Leestereo's Restoration/Upgrade Of A Sansui AU-7500

Leestereo

Super Member
A nice Sansui AU-7500 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).
2. Replace all of the signal path polarized electrolytic capacitors with film types (whenever possible) or bi-polar electrolytic types.
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 noisy failure-prone 2SA726 and 2SC1313 transistors.
6. Replace non-original mis-matched output transistors.


Part 1: Tone Control Block (F-2014)

The tone control circuit in the AU-7500 is always in the signal path as there is no "tone-defeat" switch
on this model.

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Signal Path:
The signal first passes through the input high-pass filter capacitors (C701/C702) which were 0.22µF polyester film types; these were upgraded to 0.47µF Panasonic polypropylene film types. The low leakage electrolytic capacitors at C709/C710; C719/C720; C729/C730; C739/C740 and C741/C742 were replaced with WIMA polyester film types (at the same capacity as the originals). The polarized electrolytic capacitors (C721/C722) in the local feedback loop were replaced with Nichicon ES bipolar types (at the same capacity as the originals).

The stock polyester capacitors at C711-C718 (tone controls) and at C731-734 (low filter) were replaced with polypropylene types. Although the polyester capacitors for the low filter can be switched out of the signal path, they were also upgraded to polypropylene types to match the aesthetic of the rest of the board.

The transistors at TR701/TR702, TR705/TR706, TR707/TR708 and TR709/TR710 were the failure prone 2SA726 types (rather than the 2SA493 shown in the schematic) and these were replaced with Fairchild KSA992 types. The transistors at TR703/TR704 were 2SC1313 types (rather than the 2SC1000 shown in the schematic) and they were replaced with Fairchild KSC1845 types.

Non-signal Path Capacitors and other Components

The emitter by-pass capacitors at C703/C704, C707C708 and at C723/C724 were replaced with low ESR types (at the same capacity as the originals). The local decoupling capacitor C743, originally rated at 220µF/50V, was replaced with a low ESR type rated at 330µF/50V.

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As always amazing documentation and write up. Very curious to hear your listening impressions too. I just had a stock version in my rig a week ago. It seems a bit accentuated on the bottom and top. At first it gives impression of great clarity but after a while it becomes too much of a good thing with a bit of listening fatigue kicking in especially on the high notes. But gain - totally stock and probably in dire need of a rebuild.
 
Part 2: Driver Block (F-2034)

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Signal Path:
The stock capacitor in the input high-pass filter (C801/C802) was a 4.7µF/50V low leakage electrolytic capacitor and this was replaced with a 3.3µF/50V WIMA stacked film type (F3 with the film capacitor is <1.1Hz). The stock capacitor in the low-pass filter (C803/C804) was a garden-variety 47pF ceramic capacitor; this was replaced with a 10pF C0G capacitor (the lower value raises the F3 and eliminates any high frequency phase shift within the audible range).

The input differential pair transistors (TR801/TR803 and TR802/TR804) were the failure-prone 2SA726 (rather than the 2SA493GR shown in the schematic); these were replaced with hFE matched Fairchild KSA992 pairs.

The 47kohm resistors at R821/R822 and R823/R824 are part of the negative feedback loop and were replaced with 47.5kohm 1% metal film types (note that R823/R824 is shown as 220kohm in the schematic). The phase compensation capacitor (C809/C810) was an ordinary 6pF ceramic type and its replacement was a same value C0G type (note that C809/C810 is shown as 3.3pF in the schematic). The C807/C808 capacitor, which sets the low end cut-off of the negative feedback loop, was originally a 100µF/10V polarized electrolytic capacitor (shown as 47µF/10V in the schematic). This was replaced with a 100µF/25V Nichicon ES bi-polar capacitor.

Non-signal Path Capacitors and other Components:
The power supply capacitor (C805/C806) for the input differential pair was rated at 100µF/50V (rather than 47µF/50V as shown in the schematic) and was replaced with a low ESR type of the same rating. The decoupling capacitors C815/C816 and C821/C822 were originally rated at 100µF/50V and these were replaced with 270µF/50V low ESR types. The 220µF/6.3V emitter by-pass capacitor (C819/820) for TR805/TR806 was replaced with a low ESR type of the same capacity. The variable resistor (VR801/VR802) for adjusting the DC offset was rated at 4.7kohm and was replaced with Bourns single-turn cermet type rated at 5kohms. Similarly, the 1.0kohm variable resistor (VR803/VR804) for adjusting the output stage bias was replaced with Bourns single-turn cermet type.

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Part 3: Output Stage

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The original output stage for the AU-7500 consists of the 2SA663/2SC793 complementary pair for each channel. However, in this particular unit, the output transistors in the right channel (TR814, TR816) were 2N3716 and 2N3792, respectively. Also, although TR813 was a 2SC793, it had a "R" hFE rating whereas the original TR815 transistor had a "Y" rating. In order to have matching transistor complementary pairs in both channels, MJ21195/MJ21196 transistors were installed.

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On the underside of the chassis, although not in the schematic nor the parts list, local decoupling capacitors (3.3µF/50V) were present for the +ve rail (the right channel capacitor is designated C828; the left channel C827 capacitor is not shown). Similarly, 3.3µF/50V capacitors were present across the bias transistors (TR807, TR808) on the underside of the F-2034 driver board (designated as C825, C826). The use of 3.3µF capacitors across the bias transistor is unusual since Sansui typically specifies a 1µF capacitor for this position in their designs. This anomaly, together with the absence of the -ve rail decoupling capacitors in this unit (which are present in the other models in the AU-x500 series) suggests that the installation of the 3.3µF capacitors across the bias transistors was a production line error. To correct this error, 1µF low ESR capacitors were installed across the bias transistors and the local decoupling capacitors were installed for the -ve rail (C829, C830). The replacement decoupling capacitors were low ESR types rated at 47µF/50V.

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Part 4: Equalizer Block (F-2028)

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Signal Path:
The stock capacitors (C601, C602) in the input high-pass filter for the phono input were 2.2µF tantalum electrolytic capacitors; these were replaced with 1.0µF WIMA film types (note that the F3 with the film capacitor is <1.8Hz). The capacitors (C623/C624) which feed the RIAA feedback were 1µF/50V polarized electrolytic types (rather than 10µF/50V as shown in the schematic) and the replacements were 4.7µF/25V bipolar types. The stock output capacitors (C625, C626) were originally 1.0µF/50V "low leakage" electrolytic types and the replacements were 2.2µF/50V stacked (polyester) film types.

The transistors at TR601/TR602 and TR605/TR606 were the failure-prone 2SA726 transistors (rather than the 2SA493 shown in the schematic); the replacements were KSA992 transistors. The transistors at TR605/TR606 were 2SC1313 types (rather than the 2SA493 shown in the schematic), and were replaced with KSC1815 types. Note that the pin-order for 2SA726 and 2SC1313 is "BCE" whereas the pin-order for KSA992/KSC1815 is the current "ECB" standard.

Non-signal Path Capacitors
The local DC filtering capacitors (C603/C604) were 10µF/25V capacitors and the replacements were low ESR types of the same capacity. The stock emitter-bypass capacitors (C611, C612) were rated at 33µF/10V; the replacements were 33µF/50V low ESR types.

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RIAA Feedback
The stock AU-7500 phono stage has a RIAA equalization error that is similar to that previously noted for the AU-9500 (see Sansui AU-9500: Phono Stage RIAA Equalization Error for details). As can be seen in the graph below, there is a rising bass response beginning at ~160Hz which reaches +3.3dB at 20Hz. This deviation was corrected by replacing the 560kohm resistors at R637/R638 with 332kohm 1% metal film resistors. This single change will improve the RIAA equalization dramatically; the corrected response fits within a 0.25dB window.

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The performance of the phono stage was further enhanced by upgrading all of the components in the RIAA feedback circuit with higher quality counterparts (note that replacement resistor and capacitor values were adjusted to optimize the RIAA equalization). The stock 27kohm carbon film resistors at R635/R636 were replaced with 27.4kohm 1% metal film types. The polyester film RIAA capacitors were upgraded to C0G types. For the C617/C618 + C619/C620 combination (2670pF), a single 2700pF C0G capacitor was used. For the C621/C622 + C627/C628 combination (9700pF), 6800pF + 2700pF C0G capacitors were installed. The capacitors C609/C610, which set the low end cut-off of the RIAA equalization, were 47µF polarized capacitors; the replacements were 47µF bi-polar types.

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As can be seen in the graph above, the fully modified AU-7500 phono stage has a RIAA equalization curve that is very "flat"; it is ±0.1dB from 20Hz-20kHz.

Note that such RIAA equalization errors are not limited to Sansui phono stages, but rather are all too common in vintage phono stages designed prior to the late 1970s. The landmark Lipshitz RIAA paper, published in the June 1979 issue of the Journal of the Audio Engineering Society, pointed out that "Most current disc preamplifiers have audibly inaccurate RIAA equalization. These errors are due in part to the perpetuation in print of incorrect formulae for the design of the RIAA equalization networks..."
 
Part 5: Protector Block (F-2041 & F-12515A)

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In the protection circuit, the replacement electrolytic capacitors were of the same capacity as the originals. The bi-polar capacitors at C901 and C907 were replaced with 47uF/25V and 1µF/50V capacitors, respectively. The capacitor (C902) for the relay turn-on delay was replaced with a 220µF/25V low ESR type. The capacitor (C903) which provides local filtering for the protection circuit, was replaced with a 100µF/50V low ESR type. Similarly, the capacitors C906 and C908 were replaced with 1uF/50V and 220µF/35V low ESR types, respectively. The protection relay (RL901) was replaced with an OMRON MY4-02-DC24 which is a "drop-in" replacement for the original.

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Part 6: Regulated Power Supply (F-2013-1) and Chassis-mounted Capacitors

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Since modern electrolytic capacitors are physically smaller than their vintage counterparts, an advantageous increase in the filtering capacity of the regulated supply can be easily implemented (without any modification to the PCB). Accordingly, C009 was increased from 10µF/100V (rather than the 4.7µF/63V value shown in the schematic) to 33µF/100V. Similarly, C010 was increased from 10µF/50V to 47µF/63V. The zener shunt capacitor, C012 was increased from 10µF/50V to 22µF/50V. The capacitor C104 was increased from 220µF/50V to 330µF/50V.

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The large chassis-mounted capacitors in the unregulated power supply for the main amplifier stage, C004 and C005, were rated at 6,800µF/50V and the replacements were 15,000µF/63V low ESR types. The 0.22µF PIO bypass capacitors (C006, C007) were replaced with 1.5µF polypropylene types. The smaller chassis-mounted capacitors, C008 and C015, were originally rated at 2200µF/80V and 1000µF/50V (respectively), the replacements were low ESR types rated at 2700µF/80V and 2200µF/63V (respectively). The "bleeding" resistor R009 on C008 was replaced with a 12kohm resistor rated at 2W. Also note that the bridge rectifier (D001) was a factory upgrade to a 6A RB60 type (rather than the 5B2 type shown in the schematic)

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I cannot thank you enough for these improvement threads, as shown in my own experience with the 717 the results are startlingly good :)
If you ever get a AU9900/CA2000 across the bench and feel inclined to work your magic will be stuck to the thread like glue. I know, lets make LeeStereo a stickie :bigok:
-Lee
 
Yep. Wish I could see the pics. They're always so well documented with labels, etc.
 
Every photo I've posted on here is also gone........

Photo bucket changed their terms without telling anyone, then effectively barred access until you pay up......
 
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