BA-5000 Rework

Discussion in 'Exclusively Sansui' started by HDJR808, May 10, 2016.

  1. HDJR808

    HDJR808 Active Member

    The problem child continues to perplex.

    As noted, this unit has now been willing to power up with the driver boards from my working unit. No explanation, no change from the circumstances under which the unit previously refused to power up with these same boards after it started exhibiting the pulsing behavior on the dbt with either the working unit boards or the rebuilt boards (which had been reverted back to their original semiconductors).

    Next, to confirm that the NOS 2sc984's I was using were suitable, I swapped them onto the working unit boards and the bias dialed in fine. Then, going back to the rebuilt boards, I once again built them up with the same new (commonly accepted substitute) transistors as before with the exception of keeping the original TR04 2sa818 associated with the 984 bias transistor, thinking that since my initial issue was in the bias setting possibly the previous substitute was not well suited in this position.

    Also, in an effort to reduce the high DC offset readings that were present with the first testing with these rebuilt boards, prior to the crazy dbt performance, I have attempted a new round of scrupulously matching pairs ksa992's on the curve tracer. I happened to have a number of 992's marked as "fc" -not a gain grouping currently in the datasheet, with hfe readings around 550, 100 points higher than the batch of "fa" group that I used before. Remembering some reference to higher gain being good in this instance, I matched my 2 pairs from these "fc" devices.

    Today, these re-re-rebuilt boards went back in the amp, and with only the briefest of glowing from the 500 watt bulb, it was up and running. I truly have no idea what might have been the source of all of the previous bad behavior. It's almost like wiggling the rabbit ears on the televions of yesteryear- things just start working. Did not take the time today to properly fine tune a hopefully stable bias setting, but was very happy to see that the 60mv offset readings from when things were previously running have now come down to 1.2 and 6mv. The newly matched 992 pairs seem to be the ticket.

    Now, keeping my fingers crossed that a stable bias setting can be maintained along with reasonable temps on the outputs...


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  2. kevzep

    kevzep Its all about the Music Subscriber

    Nice work, but don't you hate that, no idea what on earth caused all the shenanigans?

    Fingers crossed everything holds....
  3. JOMARK911

    JOMARK911 Super Member

    Athens Greece
    I keep mine crossed.
  4. HDJR808

    HDJR808 Active Member

    The amp has been idling for several hours now as I slowly dial in the bias. I've gotten it within a mv or 2 so far by taking very slight tweaks, put the cover back on, wait an hour, tweak again, etc.

    The temperature seems to be holding at around 110F so far, far better than the slow continual uptick from when it was previously up and running. Although I had hoped for it to idle a little cooler, this may just be what I'm going to get...

    Is anyone familiar with the "fc" category on the KSA992's? Google only results in the barrage of online component resellers, with no actual info on the device. I have no recollection of when I got the ones that I have, and am curious about what the official gain range was for the "fc" designation.
  5. HDJR808

    HDJR808 Active Member

    After a year or so on this project, I think it has finally wound down...

    As last mentioned, I was working on getting the bias dialed in, and with that complete, and idle temperature remaining comfortable, the front plate and top cover went back on. Still a bit of an ugly duckling, but no longer a basket case, I brought it downstairs (literally one step at a time) and into the main system it went.

    Happy to report that it sounds great and so far appears to be performing effortlessly. And one of the pleasant results to come out of this is that there is no "thump" at power up. Just the sound of the soft start relay clicking in and a very faint "tick" through the speakers that I assume is the reed relay on the buffer board as the protection circuit goes green.

    I want to thank all who posted encouragement and assistance during the course of this project, and use this opportunity to encourage others to tackle projects that might seem daunting. With all of the valuable input from this forum, there is so much that can be learned. While there are aspects of this rebuild that I still have to scratch my head about, the frustrations were more than counterbalanced by the enlightenment provided.

    So, let's see, what can I work on next...
    Deubi, DougBrewster and Hyperion like this.
  6. ghamilton

    ghamilton Super Member

    :needpics: CONGRATULATIONS!!!
  7. HDJR808

    HDJR808 Active Member

    Okay, well I do have a shot of it back in its natural habitat:


    The power switch knob is from who knows what- it was on the amp as I got it. The level settings are at full, I just oriented them on the shaft that way because I like to see the machined witness marks. And the meter lamps are leds that were in my already working BA, so the collection of original green incandescent lamps that I cobbled together from the basket case and the parts carcass are now in that unit.

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