Leestereo Mods, Tweaks and Tips for the Sansui 771

[Leestereo]

...Also , would like to know your thoughts on the phono stage of 881 /771 in comparison to modern stand alone budget phono preamps like rega ,schiit, ifi etc
Thanks

The phono stage of the 881 utilizes the BA312 IC and hence its topology is quite different from that of the 771 which is fully discrete. And since I have no personal experience with the 881, I cannot give a credible opinion as to how it performs. As previously mentioned in post #29 above, the 771 phono stage has the same topology as that in the AU-555A and this last one has been compared to the Lounge LCR as well as the Schiit Mani stand alone phono stages. In both cases, the 555A owners reported that the restored and upgraded Sansui phono stage outperformed the stand alone ones: https://audiokarma.org/forums/index.php?threads/new-addition-sansui-au-555a.852662/post-12542950 and https://audiokarma.org/forums/index...rade-of-a-sansui-au-555a.816754/post-11840483.

 

[Scott Muni]

Part 9: Main Power Supply Capacitors and AC Safety Components

The 4.7nF capacitor (C701) across the power switch was replaced with a more appropriate snubber (0.1uF safety capacitor + 100ohm 0.5W resistor); due to space

View attachment 3604854

View attachment 3603884



View attachment 3604306
Image from

https://www.reddit.com/r/vintageaudio/comments/25v17k/sansui_771_8010_one_receiver_two_faces

Is there an easy method to calculate the value of the snubber ? Looking over various schematics of amps & recievers the stock uF is different, and the snubber resistor is available in several values. I would like to add snubbers to my rebuilds.

 

[Warren-Oates]

I think there is a range that works as intended. The arc being suppressed on a switch for 120VAC line can be snubbed by a 100-300ohm resistor in series with a .1uf film capacitor. I'm not an EE, and I'm not very smart, this is just what I've gathered from reading on AK.

I bought a box of Kemet

P409CP104M275AH151​

They have worked nicely for me, easy to attach to switches, and from what I see do the same thing as the Quencharc products for much less money.

 

[Scott Muni]

I think there is a range that works as intended. The arc being suppressed on a switch for 120VAC line can be snubbed by a 100-300ohm resistor in series with a .1uf film capacitor. I'm not an EE, and I'm not very smart, this is just what I've gathered from reading on AK.

I bought a box of Kemet

P409CP104M275AH151​

They have worked nicely for me, easy to attach to switches, and from what I see do the same thing as the Quencharc products for much less money.

Thanks, that's good to know. Yeah, the quencharc's are pricey.

 

[ganteusz]

Hello,
I'm restoring my 771 and implemented almost all modifications mentioned here. After all, I tried to set the bias current and speaker DC offset. Unfortunately, the bias current value doesn't change (regardless the VR position - I used multiturn pots). I can't set the recommended 19mA. What could be the reason of this behaviour? DC value changes constantly and I can't set the steady 0V.

Pawel.

 

[Leestereo]

....I tried to set the bias current and speaker DC offset. Unfortunately, the bias current value doesn't change (regardless the VR position - I used multiturn pots). I can't set the recommended 19mA. What could be the reason of this behaviour? DC value changes constantly and I can't set the steady 0V.

For the bias, are you able to measure a voltage change on the emitter resistors as the trimmer is adjusted? Regarding the DC offset, were the differential transistor pairs replaced?

 

[ganteusz]

For the bias, are you able to measure a voltage change on the emitter resistors as the trimmer is adjusted? Regarding the DC offset, were the differential transistor pairs replaced?

I measured the current using the ampmeter instead od fuses F01 and F02. I'll check the voltage on the resistors. I'm also going to recheck the transistors and match the pairs. Thank you for this hint.

 

[CrSens]

I measured the current using the ampmeter instead od fuses F01 and F02. I'll check the voltage on the resistors. I'm also going to recheck the transistors and match the pairs. Thank you for this hint.

The low range fuse in the ampmeter might be bad, it happens frequently.

 

[ganteusz]

The low range fuse in the ampmeter might be bad, it happens frequently.

That helped I set approx. 3,13mV on emiter resistors for each channel. This should give 19mA in total per channel (as it's recommended in the manual).
Now I'm going to check the transistors to calm down the DC voltage fluctuations.

 

[Ray66]

I'm restoring a 661 and implemented Ben's noise reduction mods described in post #7 Part 2: Constant Background Hiss & Hum (Conclusion). The 661 uses the same driver PCB as the 771, F-1499. All ecaps, trimpots, and transistors (except TR07, TR08) were replaced. Power-up was OK, and offset and bias were set per the SM. I then went ahead and made the noise reduction changes; R21/R22 reduced from 100kohm to 47kohm, R25/R26 increased from 470ohm to 1.2kohm, and feedback components R23/R24 and C07/C08 were removed. I turned the voltage up slowly and saw the DBT complaining. Confirmed there were no erroneous solder bridges. Decided to back off the bias trimpots by several turns and try again. The unit powered up OK (still on Variac/DBT). Bias currents were in the 10-12mA range. I slowly brought one of the channels up to around 15-18mA, then it suddenly shot up to 300mA with associated glow on the DBT. Turning the trimpot back did not reduce the bias current. It seems like the mod has introduced some instability in this particular unit. Guess I should put the scope on the outputs to confirm if the problem is oscillation but I prefer to not let it sit in that high current state for too long. So, I'm at a bit of a loss. Has anyone else successfully implement this mod on a 661? The PCB in the unit I'm working on is labeled F-1499, while the SM found on HFE identifies it as F-1499A. I haven't been able to find any info on any variants of this PCB.

 

[Leestereo]

The 771 driver board that was modified is also designated as F-1499. Confirm that the components on the 661 driver board match that of the 771 board.

 

[Ray66]

The 771 driver board that was modified is also designated as F-1499. Confirm that the components on the 661 driver board match that of the 771 board.

I compared the parts list in the service manuals for the 661 and 771 and they appear to be identical. Below is a summary of the transistor replacements made prior to the noise reduction modification.

Part# Original Replacement
TR01-04 2SA726* (hfe~300) KSA992 (hfe~300)
TR05,06,09,10 2SC1124 (hfe~90) KSC2690AYS (hfe~210)
TR11,12 2SA706 (hfe 130-170) MJE15033S (hfe~200)

* Noted a discrepancy, TR01-04 were 2SA726 on the board I'm working on, SM lists 2SA640
I'm thinking the large increase in DC current gain for the TR05,06,09,10 replacements might be an issue here.

 

[Leestereo]

...
TR05,06,09,10 2SC1124 (hfe~90) KSC2690AYS (hfe~210)

I'm thinking the large increase in DC current gain for the TR05,06,09,10 replacements might be an issue here.

IMO, the oscillation is more likely due to a difference in fT. Although in this case, the fT for both the stock transistor and the replacement are similar: 140MHz (2SC1124) and 155MHz (KSC2690). I would suspect that the VAS (TR05/TR06) replacement is a probable culprit. Can the stock 2SC1124 be reinstalled? Or try installing a different transistor for the VAS, e.g., KSC2383 which has a lower fT. Another possibility is increasing the Miller capacitor, C11/C12.

 

[mbz]

I did the same mod on my 661 a while back. My notes show board F-1499 used. I also (incorrectly??) removed R23, 24 and C07 and 08 as per a post by Leestereo.

The mod certainly killed hum and background noise but the reduction in gain seriously limits my speaker selection.
Volume needs to be on about 3 o'clock position for comfortable listening level with budget altecs (Santana, spl 102db?). I plan to reinstall R23,4 C07,8 and live with the hiss/hum or maybe go for a smaller gain reduction???

 

[mbz]

Post #7 of this thread

Also, note that extra feedback components at R23/R24 and C07/C08 have been removed. Failure to remove these components when R25/R26 is increased to 1.2kohm will cause the amplifier stage to become unstable and oscillate.

 

[Leestereo]

...The mod certainly killed hum and background noise but the reduction in gain seriously limits my speaker selection.
Volume needs to be on about 3 o'clock position for comfortable listening level with budget altecs (Santana, spl 102db?). I plan to reinstall R23,4 C07,8 and live with the hiss/hum or maybe go for a smaller gain reduction???

The gain reduction doesn't reduce the maximum power available. As long as the input signal level is sufficient to drive the power amp up to clipping. In this case, it may be that the 661 isn't powerful enough to provide the loudness you desire for the room size?
The R23/R24 and C07/08 feedback components only impacts very high frequencies; the capacitor is a high pass filter. Reinstalling these components will cause oscillation. A better alternative would be to increase the feedback resistors installed at R21/R22 and/or reduce R25/R26.

 

[Ray66]

IMO, the oscillation is more likely due to a difference in fT. Although in this case, the fT for both the stock transistor and the replacement are similar: 140MHz (2SC1124) and 155MHz (KSC2690). I would suspect that the VAS (TR05/TR06) replacement is a probable culprit. Can the stock 2SC1124 be reinstalled? Or try installing a different transistor for the VAS, e.g., KSC2383 which has a lower fT. Another possibility is increasing the Miller capacitor, C11/C12.

Thanks for the tips. I'll start by replacing the original TR05, 06 (2SC1124) to see if that stabilizes things. If it does I'll try the suggested KSC2383. Stay tuned...

 

[Ray66]

OK, so I started by replacing TR05, 06 with the original 2SC1124 transistors. Powered it up on the DBT and it seemed stable. I re-set the bias, fed a 1KHz/0.5V test signal into the AUX input and measured the output into 8ohm dummy loads. With the volume set at 50% I measured less than 0.1W at the outputs. Seemed a bit low. I reverted to the original circuit configuration (R21/R22 back to 100K, R25/R26 back to 470ohms, and replaced replaced R23/R24 and C07/C08). Under the same test conditions the unit put out around 1W. Finally, I switched TR05, 06 back to KSC2690AY. Again, all good. In this configuration I can get this unit up to around 30W before clipping sets in. This occurs with the volume at around the 3 'o clock position. Maybe the 10X lower power with the volume at 50% was the intention of this modification, but for now I think I'll leave the driver PCB as is. While I did convince myself I hear some noise on headphones with no input prior to restoration, perhaps the benefit of this mod is not as apparent on the lower power 20W 661 vs the 32W 771. Would be interested to hear if any other 661 owners out there successfully implemented this change.

 

[Leestereo]

...Maybe the 10X lower power with the volume at 50% was the intention of this modification, but for now I think I'll leave the driver PCB as is. While I did convince myself I hear some noise on headphones with no input prior to restoration, perhaps the benefit of this mod is not as apparent on the lower power 20W 661 vs the 32W 771. Would be interested to hear if any other 661 owners out there successfully implemented this change.

The feedback modification was designed to address the obvious background noise in the 771, which originates in its preamplifier stage. I do not have any experience with the
661, and it may not have any background noise issues since its preamplifier stage topology is different than that of 771.

 

[ganteusz]

That helped I set approx. 3,13mV on emiter resistors for each channel. This should give 19mA in total per channel (as it's recommended in the manual).
Now I'm going to check the transistors to calm down the DC voltage fluctuations.

OK, small update. I replaced the differential transistors with the new pairs. DC voltage still fluctuates. I checked this with the speakers and I still can hear some hum and noise. Noise drops after I press the "Audio Mutting" button, but humming persists. Seems I need to check the preamp stages to eliminate the source of noise.