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Sansui 3000A - Speaker Prot. Mod.

Discussion in 'Exclusively Sansui' started by mpr2000, Mar 10, 2015.

  1. mpr2000

    mpr2000 Active Member

    Messages:
    295
    Location:
    Rochester, NY
    I am working on a 3000a that is all original and has one shorted output. This one does not have the service bulletin applied for the speaker protection.

    Aside from recapping and replacing OOT components, I'll be doing the following:

    (1) Replace driver board diodes with 1N4002 diodes
    (2) Replace all four output transistors with 2N3772G transistors
    (3) Change emitter resistors from .3 ohms to 1 ohm
    (4) Add base series resistors per the service bulletin
    (5) Change DC fuses from 4 amp to 3 amp

    And finally - my question:

    Instead of adding the two 1000uf caps per the service bulletin (output protection mod), what do you folks think of using one of those speaker protection board kits?

    Pros:
    - Reports of minor performance degradation by adding the 1000uf caps eliminated
    - Modern method to monitor for DC on speaker outputs
    - Cheap

    Cons:
    - Not original
    - Need to engineer a non-obtrusive way to provide power to the added speaker protection board

    Thoughts?
     

     

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  2. Hyperion

    Hyperion Roobarb & Custard Subscriber

    Messages:
    48,556
    Location:
    Hertfordshire, UK
    If you are worried about 'performance degradation' don't fit 1000µF capacitors, use 4,700µF instead.

    Output transistor choice is ok, (maybe you already have these on hand?) but have you thought of using MJ21194 instead?
     
  3. mpr2000

    mpr2000 Active Member

    Messages:
    295
    Location:
    Rochester, NY
    Interesting idea - thanks. What are the ramifications to using 1000uf? IE, protection vs. 'performance'.

    I chose the 2N3772G mostly due to the fT of 200khz - close to the original slow outputs. Would there be a stability concern with the (slightly) speedier MJ21194 (4mhz)?
     
  4. Hyperion

    Hyperion Roobarb & Custard Subscriber

    Messages:
    48,556
    Location:
    Hertfordshire, UK
    The 1000µF will limit the low frequency output, to combat this, and up to a point, more capacitance is better, no need to use greater than 4,700µF IMO.

    Always a good idea to try to match the replacements with the originals, but the 2N3772's are really really slow, I don't think you will have any stability problems with MJ21194's - and they are so much better in every way. ;)
     
  5. mpr2000

    mpr2000 Active Member

    Messages:
    295
    Location:
    Rochester, NY
    Ok - I'll look at options and availability on NP caps larger than 1000uf.

    Also - would bypassing these electrolytics with film caps provide any benefit?
     
  6. Sam Cogley

    Sam Cogley Last of the Time Lords Subscriber

    Messages:
    30,618
    Location:
    SW Missouri
    I honestly haven't noticed much, if any, degradation of low-frequency response with the 1K caps. The idea of using a protection relay board instead of the cap coupling is interesting, and quite possibly a better idea. The cap coupling was introduced by Sansui to stop 3000/3000A receivers from setting woofers on fire.
     

     

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  7. super98lsc

    super98lsc Well-Known Member

    Messages:
    983
    For output coupling caps its also impedance dependent. The output load. Then take into account whether or not the driven pair of speakers can reproduce the extreme lows where the cap size has effect. If the F3 on the woofers is 60Hz your not likely to notice it much if at all. 8 Ohm even less so than 4 Ohm etc.

    8 Ohm load with a 6800µF cap = F3 of 2.93Hz with an optimal low (flat until) 29.96 Hz

    4 Ohm load with a 6800uF cap= F3 of 5.85Hz with an optimal low (flat until) 58.51 Hz
     
  8. mpr2000

    mpr2000 Active Member

    Messages:
    295
    Location:
    Rochester, NY
    Great info -thanks.

    Getting NP electrolytics of that size - even paralleled might be a challenge.

    Still thinking about the speaker protection module...
     
  9. Hyperion

    Hyperion Roobarb & Custard Subscriber

    Messages:
    48,556
    Location:
    Hertfordshire, UK
    They don't need to be non-polar, standard polar is ok.
     
  10. mpr2000

    mpr2000 Active Member

    Messages:
    295
    Location:
    Rochester, NY
    I think on this amp the two speaker protection caps do need to be BP. This one has +/-voltage rails, and the output can swing from positive to negative referenced to ground.
     
  11. Hyperion

    Hyperion Roobarb & Custard Subscriber

    Messages:
    48,556
    Location:
    Hertfordshire, UK
    Ok, you won't get non-polar of that capacitance, you will have to use polar capacitors in series (connect +ve's together). Because you now need 2 for each channel you may have space constraints, as well as halving the capacitance (by placing them in series).

    The speaker protection relay idea is beginning to sound like a better solution...
     

     

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  12. mpr2000

    mpr2000 Active Member

    Messages:
    295
    Location:
    Rochester, NY
  13. mpr2000

    mpr2000 Active Member

    Messages:
    295
    Location:
    Rochester, NY
    I've made some progress. The new outputs are in and the 1000uf caps for the output mod are installed. I ended up using 3 caps per side, for 3000uf each channel.

    I did find what seems a workable method to bias this thing and get DC offset where it should be.

    - Remove both fuses from one channel.
    - using two meters, connect to measure mA across each of the two fuse holders
    - Use a third meter to measure DC offset at a point before the cap mod
    - adjust both bias pots for 80mA while watching the 3rd meter for offset
    - you'll find a sweet spot where 80mA is achieved on both sides of the one channel while the DC offset lands where it should be.
    - Repeat above steps for the second channel

    I was able to do both channels this way and have a rock solid 80 mA on both channels and a DC offset of around 1mV each.

    It looks like I need a new volume pot, as at full off there is enough volume to hear program material quite well. Measuring the low wiper on the pot shows 180 ohms and 320 ohms (L/R) to ground, where it should be at or closer to 0/ground.

    However - it sounds great! A very sweet sound from the little testing I've done so far. I'm very pleased.

    [​IMG]

    Pre-Power board with new diodes, resistors, and multi-turn pots for bias. And below that, the Driver amp board with new caps, pre-driver transistors, etc.
    [​IMG]

    3000uf each channel for the output mod. Still need to find some 25mm fuses...
    [​IMG]

    Ripple-Filter board with new caps.
    [​IMG]

    Phono preamp with new caps. Input cap replaced with a film.
    [​IMG]

    I don't have a picture of it, but I also pulled and recapped the Control amp board and did the caps and transistors there also.
     
    McGowdog likes this.
  14. elguapo511

    elguapo511 New Member

    Messages:
    12



    Hi, How did this end up?
    I just found one of these that seems very new (but old) with the blue tape still on it.
    Im only getting sound from the radio.

    So, I am unable to fix things myself and am going to take it to a repair shop.
    They will probably confirm it needs fixing so I can give them more money, but wondering if I might lead them to the best end result.

    I see you had some options and opinions as you went through your fix....

    do you think you could give me requests to give my electrician before I agree to get it fixed, as far as what cap types. etc..

    PS I don't have the mod paperwork either. Do you?

    Thanks in advance.
     
  15. markuus3889

    markuus3889 Diatom, what's that?

    Messages:
    311
  16. mpr2000

    mpr2000 Active Member

    Messages:
    295
    Location:
    Rochester, NY
    I'd ask your tech to replace the PS caps to start with. The Mod may have already been done and your tech can check for that. If not, and after testing basic operations after doing the PS caps, they can add the factory mod (to protect your speakers and avoid a fire) and assess additional cap replacement as needed and within your budget.
     

     

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  17. The Fuxtor

    The Fuxtor AK Subscriber Subscriber

    Messages:
    2,811
    Location:
    Calgary, Alberta
    Thanks for this post! I have myself a similar situation regarding one of these guys... I replaced the coupling caps for the safety mod with 3 - 1000 uf bipolar xaps as well,but washaving issues measuring the dc offset. Looking at your fine pics, could you remeber offhand where exactly one should be measuring the dc at??
     

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