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niss_man

Sansui AU555a Rebuild, restore, modifications and distortion results thread

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I recently was able to aquire an Early Sansui (most likely 1971 model) AU555a from Gumtree. I have previously helped a mate get his late fathers AU555a rebuilt and modded a few months back and really enjoyed the sound these things produced so jumped at the chance to get one for myself with intention of holding onto it for many years to come. Another overhaul was on the cards before it even was in the house.

     Just recently I did learn to test for distortion measurements by reading this thread (mods please remove if you don't allow links to other forums but it is useful info). https://www.diyaudio.com/forums/solid-state/338511-howto-distortion-measurements-rew.html

Also these mods not done out of my own genius so I take no recognition for that. Other more knowledgable folk have done rebuilds on these before and I found a handy thread over here https://audiokarma.org/forums/index.php?threads/leestereos-restoration-upgrade-of-a-sansui-au-555a.816754/ . 

Anyway this thread will be mainly photos and maybe some useful info so that other prospective diy incensed individuals want to have a go at something like this, it may help them. 

 

front.thumb.jpg.f282dd666c05225f83c36c3fe32d0b9b.jpg

As you can see the front face is in spectacular condition. No scratches for something nearly 50yrs is pretty good I must say.

 

 

back1.thumb.jpg.681f37d91aa3a8f2ebd46c78618e5395.jpg

 

 

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Looks like it has been services a couple of times. Last time in 04.

 

 

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Cover in excellent nick apart from a scratch.

 

 

cover2.thumb.jpg.cbd282564755d02b2d5f0f671749c021.jpg

 

 

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Ends of cover have splits (delamination of ply). I ended up putting some super glue in the split and clamping them together.

 

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Original inside before clean. This is for it's age a very clean amp with not much dust build up. It must have lived a good life. The only down side I can see is a bit of tarnishing on the gold metal base plate and tone control cover.

 

1838088811_ampboardbefore.thumb.jpg.9d466df88e890d023ea60802f47cdc6c.jpg

Amplifier board before touching.

 

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Now comes the fun stuff

parts.thumb.jpg.2970496be64f9abac77cb737feef8a9f.jpg

Time to put some new parts in this baby and get her back to new spec.

 

986730414_externaloscdist.thumb.png.e8dd0c1ba68c975e3f08ead34c9c7354.png

This is a measurement of distortion of my chinese signal generator. it shows 1.22% THD on top left corner. So if I measure my amplifier and get 2% distortion. The actual amp distortion is 2 - 1.22 = 0.78% THD. See how crappy my signal generation distortion is I have decided to build another super low distortion signal generator, but that is a project for another day being put under an ever increasing project pile.

 

123302445_AU555aLCbeforemodsextoscilator.thumb.png.72bc225bd3e0d4b989f77ba668aec532.png

Left channel before restore. THD = 1.5 - 1.22 = 0.28% THD. all measurements were taken at 1v output into my external Behringer soundcard.

 

2046751487_AU555aRCbeforemodsextoscilatorafterwarmup.thumb.png.de5549def61d7ba27745b74bef8eb64c.png

Right channel. = 2.11 - 1.22 = 0.89% THD. Now thats a bit different to the left channel. I am begining to suspect dodgy transistor/s somewhere. Maybe in the Tone control/pre stage board.

Also with this test you can see this channel has more 3rd order harmonics a 3KHz compared to the left.

 

 

 

 

 

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1033040584_LCPowersuplyharmonicsbeforemods..thumb.png.39070ef192f04ac50eac6a7053c93f73.png

This is the power supply harmonics as tested on the LC output before any mods.

 

 

1317734713_RCPowersuplyharmonicsbeforemods..thumb.png.43bcb071a1ecb864354bd59fa47bcaf4.png

This is the RC power supply before mods. Pretty much the same.

 

1674704484_powersupplyparts.thumb.jpg.0664888271a676393a715e291fb4c451.jpg

 

Parts replaced in the power supply include all the capacitors, a out of spec resistor and a couple of old diodes. It is funny how large the old capacitors used to be. The replacements are the same diameters (I wanted them to fit in the same clamps) but are not as long but still the new ones are all of greater capacitance and voltage ratings than the old ones. These caps have 2 - 3 times the capacitance of the originals with a smaller form factor. Upgraded PS. Tick!

 

1570779463_LCPowersuplyharmonicsaftermods..thumb.png.7009afe194b23f3b74e439c57cc7c37b.png

579859804_RCPowersuplyharmonicsaftermods..thumb.png.4049d4f8f7976f07cdd91348915ded5d.png

A big difference now. The harmonics have significantly dropped especially that 2nd harmonic at 100Hz gone into the noise floor. The noise floor has also dropped all across the frequency band. 

 

IMG_20190613_133622.thumb.jpg.f2b69c53b066169eafcaed658e407432.jpg

 

New Nichicon caps in power supply at top.

 

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199440658_toneparts.thumb.jpg.eb2e8ee62cbb8f2ef327a61c57cfd4c7.jpg

Tone control board parts. I also replace the transistors on this board with new matched transistors. 

 

1491458155_npngood1.thumb.jpg.fed49c0d4335e1f84fe77196fb3818a6.jpg

1013921376_npnbad.thumb.jpg.946e6e20384c06b1c500994f6987ae67.jpg

Well looky here. As suspected one of the transistors thought it would start to identify as a diode, not that I have a problem with that, just not in my amplifiers thank you very much😋.

 

1675811155_npngood.thumb.jpg.b676a0a9be37aefdad6f3ceec5aaaa52.jpg

The funny thing is when I bent the pins of this transistor (as a dodgy trans..istor identifier), it now showed ok. 

 

1826839089_AU555aLCpresectionaftermods.thumb.png.c78561af520ab86edb565cf937b84825.png

This LC pre section only (links removed at back of amp). This is nice 1.26 - 1.22 = 0.04% THD out of the pre section LC.

 

1929943005_AU555aRCpresectionaftermods.thumb.png.43ccdd1522f10f9f985c8edc1a7a433e.png

RC has 1.24 - 1.22 = 0.02% THD. Now an excellent result. Again this is pre section only.

 

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892106920_phonoparts.thumb.jpg.7c7d580257ca55d48267129a683b8842.jpg

This is the Phono stage board. It has been modded to have a more acurate riaa curve and is the same topology as found in the AU-666, AU-777 and later Sansui phono stages.  The modified AU-555A RIAA equalization is accurate ±0.2dB from 20Hz-20kHz.

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779057259_psparts.thumb.jpg.469f2c9fba9bd10dcfa178dede11b5d2.jpg

Amplifier board parts. 

 

Finished.thumb.jpg.59159e2da0106be90871548137140968.jpg

Its had a nice clean inside as well. Note the early models have these UFO ceramic transistors. I left them alone as they are excellent Mil spec ones and I like them and they test fine and they look really cool. Pots are changed to multi turn types.

 

After amp was adjusted and bias set now.....

127240089_AU555aLCafterallmods.thumb.png.f4f94ff53567d5ccf3425cd120a03ca1.png

Final LC measurement is 1.43 - 1.22 = 0.21 % THD back to correct specs

 

769419661_AU555aRCafterallmods.thumb.png.1739c0c18bdc9590bce6c55e64bc9d15.png

RC 1.45 - 1.22 = 0.23% THD also excellent.

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Some final pics before it gets put back into use as the lounge room amplifier.

 

1377666582_woodsplitgone.thumb.jpg.a76ec75dc9779e905d8aa68600332ced.jpg

Wood split gone.

 

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After fixing the splits, I gave the wood cover a light sand and sprayed on a few coats of clear laquer satin finish.  The scratch is still visible in the right light/angle but is better than before.

 

1770953160_woodon1final.thumb.jpg.38b2f24d874ca484c8c59b19ae088e43.jpg

 

1932879164_sexywoodfinal.thumb.jpg.a8618f9a672b840da9e58ed64b330bfc.jpg

That cover is sexy

 

1493918057_ampbackfinal.thumb.jpg.6e3e3f6b9f285aa7eaa17a40653f0695.jpg

A clean up and de-oxit of terminals and connections. The Black base plate had a sand, service stickers removed and re-painted satin black.

 

303419033_ampopenfinal.thumb.jpg.1b812f64c216f6ab58af88bd02f92f43.jpg

As good as I can get it. Its funny those marks on the tone control metal cover look like an actual hand print. Maybe from the manufacturing process? who knows..

 

926694036_amppcbfinal.thumb.jpg.6ef2c4359d5d01cd7078c171fda590d5.jpg

Power amp board.

 

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The pots were originally all quite stiff in their action. Like someone may have put lots of wd40 that had gone tacky with time. So all the pots and switches were removed and cleanned properly to give a nice smooth action.

 

 

Cheers

 

Simon

 

 

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Some final power measurement through the oscilloscope using  7.3ohm resistors as load.

 

1829224985_poweroutbothchanelsdriven.thumb.jpg.870896dbf9ebdf9f58b022d0564e2908.jpg

Power out both chanels driven just before clipping. 18.8v peak. 18.8 x 0.71 = 13.35V Rms

                                                                                                     Power = 13.3 x ( 13.35 / 7.3)   = 24.2 W both channels driven

 

109541764_clippingbothchanels.thumb.jpg.71daca8e2df32c9dc1450bfbfe6bd657.jpg

Clipping on both channels

 

328443102_poweronechanel.thumb.jpg.5f3a33d157eb5af1dee7755efc337925.jpg

Power output one channel driven = 29.6 W

                                                                                

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As a final post. There is one other mod that can be done to the power amp stage circuit of this amp called a "Baxandal diode mod". I have performed this mod to @lebone 's amplifier so I will be taking this over to his place and making a direct comparison between the 2 before deciding to do this last mod on this amp. The mod is supposed to drastically improve distortion levels. From what audio memory I have, I thought it made the bass slightly tighter and clarity across the audio band seemed to improve. A side by side comparison should be good because audio memory quickly fades especial in between performing hours of modification.

  

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Great work!!!

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That's quite some work to bring this amp back to top spec!

 

Beautiful looking for something that's a few decades old

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Very impressive niss_man. 

I am watching this Sansui on eBay. I was wondering if you could share your opinion on this model (AU-519), in terms serviceability and performance. 

I have the AU-505. It’s great but would like more power for my floorstanders.

 

https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.com.au%2Fulk%2Fitm%2F153529751613

 

 

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If this the same model I hear at the Sansui meet with more power and sound like the 555a.  It had the issue of dropping one channel I remember. 

It was very good.

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4 hours ago, davidSun said:

Very impressive niss_man. 

I am watching this Sansui on eBay. I was wondering if you could share your opinion on this model (AU-519), in terms serviceability and performance. 

I have the AU-505. It’s great but would like more power for my floorstanders.

 

https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.com.au%2Fulk%2Fitm%2F153529751613

 

 

 

The 519 is a fine amp - it features the diamond differential circuit and slightly improved resolution over the 517/717, whilst maintaining that lovely Sansui sound. 

 

The 519, like many Sansuis of that era, suffers from corrosive glue on some of the driver board and power supply capacitors which needs to be addressed. The ‘black flag’ capacitors also need to be changed.  

 

The good news is that this amp is worthy of a restoration. If it works fine now, have a listen and then contemplate whether you intend keeping it. If so, go down the restoration route. 

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