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Leestereo's Restoration/Upgrade Of A Sansui AU-717

Discussion in 'Exclusively Sansui' started by Leestereo, Feb 10, 2015.

  1. Leestereo

    Leestereo Super Member

    Messages:
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    A very nice Sansui AU-717 was on the workbench for a complete restoration and upgrade.

    The restoration/upgrade plan included the following:
    1. Replace all power supply electrolytic capacitors with low ESR types (increasing capacity where appropriate and physically possible).
    2. Replace all of the signal path electrolytic capacitors with either "audio grade"/bi-polar electrolytic types or film types (whenever possible).
    3. Upgrade original polyester film capacitors to polypropylene film types (as appropriate).
    4. Replace signal path ceramic capacitors with film or C0G types.
    5. Replace fuse resistors with metal film types.
    6. Replace VD1212 (dual diodes) with pair of 1N4148 diodes (in series).
    7. Replace any components damaged by the infamous corrosive board glue.

    Although cosmetically near mint, there was a noticeable "hum/buzzing" noise on both channels (independent of the input source and the volume control setting). Further investigation revealed that the noise originated from the preamplifer stage; specifically, failing capacitors in the regulated 35V power supply for the preamplifier. Accordingly, the first board to be restored/upgraded was the power supply board.


    Part 1: Regulated Power Supply & Protector Circuit Board (F-2663)

    The F-2663 board contains 2 independent regulated 35V power supplies (one for each channel). Unlike the main amplifier power supply, the regulated power supply uses rather modestly sized capacitors; this is one of the few weak spots in this otherwise excellent design. Fortunately, since modern electrolytic capacitors are physically smaller than their vintage counterparts, the capacitance reserve can be significantly increased with the replacement capacitors.

    [​IMG]

    Capacitors C05, C06, C07 and C08 filter the raw DC from a diode bridge, and were originally rated at 220µF/63V; the replacements were low ESR types rated at 820µF/63V. Similarly, capacitors C09, C10, C11 and C12 were originally rated at 100µF/63V and the replacements were low ESR types rated at 330µF/63V. Capacitors C17 and C18 which shunt the 13V zeners were originally rated at 470µF/16V and were replaced with low ESR types rated at 470µF/25V. The small value (0.47µF) low leakage capacitors, C19, C20, C21 and C22, were replaced with 0.47µF stacked film types. The final filtering capacitors in the regulated supply, C23, C24, C25 and C26 were originally rated at 100µF/35V and the replacements were low ESR types rated at 220µF/50V. Note that the voltage rating of these capacitors must be increased to 50V from the original 35V since these capacitors are subject to ~35V. The VD1212 double diode was replaced with a pair of 1N4148 diodes in series. Also, since the integrity of a number of solder joints appeared suspect, in particular the ones for the regulator transistors (TR01-TR04), these were re-flowed/renewed.

    [​IMG]

    [​IMG]

    Due to the cramped installation of the components on this board and the location of the board within the amplifier, this circuit runs rather warm. In order to improve the air cooling of this circuit, most of the replacement capacitors were selected to be physically smaller than the originals, except for C23-C26 which were the same size as the originals.

    A number of components had to be replaced due to damage from the corrosive glue used to secure the larger capacitors on this board. The damaged diodes, ZD01 and ZD02, were replaced with precision (2%) 13V BZX55B13 zeners. Similarly, the glue damaged 10kohm resistors (R7-R14) and the 22kohm resistors (R33, R34) were replaced with same value metal film types. Note that all remnants of the corrosive glue was removed from the board prior to the installation of the new components.

    [​IMG]

    The measurements of the restored/upgraded regulated circuit confirmed very stable ±35V rails, with no measurable AC ripple. Also, a quick listening test confirmed that the "hum/buzzing" noise was completely eradicated.
     
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  2. bktheking

    bktheking Gitter Done!

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    The master is at it again.
     
  3. SoundOfSound

    SoundOfSound Super Member

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  4. super98lsc

    super98lsc AK Subscriber Subscriber

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    This is great!! Thanks very much for sharing, I have my 2 AU-717s sitting in line to rebuild next,as soon as I button up my 2000x. ( had to have a good tuner in the lab :).
     
  5. Soundphile

    Soundphile Active Member

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    Sharp pictures, clear notes. A great thread. Thanks.
     
  6. ConradH

    ConradH AK Subscriber Subscriber

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    Very nice, especially the mechanical fit. It always bugs me when people shoehorn in poorly chosen parts that don't really fit. Going up a bit on voltage automatically gets you a lower esr and longer life. On top of that, it's a Sansui- what's not to like!
     
  7. pete_mac

    pete_mac Super Member

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    Nice work!
     
  8. Leestereo

    Leestereo Super Member

    Messages:
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    Location:
    Ottawa, ON
    Thanks for the kind words, everybody; its much appreciated!


    +1 on mechanical fit, its also a pet peeve of mine! Whenever possible, I match the board lead spacing for the replacement capacitors: mostly for mechanical stability but also for aesthetics. The standardization of the lead spacing and capacitor diameter makes it simple to choose a capacitor with a proper "foot print". Particularly advantageous is the the 5mm lead spacing for the 10mm and 12.5mm diameter capacitors: I chose 10mm capacitors to replace the original 12.5mm capacitors on the power board to provide better air cooling.
     
  9. Karl vd Berg

    Karl vd Berg Super Member

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    :thmbsp::thmbsp::thmbsp:
     
  10. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Thank you for the nice clear pictures and write-up - your careful approach to component selection is refreshing. ;)
     
  11. trueno92

    trueno92 Active Member

    Messages:
    366
    Location:
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    that job looks totally professional!

    I tried to get some of the glue off of my F-2980, but I am afraid of cracking or breaking the pcb. How did you get the glue off? is there a substance that can dissolve it or just gently scrape?
     
  12. bktheking

    bktheking Gitter Done!

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    Nail polish with acetone.
     
  13. Leestereo

    Leestereo Super Member

    Messages:
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    Location:
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    Part 1: Regulated Power Supply & Protector Circuit Board (F-2663) - Cont'd

    [​IMG]

    The C601 capacitor filters the raw DC from the diode pair (D601, D602) in the protection circuit and was originally rated at 47µF/50V; the replacement was a 100µF/63V low ESR type. The C604 capacitor, which with R609 determines the turn-on delay of the protection relay, was originally rated at 470µF/10V (listed as 330µF/10V in the schematic) and was replaced with a 470µF/25V low ESR type. The C605 capacitor which shunts the ZD602 zener, was originally 33µF/50V and was replaced with a 47µF/50V low ESR type. The capacitors C606 and C607, both rated at 4.7µF/50V, are part of the power/protection LED indicator circuit and were replaced with low ESR capacitors of the same rating. The bi-polar capacitors C29 and C30, rated 220µF/6.3V (listed as 100µF/6.3V in the schematic) are part of the abnormal output voltage sensing circuit, for the left and right channels, respectively. The capacitors were replaced with 220µF/25V bi-polar types. The capacitors C606 and C607, rated at 4.7µF/50V, are part of the power/protection LED indicator circuit and were replaced with low ESR capacitors of the same rating. The VD1212 dual diode (D604) was replaced with a pair of 1N4148 diodes in series. The protection relay (RL601) was replaced with an OMRON MY2-02-DC24 which is a "drop-in" replacement for the original Taiko RAB-K2.




    Part 2: Tone Control Circuit Board (F-2720)

    [​IMG]

    [​IMG]

    [​IMG]

    Signal Path Capacitors:
    With the tone controls "defeated", the signal first passes through the input high-pass filter capacitors (C05, C06) which were 0.33µF polyester film types; these were upgraded to 0.33µF Panasonic polypropylene film types. The stock capacitors in the low-pass filter (C07, C08) were ordinary ceramic capacitors at 22pF (listed as 10pF in the schematic); these were replaced with same value C0G capacitors. Similarly, C23 and C24 were ordinary ceramic capacitors at 22pF (listed as 10pF in the schematic) and these were also replaced with C0G cpacitors of the same value. The output high-pass filter consists of a pair of polarized 100µF/6.3V capacitors in series (forms a "bi-polar" capacitor) for each channel (C27/C29 and C28/C30). Each capacitor pair was replaced with a single 47µF/16V Nichicon ES bi-polar "audio grade" capacitor (jumpers were installed in the vacant capacitor positions). Also, the film by-pass capacitors for the output capacitors, C25 and C26 which were 0.47µF polyester film types, were upgraded to 0.47µF Panasonic polypropylene film types.

    With the tone controls "on", the signal additionally passes through the high-pass filter capacitors (C21, C22). These capacitors were originally polarized types rated at 10µF/25V; the replacements were 10µF/35V Nichicon ES bi-polar "audio grade" capacitors. Similarly, the capacitors C45, C46 capacitors in the output high-pass filter of the tone control circuit were originally low-leakage polarized types rated at 10µF/25V; the replacements were 10µF/35V Nichicon ES bi-polar "audio grade" types.

    With the subsonic filter engaged, the signal additionally passes through a pair of polarized 4.7µF/50V capacitors in series (forms a "bi-polar" capacitor) for each channel (C33/C35 and C34/C36). Each original capacitor pair was replaced with a single 2.2µF/50V stacked film capacitor. Also, the film by-pass capacitors for subsonic filter, C31 and C32, were removed.


    Non-signal Path Capacitors and other Components
    The local decoupling capacitors C39, C40, C41 and C42 were originally rated at 47µF/50V; the replacements were low ESR types rated at 220µF/50V. The VD1212 dual diodes (D01, D02) were each replaced with a pair of 1N4148 diodes in series. And the notorious failure-prone 82ohm fusitors (R37-R40) were replaced 82.5ohm/0.25W metal film resistors.

    [​IMG]

    [​IMG]
     
    Last edited: Feb 11, 2015
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  14. super98lsc

    super98lsc AK Subscriber Subscriber

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    Your work in the Tone control section brought up some "ideas" I had while redoing my CA-3000. I have not been changing many of the green mylar caps out. I did swap the input caps on my driver boards to Elna Silimic II on the 5000x.

    Do the Panasonic Polys have a better sound?
     
  15. Leestereo

    Leestereo Super Member

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    Regarding film capacitors, polypropylene is a much better dielectric than polyester/mylar and given the choice between the 2 would always prefer a polypropylene one. Note however that polypropylene capacitors are generally physically larger than a polyester capacitor of the same rating and manufacturing age. In the AU-717, the vintage polyester/mylar capacitors were all physically larger than the polypropylene replacements.
     
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  16. super98lsc

    super98lsc AK Subscriber Subscriber

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    Excellent, thanks again for the insight.
     
  17. Leestereo

    Leestereo Super Member

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    Part 3: Driver Circuit Board (F-2721 & F-2722)


    [​IMG]

    [​IMG]

    [​IMG]

    The capacitors (C02, C04) which shunt the 22V zeners were originally 33µF/25V; the replacements were low ESR types rated at 100µF/35V. Also, the zeners (ZD01, ZD02) themselves were replaced with 22V/1.3W types (BZX85C22). The local decoupling capacitors C11 and C14, originally rated at 470µF/63V, were replaced with 1000µF/63V low ESR types. The C12 capacitor was 1µF/50V and was replaced with a low ESR type of the same value. The variable resistors VR1, VR2 and VR3 (100ohm, 2.2kohm and 1kohm, respectively) were replaced with Bourns single turn cermet types. The failure-prone 150ohm fusitors (R23 & R25) were replaced with 150ohm/0.25W metal film resistors. The negative feedback resistor (R16) was also replaced with a 27.4kohm/0.5W metal film resistor.

    A couple of additional components were also replaced due their proximity to the C11 or C14 capacitors that were attached to the board with the corrosive glue: R15 (10kohm) was replaced with a same value 0.25W metal film resistor on F-2721 and D02 (1S2473) was replaced with a 1N4148 diode on F-2722.

    [​IMG]

    [​IMG]
     
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  18. Leestereo

    Leestereo Super Member

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    Part 4: Pre-Main Switch Circuit Board (F-2670)

    [​IMG]

    [​IMG]

    When the AU717 is used a stand alone power amplifier, the pre-main switch allows one to use it either in the direct-coupled mode or via the capcitor-coupled (high-pass filter) mode. The high-pass filter consists of a pair of polarized 10µF/25V capacitors in series (equivalent to a 5.0µF "bi-polar" capacitor) in the left and right channels (C01/C03 and C02/C04, respectively). Each capacitor pair was replaced with a single 2.2µF stacked film capacitor. The replacement capacitors were installed in the previously vacant spaces for film by-passes, C05 and C06.

    [​IMG]
     
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  19. super98lsc

    super98lsc AK Subscriber Subscriber

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    :thmbsp::beerchug:
     
  20. Leestereo

    Leestereo Super Member

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    Part 5: Equalizer Circuit Board (F-2723)

    [​IMG]

    [​IMG]

    Signal Path Capacitors:
    The stock capacitors in the input high-pass filter (C01, C02) for the phono input were 1.0µF polyester (mlyar) capacitors and these were upgraded to polypropylene film types of the same capacity. The stock capacitors in the input low-pass filter (C03, C04) were ordinary 100pF ceramic capacitors; these were replaced with Wima FKP2 polypropylene capacitors of the same value. The capacitors C05 and C06, which determine the low end roll-off of the RIAA equalization, were 220µF/6.3V polarized capacitors; the replacements were 470µF/16V bi-polar types. The "speed up" capacitors (C19, C20) were originally 10µF/16V and the replacements were 10µF/35V Nichicon "Audio Grade" types. The output high-pass filter consists of a pair of polarized 10µF/25V "low leakage" capacitors in series (equivalent to a 5.0µF "bi-polar" capacitor) for each channel (C27/C29 and C28/C30). Each capacitor pair was replaced with a single 4.7µF stacked polyester film type; the replacement capacitors were installed in place of the original film by-passes, C35 and C36.

    Power Supply Capacitors and other Components
    The local decoupling capacitors C31-C34 were originally rated at 10µF/50V and the replacements were low ESR types rated at 47µF/50V. The VD1212 dual diodes (D01-D04) were each replaced with a pair of 1N4148 diodes in series. And the failure-prone 82ohm fusitors (R47-R50) were replaced with 82.5ohm/0.25W metal film resistors

    [​IMG]
     
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