Leestereo's Restoration and Upgrade Of A Sansui AU-919

Leestereo

Super Member
AK member Barclay t. sent in a really nice Sansui AU-919 for a complete restoration and upgrade.

The restoration/upgrade included the following:
1. Replacement of all of the failure-prone "Black Flag" capacitors with C0G types.
2. Replacement of all of the small electrolytic capacitors with low ESR types (increasing capacity where appropriate and physically possible).
3. Replacement of all of the signal path electrolytic capacitors with either "audio grade"/bi-polar electrolytic types or film types (whenever possible).
4. Upgrade of the original polyester film capacitors to polypropylene film types (whenever possible and appropriate).
5. Replacement of output relays.
6. Replace any components damaged by the infamous corrosive board glue.

Here are couple of pictures of the unit (with the covers off) before the restoration:







 
Regulated Power Supply For Preamplifier Stages (F-2844)

The F-2844 board contains the regulated ±40V power supplies for the Head Amp (MC), Head Amp (MM) and the Flat Amp boards. In many integrated amplifiers, the preamplifier power supply is too often modestly sized, and although the one in the AU-919 is quite sophisticated, it would nevertheless benefit from a larger capacitance reserve. Fortunately, since modern electrolytic capacitors are physically smaller than their vintage counterparts, the capacitance reserve can be significantly increased in the restoration process.



Capacitors C02 and C03 filter the raw DC from a diode bridge, and were originally rated at 1000µF/63V; the replacements were Nichicon KW, rated at 3300µF/63V. The C06 capacitor which shunts the zener was originally rated at 100µF/25V and was replaced with a 120µF/50V low ESR type; note that the voltage rating of this replacement capacitor should be increased to at least 35V since the original was subjected to ~24VDC. Capacitors C07 and C08 were originally rated 100µF/35V (schematic erroneously lists 10V parts) and the replacements were low ESR types rated at 120µF/50V. Also note that the silk-screened polarity for C07 and C08 are incorrect. The final filtering capacitors in the regulated supply, C11 and C12, were originally rated at 100µF/50V and the replacements were low ESR types rated at 270µF/50V. The ZD01 24V zener was replaced with precision (2%) BZX55B24 type.





 
Power Supply For the Main Amplifier Stage (F-2845)

The F-2845 board contains 2 independent (one for each channel) regulated ±56V power supplies as well as the 2 independent unregulated power supplies for the output stage.





As with the preamplifier power supply, the capacitive reserve of the regulated supply was increased during the restoration. Capacitors C07-C10, which filter the raw DC from the diode bridge, were originally rated at 220µF/80V and these were replaced with low ESR types rated at 680µF/80V. The capacitors in the regulator circuit, C11-C14, were originally 100µF/63V and the replacements were low ESR types rated at 390µF/63V. The final filtering capacitors in the regulated supply, C15-C18, were originally rated at 220µF/63V and the replacements were low ESR types rated at 680µF/63V.





The 4 main oval power capacitors, C703-C706, were tested and found to be in excellent condition: with a mains voltage of 121V, the rail voltages were stable at ±58.5VDC and the measured AC ripple on each capacitor was <17mV AC (comparable to that of a new capacitors of similar capacity). Following a discussion with the AU-919's owner, it was decided to forgo replacement of the main capacitors at this point.
 
I was happy when LeeS told me that those four large caps were in good working order and that we would be able to keep the originals in place. Besides the work which would have been involved in replacing or overhauling them, I suppose I also just enjoy them for aesthetic reasons -- with that distinctive oval shape, they seem somehow iconic of this unit.
 
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Flat Amplifier (F-2841)

The F-2841 board contains the Flat (Pre) Amplifier stage. Note that the schematic below does not match exactly the board installed in this particular AU-919. Specifically, the 5pF capacitors C15 and C16 were not installed (although there are vacant positions for these). Similarly, the capacitors C31-C34 (also 5pF) are not present (these are typically "Black Flag" types when installed). Other differences include the numbering of some of the capacitors.





Signal Path Capacitors:
The capacitors in the input low-pass filter (C01, C02) were 33pF polystyrene film capacitors (the failure-prone "Black Flag" types). These were replaced with same capacity Murata C0G capacitors rated at 100V and 125°C (Automotive Grade). The output high-pass filter consists of a pair of polarized 100µF/25V capacitors in series (forms a "bi-polar" capacitor) for each channel (C19/C21 and C20/C22). Each capacitor pair was replaced with a single 47µF Nichicon ES bi-polar "audio grade" capacitor (jumpers were installed in the vacant capacitor positions). Note that a single bi-polar capacitor is preferred to a pair of polarized capacitors in series; the latter does mimic a bi-polar type but is not its equivalent. Also, the film by-pass capacitors (C17 and C18) for the output capacitors which were 0.33µF polyester film types, were upgraded to 0.47µF Panasonic polypropylene film types. Note that although capacitors C03 and C04 use polystyrene film, they are not "Black Flag" types and hence were not replaced.

Non-signal Path Capacitors and other Components
The capacitors C07 and C08 that shunt the zeners ZD01 and ZD02, respectively, were originally rated at 220µF/25V; the replacements were low ESR types of the same capacity. Similarly, the capacitors C13 and C14 which shunt the zener diodes, ZD03 and ZD04 (respectively) were originally rated at 33µF/25V and their replacements were low ESR types rated at 100µF/35V. The capacitors C09 and C10 were originally 1µF electrolytic types and these were replaced with same capacity stacked film types. Consequently, the 0.047µF film by-passes (C11, C12) for C09 and C10 were no longer necessary and were hence removed. The local decoupling capacitors C23-C26 were originally rated at 100µF/50V and the replacements were low ESR types rated at 270µF/50V. The capacitors C35-C38 were 5pF "Black Flag" types and these were replaced with same capacity Murata C0G types. The 9.1V zeners (ZD01, ZD02) were replaced with precision (2%) BZX55B9V1 types and similarly, the 6.2V zeners (ZD03, ZD04) replaced with precision (2%) BZX55B6V2 types.

 
Head Amplifier (F-2835)

The F-2835 board contains the Head (Phono) Amplifier for moving magnet cartridges.





"Black Flag" Polystyrene Capacitors
The F-2835 board contains 12 "Black Flag" polystyrene film capacitors. Each of these failure-prone types were replaced with Murata C0G capacitors of the same capacity: C03, C04 and C33-C36 were 22pF; C23 and C24 were 9pF; and C19-C22 were 6pF.

Signal Path Capacitors:
In each channel, the output high-pass filter consists of a pair of polarized 100µF/35V capacitors in series (C55/C57 and C56/C58); this is equivalent to a 50.0µF "bi-polar" type. Each capacitor pair was replaced with a single 47µF Nichicon ES bi-polar "audio grade" capacitor (jumpers were installed in the vacant capacitor positions). Also, the film by-pass capacitors (C43 and C44) for the output capacitors which were 0.47µF polyester film types, were upgraded to 1.0µF polypropylene film types.

Non-signal Path Capacitors and Other Components
The capacitors, C05 and C06, which shunt the zener diodes, ZD01 and ZD02 (respectively) were originally rated at 33µF/50V and their replacements were low ESR types rated at 120µF/50V. The 27V zeners (ZD01, ZD02) were also replaced with precision (2%) BZX85B27 types. The capacitors C09-C12, which shunt the diodes D01-D04, were rated at 1µF/50V and their replacements were low ESR types of the same capacity. Similarly, the capacitors C25 and C26 (rated at 1µF/50V) shunt the diodes D05 and D06, and their replacements were low ESR types of the same capacity.
The local decoupling capacitors C47-C50 were originally rated at 330µF/50V and the replacements were low ESR types rated at 680µF/50V. The original 0.22µF film by-passes for the decoupling capacitors, C51-C54, were damaged from the application of the infamous corrosive glue and were replaced with 0.33µF polyester film types. Similarly, the leads of resistors R49 and R50 were also damaged by the corrosive glue and were replaced with 121ohm/0.25W metal film resistors.

Local Regulated Power Supply
The F-2835 also contains the local regulated ±10V power supplies for the moving coil Head Amplifier board (F-2833). The capacitors C101-C104 were originally rated 100µF/35V and the replacements were low ESR types rated at 220µF/35V. The final filtering capacitors in the regulated circuit, C105-C108, were originally rated at 220µF/35V and the replacements were low ESR types rated at 330µF/35V.



Note that all remnants of the corrosive glue was removed from the board prior to the installation of the new components. This is not just a case of aesthetics, as the old glue sometimes prevents good coupling of the new components to the board. Note that the picture of the board in original condition demonstrates that it is sometimes difficult to see the extensive use of glue with the components in place.



 
Been away for a couple of weeks, another AU-919 having the treatment. Excellent read.
 
The methodical, systematic, logic driven upgrades and restorations is what makes Leestereo's works such an involving and learning thread. Even other gurus become instant followers. The HQ photos and schematics makes it easier to follow. Thanks for sharing these wonderful experiences ... And keep 'em coming.
 
Moving Coil Head Amplifier (F-2833)

The F-2833 board contains the Head (Phono) Amplifier for moving coil cartridges.





Signal Path Capacitors:
In each channel, the feedback loop included a pair of 100µF polarized electrolytic capacitors (C03/C05 and C04/C06) and these were upgraded to Nichicon ES bipolar types of the same value. Similarly, the output high-pass filter of each channel included a pair of 1.0µF polyester film capacitors (C07/C09 and C08/C10) and these were upgraded to polypropylene types of the same value.

Non-signal Path Capacitors and Other Components
The local decoupling capacitors C11-C14 were originally rated at 10µF/50V and the replacements were low ESR types rated at 47µF/50V.





Pre-Main Amplifier Switch Board (F-2838)





When the AU-919 is used as a stand alone power amplifier, the pre-main switch allows one to use it either in the direct-coupled mode or via the capacitor-coupled (high-pass filter) mode. The high-pass filter consists of a pair of polarized 10µF/50V electrolytic capacitors in series (equivalent to a single 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. Note that the F3 of the high-pass filter with the smaller 2.2µF capacitor is still more than sufficiently low enough at 1.45Hz.

 
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