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Leestereo's Restoration/Upgrade of a Sansui AU-555A

Ok, started some forum searches. Seems they diode mod is suitable for many of these npn/pnp amplifiers. I guess the trick is understanding the right values. What a great forum.
 
Kolner said:
Question re the Baxandall diode and Vbe bypass cap mods, which are seemingly in universal opinion that these make the largest difference…

Are these suitable mods for the AU-555 also?

Second, what about Sansui receivers? Any receivers use the same circuit as the 555 and 555A?
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I did those mods on my 555. There are some differences but they're covered in this thread too.
 
Kolner said:
Thanks. I will have a look in this thread.
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I made a thread on my 555 too and there are some details on what I did in there too.

Brian's AU-555 restoration

I'm pretty new at this hobby but was curious about the early Sansui "tube-like sound" so I found this AU-555 and took a chance on it. I plan to do a restore on it and apply some of the upgrades leestereo documented so nicely in his 555A thread. It's in pretty original condition as far as I can...
audiokarma.org
 
Thoroughly enjoying seeing this detailed restoration. Amazing work. I have a question: if I can't source the 2SA726, 2SA532 and the 2SA875, which is proving difficult to find, what are good substitute transistors? TIA
 
Aircooled said:
Good day fellow AKers, I just finished my 3rd attempt of a 555A restoration Leestereo style (thank you @Leestereo ) and made an interesting observation, thought it might be worth sharing here.

I used mainly Nichicon KG 6800uf capacitors for the power supply filter and output capacitors in my first 2 amps, and I was satisfied with the sound.

This time however, I used general purpose Chemicon SMH capacitors in all 3 areas of the amplifier and the sound quality is truly astounding. All other capacitors remained the same as the first 2 amps but I can really hear a positive difference in the sound quality in this 3rd unit. It seems odd that these general purpose capacitors perform better (in my amplifier) compared to the nichicon KGs, perhaps someone with more experience might be able to provide further insights
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Interesting indeed. I recall a Japanese refurbisher who notes that more than the 'grade' of the capacitor, it is the impendence and voltage handling capacity of the capacitor appropriate to that specific circuit which is more important. Also he is against increasing the value of the power supply capacitors too much as the higher inrush current will put too much stress of the first stage of the main amplifier, making it more likely to break down. Interesting :-)
 
Revisiting my first and favorite restoration. One of my forever amps. Forgot how much I love this amplifier with the classic and very classy wood case and the tighter and improved sonics thanks to @Leestereo and his thread.

Unfortunately I powered it up and the left channel is suffering from low volume and a lot of distortion. Doesn't sound like dirty controls I did perform a thorough cleaning though. Left channel, all inputs and A or B speakers. To me it "sounds" like a small transistor failure, but I am far from an expert when it comes to troubleshooting.

Any advice on where to start?

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JimEGR said:
Unfortunately I powered it up and the left channel is suffering from low volume and a lot of distortion. Doesn't sound like dirty controls I did perform a thorough cleaning though. Left channel, all inputs and A or B speakers. To me it "sounds" like a small transistor failure, but I am far from an expert when it comes to troubleshooting.

Any advice on where to start?
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Are left channel issues present when using headphones? Also try the following: remove the pre-out/main-in jumpers and use a cable to connect the left pre-out to the right main-in and the right pre-out to the left main-in. If the sound issue is still in the left channel speaker, the problem is in the left main amp; if the sound issue in now in the right channel speaker, the problem is in the left premap stage.
 
Leestereo said:
Are left channel issues present when using headphones? Also try the following: remove the pre-out/main-in jumpers and use a cable to connect the left pre-out to the right main-in and the right pre-out to the left main-in. If the sound issue is still in the left channel speaker, the problem is in the left main amp; if the sound issue in now in the right channel speaker, the problem is in the left premap stage.
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L channel failure present with headphones
Feeding L and R preamp to L and R power amp, L power amp is distorted fed from either L or R preamp.

Sounds like a L power amp failure. Where should I start? Small transistors? Or?

And much appreciated!
 
Hopefully I measured that correctly - it's been a while! The schematic shows for TR804 1.2V and I have 1v.

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Sorry, wasn't clear in my request: the voltages for the emitter, base and collector for each transistor. Nevertheless, it appears that except for TR801, all of the voltages for the left channel are much lower than that of the right channel. Something is dragging the B+ for the left channel voltage down. What if you pull the output transistors for the left channel, do the voltage increase?
 
Pulled L outputs. New readings to the left. Everything came up a little, but not matched to R. TR801/TR802 still measuring same voltage.

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Is it possible that the left fuse has high resistance? If not, there is likely failing transistor in the left channel. Voltage measurements on the emitter, collector and base of each transistor will help identify the culprit.
 
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