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AU-317 - recap/upgrade of F-2754 and F-2752 boards

Discussion in 'Exclusively Sansui' started by Karl vd Berg, May 17, 2017.

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  1. Karl vd Berg

    Karl vd Berg Super Member

    Messages:
    1,807
    Location:
    A'dam
    Hello to all Sansui fans,

    A quick thread with some pictures of a classic "little" integrated, one my favorites... I want to thank to all those who shared previously info on various threads... :)

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    ... and I have had another (3rd unit, so far) AU-317, first version, on bench and this one is in quite good cosmetic condition. However, the sound was "cutting off", sometimes on left channel, sometimes on the right. Chances to be only the relay is remote, in all the other units I had the relay worked pretty good.
    So I decide to work on it, inspect where the problem was and rebuild the PSU, plus the regulator to the preamp part and the protection circuit by replacing and upgrade all the electrolytic caps, some ceramic ones, the Zeners and small signal diodes, the trimpots, some resistors and other components we'll see along the posts...

    I'm working on F-2754 and F-2752 boards here, since most of the time I use "Line In" stuff (tuners, CD players, DACs, other preamps, etc).

    So, this is the unit under the cover...

    [​IMG]

    Voltage masurements on TM22, TM23 and TM24 cable connectors showed different DC voltage to the preamp boards, something like 27Vdc in one channel to ~31Vdc in the other, so I decided to start around the regulator.
    [​IMG]

    Disconected and cleaned the main F-2754 board and there were cold and broken solder joints everywhere...
    [​IMG]

    On the left, TR601 (2SD356), at regulator circuit...
    [​IMG]

    Cold/broken solder joints in other locations...
    [​IMG]
    [​IMG]
    [​IMG]

    Before reflow the cold/broken solder joints, I started checking the electrolytic caps and their values to see how they drifted over the last 4 decades.
    Below, C608 (220uF/35V):
    [​IMG]

    The cap was replaced to 270uF/63V that fits in size... (thanks to Leestereo for the suggestion here)
    [​IMG]

    Then, ZD601 and ZD602 - 30V Zeners - had to be replaced to BZX55C30 ones (thanks to AUD101 for the suggestion in previous threads).
    [​IMG]

    Next, the four capacitors (C609~C612) that feed the preamp board are essentially a couple of 147uF caps arranged in parallel (100uF + 47uF / 35V). I upgraded these with two of 150uF/ 63V, low ESR.. (thanks to Leestereo for the suggestion here)
    [​IMG]

    And here we have the regulator rebuilt, fresh solder joints (the new Zeners are in a next picture)...
    DC voltages are +/- 30V! :)
    [​IMG]

    Now, C605 and C606 (470uF/50V) with the dreadful glue, that "contaminates" the the surrounding components, were also below capacitance value...
    [​IMG]

    These - dated 1975 - were replaced and upgraded to a higher value, 820uF/50V. (Thanks, Leestereo).
    I wanted to use this "new product" (dated 2016) from Nichicon, UHW series, low ESR.
    [​IMG]

    Their rectifiers (D606~D609) were the 10D1 types and were replaced and upgrade to STTH208 types (thanks to Hyperon and Avionic for suggestion in previous threads)
    [​IMG]

    The same STTH208 replaced also D601 (rectifier for the protection relay circuit), and D610 as the original was also 10D1 types...
    [​IMG]
    [​IMG]
    [​IMG]
    [​IMG]

    And here we have the power supply section rebuilt, plus the relay and protection circuit and regulator for the preamp... My only doubt here is that the bypass caps 1uF/100V (C901, C902) should remain in parallel with the big ones, or they are not necessary with modern 12,000uF/63V capacitors...
    [​IMG]
     
    skippy124, Kale and Hyperion like this.
  2. Karl vd Berg

    Karl vd Berg Super Member

    Messages:
    1,807
    Location:
    A'dam
    Moving to the power amp section, here a partial shot of it.
    [​IMG]

    The 150 ohm fusistors (R35, R36, R45, R46) were above their stock value and will be replaced with new ones, rated 1/3W (Thanks to Skippy124 here).
    [​IMG]

    The "fine-tuning" trimmer (VR03) showed max resistance value under its stock value..
    [​IMG]

    VR01, instead, showed max value above its stock value...
    [​IMG]

    For C25 and C26 (1uF/50V) this time I wanted to try Nichicon MUSE.
    And for the Mylar caps C27 and C28 (0,047) I used some PP caps from Wima...
    [​IMG]
    [​IMG]

    Here, a partial shot showing some components soldered, includind the 22K negative feedback resistor (R31 and R32) that was posted a while ago here.
    Capacitors C15 and C37 (3pF) were "upgraded" with C0G ones. DC offset is pretty stable. :)
    [​IMG]

    Below, part of the power amp section rebuilt, showing the 18V Zeners (ZD603~ZD606) that were replaced with TZX18B-TR types.
    [​IMG]

    Before I replace any transistor, I want to be sure if it is really needed when all the electrolytics were replaced in this board and the solder joints are now fresh. I have, however, ordered some parts which could replace some of them in case of failing... I can change yet some parts in the F-2754 after fire it up for tests...
    KSC2690AYS, KSA1220AYS, KSC2316YTA, KSA916YTA, 2SC945, 2SC2240, etc...
    [​IMG]
    [​IMG]
    [​IMG]

    Coming next, the F-2752 board... :)
     
  3. vsat88

    vsat88 Like my Bluetooth ?

    Messages:
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    Your documentation is beyond belief. I mages are clear as a bell. It takes time to do such good work. I am amazed at you guys who can do such as this. Just you being able to remember who recommended the parts installed and giving credit to them blows my mind. I now bow to the Stereo Gods !
     

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