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Leestereo Mods, Tweaks and Tips for the Sansui 771

KeithD said:
Yes, and interesting that the “issue” with the AU-9900 volume pot was described as being “fixed” in the 9900A. Agree with your observation that the “linear” volume pot must not have been well-received. Both the Professional and Definition series had linear volume pots. Recently, some one here was servicing an AU-20000 and thought something was wrong with it because he had to turn the volume pot ”way up” to get the same loudness as on other amps. I had to explain the difference in the volume pot on the 20000 vs. all other amps.
For me, I absolutely love the volume control on my AU-9900 an the multiple levels of “muting“ it has.
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The Volume pot on the AU 9900 is an Audio taper, hence the reason it has to be turned up higher to achieve the same Volume as the later AU 9900a with the Linear Taper control. Linear Taper Pots bring the Level up much faster than Audio Taper ( Log). That is why Audio Taper was developed in the first place.
 
Leestereo said:
Part 5: Unregulated Power Circuit

The 771 uses a pair of 10DC-1 and 10DC-R diode modules in the unregulated power supply. As the F-1500 board design allows for the substitution of individual diodes, a worthwhile upgrade is the installation of low-noise soft-recovery types. The replacement diodes should be rated at for at least 3A, 100V PIV to meet the specifications of the stock ones; Vishay BYV28-200 diodes were installed in this 771. Also, note that ceramic capacitor C01 was relocated to the D01 location.

View attachment 3573901

On the F-1500, the 1uF capacitors at C21/22 decouple the power supply B- rail voltage at the F-1499 Driver Board socket. These capacitors were replaced with 22uF 50V low ESR capacitors. Additional 22uF 50V capacitors were installed on the solder side of the socket to decouple the power supply B+ rail voltage. These decoupling capacitors are necessary to counteract the parasitic inductance of the long traces on the F-1500 board between the power supply and the Driver Board socket.

View attachment 3573910
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I'm a layman, but does this apply to 230V powered devices from Europe or the USA where the voltage is lower?
 
art1555 said:
I'm a layman, but does this apply to 230V powered devices from Europe or the USA where the voltage is lower?
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Replacing the dual diodes modules with single diodes is applicable to any 771; the diodes rectify the secondary voltage which is independent of the country's mains voltage.
 
Thank you very much for your answers, I read your articles about the Sansui 771 and the noise problem, I can't fix it, but I am impressed by your knowledge and detailed descriptions, greetings from Poland
 
art1555 said:
There are two pairs of modules in my Sansui, 2x 10DC-1, 2x10DC-R, does the modification mean removing these four modules and replacing them with only four diodes?
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And don't forget to move C01 to the D01 location as @Leestereo mentioned. I've performed this mod also following his instructions.
 
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Leestereo said:
Part 4 : Regulated Power Supply & Protection Circuits

The regulated power supply circuit on the F-1500 board outputs 13.6VDC (to the AM/FM tuner stage) and 33.0VDC (to the phono and preamp stages). During production, Sansui made several modifications to this circuit: R01 was increased to 330ohm; R04 was removed, and instead a 4.7kohm resistor was installed on the solder side to bridge C02 and C05; ZD02 was also moved to the solder side to allow a proper ground connection. The corrected schematic for the regulated power supply circuit is shown below.

View attachment 3570848

As this circuit dissipates a significant amount of heat, it is advisable that this be taken into consideration when choosing replacement components. Accordingly, the replacement capacitors are rated at 105°C and have a as small as possible footprint (to improve air circulation). Also, to facilitate heat dissipation in this circuit, the plastic sleeve on TR05 (insulates collector) was removed and a heatsink was installed on TR06.

The protection circuit is located adjacent to the regulated power supply. At power on, the 771 relay typically closes almost immediately (under 2 seconds). This delay is too brief to allow the amplifier DC offset to stabilize sufficiently to avoid a turn-on "pop" noise to be heard. The relay closing delay interval was extended to ~4 seconds by increasing the values of C07, C08 and R07 to 100uF, 470uF and 100kohm, respectively.

View attachment 3570860

View attachment 3570890
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I attempted this to increase the delay time. Replaced C07, C08 and R07 as stated. Upon power up, R06 smoked. I replaced everything back to original values but unit is still stuck in protect mode. Verified voltages at collector of TR08 and DC offset. TR07 and TR08 test good, but I have no voltage at the base of TR08. Any idea what may have happened?
 
On f1500c, I'd verify polarity on c07-8, fuses f06-8, diodes d08-12. Not many parts in that area... I'd check everything associated with relay. I bumped up r07 to 120k.
 
raycarr00 said:
On f1500c, I'd verify polarity on c07-8, fuses f06-8, diodes d08-12. Not many parts in that area... I'd check everything associated with relay. I bumped up r07 to 120k.
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Thanks for the input. I triple checked C07 and C08, but will check again. Fuses and diodes all checked good (in circuit). Also pulled relay and verified operation. I agree not much going on there but I must have messed something up :rolleyes:
 
Leestereo said:
Check the solder side for accidental solder bridges; the solder pads for components R07, C07 and C08 are very close together.
View attachment 3724802
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Appreciate the input! I actually had a couple things going on, both related to R07. Solder pad had lifted and there was no connection to C07 and there was a minute solder bridge to R06/TR07/TR08. Maybe time for some magnifying glasses. Also went back to the 100uf, 470uf, 100K configuration and all works well now. Thanks again for your time.
 
Leestereo said:
The lowering of the power amp stage gain via the feedback loop modification does not affect the maximum power output; this is dependent on the power supply B+ and B- voltages (which are unaffected by the feedback modification). However, the lower gain does impact the power amplifier input sensitivity, i.e., the signal voltage required to drive the power amplifier to full output is now higher following the feedback modification. In use, it means that the volume knob has to rotated a bit more clockwise than before for the same sound loudness. This is actually beneficial since it means that the practical working range of the volume control is now in the rotation range with better inter-channel matching.
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Some inquiring minds asked me what was the change in the input sensitivity with the power amplifier feedback modification. From the Sansui Gain Level diagram in the service manual, we see that a 100mV line level signal is sufficient to drive the 771 to full power output, i.e., 19V (with the volume control at maximum).

771 gain schematic.jpg

With the power amplifier feedback modification, the gain is lowered to 32dB and since the full power output is the same, the input voltage can be calculated with this formula: dB=20log10(V2/V1). Solving for V1, the new input sensitivity is 480mV. And although this value is ~5x higher than the stock value, it is easily exceeded since the vast majority, if not all, of line level input sources can produce outputs in volts.

771 gain schematic mod.jpg
 
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