for the - side of R, tweaking pots vr803 and 804 to try and nail down that 80. Things seemed to be going well.

    Then I went to the next one, zero mA. Nothing from there on out, even when I went back to the first one.

    I ohmed out every transistor I could find from driver board, preamp, output transistors, ohmed out diodes, no obvious problems so far.

    At some time along the way, I recall reading +44 and -44 volts with my vtvm. I notice now that I get -44, but only about +32 if that's possible.

    Filter cap problem? Mains xfmr? Rectifier? Both legs of the +44 are both low.

    Another seemingly weird thing I've noticed about this rcvr from the get-go, these voltages don't bleed down in any kind of a timely fashion. I noticed this the first time I was playing around those quick acting 3a fuses. Oh looky here, the manual says I should be using 4a very quick acting fuses.

    Wonder how many hacks have messed with this thing.

    All four output transistors are not the same either. That irks me as well.
     
  19. McGowdog

    McGowdog Active Member

    Messages:
    135
    Location:
    Pueblo, Colorado
    Ok, replaced blown fuse on current meter, but still only seeing about 5 or 6 mA, not the 80 I should be seeing.

    Seeing probably about +38 volts from power supply, not the 32 I said prior, and about -44v, supposed to be about +40 and -40?

    A little taken aback by the amount of Germanium transistors in this one, also some silicons.
     
  20. McGowdog

    McGowdog Active Member

    Messages:
    135
    Location:
    Pueblo, Colorado
    Oh silly me. The supply voltages discharge just fine with fuses in. Probably the first time I hadn't blown one of the fuses.

    Seems like R+ came blown when I first removed covers, but it was left channel that seemed to have the high dc to that speaker.

    Anywho, moving along to L- and L+ next for biasing.
     

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