BA-F1 restore

Discussion in 'Exclusively Sansui' started by Hyperion, Dec 1, 2017.

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  1. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Here is the Left driver board - almost complete, I have 2 resistors to change and 2 capacitors to fit - which need to be ordered. All the Black Flags are gone, there are new trimmers and zener diodes, new electrolytics, some of these changed for 'films', some resistors have been replaced.
    IMG_2299a.jpg

    I will be dealing with the Right driver board next.
     
    Last edited: Dec 16, 2017
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  2. ghazzer

    ghazzer Sansui addict Subscriber

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    I thought that the generally accepted standard was what you have done: Replace what you find on the board ("factory") unless you observe evidence of 'recent' replacement (after factory). This standard probably has an amendment to let you consider wisdom recorded here by AKers.

    BTW: alt-159 = ƒ for Farad, but I don't see a shortcut for lower case Rho in this character set. Alt-234 = Ω.

    Good choice, given the variables.
     
  3. smurfer77

    smurfer77 AK Subscriber Subscriber

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    John if I had a BA-F1 I would crack it open and let you know what I have. I guess it probably wont make much difference going between 1 and 1.5 nF but still would be nice to get confirmation. I will let you know what I have when I acquire a BA-F1 myself, but I think going with 1nF as per manual is what I would do if I felt the 1.5nF was not factory installed.
     
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  4. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Thanks for that ;) I have gone with 1000pF, I notice exactly the same combination (1000pF + 390Ω) used in the AU-919 - that nailed it for me. :thumbsup:
     
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  5. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Here is the Right driver board, more or less as found, - replacement transistors ringed in green, not ideal in my opinion - MJE340/350 pairs as replacements for 2SC2071/2SA939 pairs, and a mish-mash of resistors ringed in red. This is the channel that has non-original OP transistors as it has obviously suffered a major failure and has been repaired.

    IMG_2301a-markup2.jpg

    And a picture of the underside showing the added 1500pF (wrong value) capacitors, the original 1000pF capacitors had been removed.
    IMG_2302.JPG
     
    Last edited: Dec 17, 2017
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  6. Overundr1

    Overundr1 AK Subscriber Subscriber

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    This is one area where I don't dare play in the sandbox (capacitance/resistor networks) and the effect on phase shift. 1000pf would be my choice as well. Kale and a few others have scopes accurate enough to pull minute ring oscillation traits off a signal however my analog tectonics probably would not. even with its 200mhz bandwidth. Heck, probably so much stray capacitance off my old interconnect cables would be spending more time chasing my tail than the ring overshoot :)
    -Lee
     
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  7. ghazzer

    ghazzer Sansui addict Subscriber

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    I have used KSA1381 and KSC3503 as subs for the 2SA939 and 2SC2071 as they appear to be a better match than the MJE 350 &340. The Ft are a little higher at 150& 150MHz vs 150 & 100 MHz, but I think that the MJEs are much slower.
     
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  8. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Thanks Steve, yes I am thinking about this, I have some others in mind, I just need to check their suitability.

    I got the right driver board stripped of old components, pins cleaned, glue removed, and washed clean of dust. I discovered more burn marks under components including the 'famous' 390Ω resistors (ref: AU-X1), I will mount replacements up on ceramic beads, and revisit the other driver board to do the same.

    I found some gold plated pins, and ring terminals, I also have TO-3P insulators, and have screw hole insulators - so I can mount the new OP transistors very soon.

    I ran out of flux cleaner - so another order placed. :)
     
    Last edited: Dec 19, 2017
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  9. super98lsc

    super98lsc Well-Known Member

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    John, I have 2 BA-F1's and matching CA-F1 here that I have yet to have time to rebuild. If you need me to look at anything in them for reference just let me know.
     
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  10. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Thank you very much ;) - I will bear that in mind - very kind of you :thumbsup:
     
  11. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    I am going with your suggestion - an excellent match. :thumbsup:
     

     

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  12. ManyMoonsAudio

    ManyMoonsAudio AKA paul79 Sponsor Subscriber

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    Yes John, the KSA1381 and the KSC3503 are excellent replacements for the pre-drivers and the current source. I have successfully used these in this amp with no problems.
     
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  13. Karl vd Berg

    Karl vd Berg Super Member

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    And only 2.6pF of Cob is excellent!
     
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  14. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    The PSU has gone from this....

    IMG_2290a.jpg

    To this, it was in a bit of a state - and I still have more cleaning to do as you can see.
    IMG_2303a.jpg

    The rebuilt driver board tests were completed successfully. I hope to further improve on bias stability by implementing Kale's innovative bias modification. All of this will be carefully checked again when I get the new OP transistors installed.
     
    Last edited: Dec 22, 2017
  15. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    And here is the cleaned up and re-installed PSU

    IMG_2305a.jpg

    I doubled the capacitance of the filter electrolytics nearest the switch shafts, and replaced the bridge rectifiers. Everything else remained as per the values that came out, although I did 'bump' the voltages of some of the capacitors.

    As you can see there is much more 'space' around the components now, which should allow better ventilation. (compare with pictures in the post above).
     
    Last edited: Dec 30, 2017
  16. ghazzer

    ghazzer Sansui addict Subscriber

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    Ain't technology wonderful!

    It is almost like you "zoomed out" on all the electrolytics and left everything else the same. Cool!, and cooler.
     
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  17. ghamilton

    ghamilton Super Member

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  18. stereofun

    stereofun Super Member

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    That is excellent ! As comparison the pre-drivers I swapped to in the 999, ksa1220/ksc2690 are 26/19 pf ! - they are rated at +1A and that's the problem - high Ic = capacitance. I am afraid to use the ksa1381/ksc3503 since they are only rated at 100mA with originals at 500/200.

    -----------------------------------------------

    John - Your restorations are always so well considered and thoughtful, striking a balance of staying true the original design while only carefully and deliberately up-rating specs when called for :thumbsup:
     
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  19. ManyMoonsAudio

    ManyMoonsAudio AKA paul79 Sponsor Subscriber

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    Just an FYI, you might want to consider replacing the power switch. I have seen these go bad a couple times in these amps.
     
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  20. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Wow - that is a compliment and a half - thank you. :thumbsup:
     
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