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ampaddict

Sansui AU-555A Left Channel Distortion

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Posted (edited)

Hey folks,

 

I held off for a few weeks to try and nut this one out myself but have hit a wall.

 

I've recapped the entire amp and replaced all small transistors. RH sounds absolutely amazing but LH is very distorted. 

 

1. Checked that it was not a speaker issue by swapping LH/RH

2. Checked that it was not a preamp issue by swapping Aux LH/RH inputs - LH amp out still distorted

3. Swapped out tone boards from a spare AU-555A - no change

4. Swapped out Preamp boards from a spare unit - no change.

 

To me, this points to only an issue with the main amp.

 

Swaps:

 

TR801/802 - XA495G ->> KSA992F

TR803/804/807/808 - 8002-1 ->> KSC2690A

TR805/806 - 2SC281 ->> KSC1815 (attached to heatsink)

TR809/810 - 9002-1 ->> KSA1220A

TR815 - 2SC627 ->> ZTX694B

 

PS caps upgraded to Vishay BC

C001 2200uF ->> 4700uF

C002/004 470uF ->> 1000uF

C003 1000uF ->> 2200uF

 

Main amp

C819/820 1500uF ->> 1800uF

All other electros swapped like for like values with Nichicon FG/Panasonic FM and WIMA MKS2. 

 

All resistors checked Left to Right and all in spec and matching.

 

Output transistors swapped for MJ21194G but not difference (old ones reinstalled)

 

Centre voltage set to 28.5V

Bias voltage set to 12mV

DC offset at around 20mV at the speaker terminals

 

Sorry, my previous post sent too early 🙂

 

 

 

20200703_135750.jpg

20200703_135731.jpg

hfe_sansui_au-555a_schematic.pdf

Edited by ampaddict

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Don't know why you decided to replace all small signal transistors as normally they'd be fine but suspect this distortion is due to an offset and/or bias issue (provided you didn't accidentally reverse a cap) so get hold of a service manual and check these.

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Posted (edited)

double and triple checked caps and replaced all transistors on both sides, that's what's very strange. Even replaced the ceramics, just in case.

 

Edited my original post with a lot more information ...

Edited by ampaddict

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Power rails the same for left and right channels?

 

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Enlarging the  photo  of the underside of the main amplifier PCB  appears to show that the replacement of the bias trim pot   has resulted in a solder bridge shorting the base of the bias transistor to R823. 

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That bridge is part of the standard PCB tracks as per SM. rails are 48v each measure at F002/003.

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Look again ampaddict. The PCB is close to a mirror view  and comparing the right to the left sections  that solder bridge should not be there. If you look at the Sansui Owner's and service manual you will see that join  is non- existent.

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38 minutes ago, VanArn said:

Look again ampaddict. The PCB is close to a mirror view  and comparing the right to the left sections  that solder bridge should not be there. If you look at the Sansui Owner's and service manual you will see that join  is non- existent.

 

 

Hmmm... looks OK to me, noting that the service manual needs to be reversed to correspond with the solder-side of the PCB:

 

 

551166408_au555aunderside.jpg.fb75ff520fa2e33dff752a3f3fffdc46.jpg1851804039_au55alayout.png.82609eca7c78c37a1c79a525ed37de67.png

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Posted (edited)

I agree Pete. I also have another PCB untouched original that is identical - photo. I'm really stumped as to where the LH distortion is stemming from.  RH is outstanding.

Listening through headphones makes no difference either. I know it's something in the main amp but I seemed to have tried everything. 

20200703_180237.jpg

Edited by ampaddict

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Consider building an audio probe (RCA cabe, series cap, probe, junk spkr) to track down. Plenty of toys on the bay which are ok for binary yes/no testing eg,

https://www.ebay.com.au/itm/DSO138-2-4-TFT-Digital-Oscilloscope-Acrylic-Case-DIY-Kit-SMD-Soldered/192004290393

 

need to get the kit with the case, need external power supply.

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Posted (edited)
40 minutes ago, mbz said:

Consider building an audio probe (RCA cabe, series cap, probe, junk spkr) to track down. Plenty of toys on the bay which are ok for binary yes/no testing eg,

https://www.ebay.com.au/itm/DSO138-2-4-TFT-Digital-Oscilloscope-Acrylic-Case-DIY-Kit-SMD-Soldered/192004290393

 

need to get the kit with the case, need external power supply.

Thanks Mike. I have a Rigol DS1052E digital oscilliscope but not that experienced in using it. I also have a digital signal generator. Would you advise to send, for instance a 1KHz sine wave into the Left Aux RCA then measure the signal from the speaker terminals back through the signal path?

 

Will begin with an audio probe as this seems less complicated 🙂

Edited by ampaddict

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2 hours ago, ampaddict said:

I have a Rigol DS1052E digital oscilliscope but not that experienced in using it. I also have a digital signal generator. Would you advise to send, for instance a 1KHz sine wave into the Left Aux RCA then measure the signal from the speaker terminals back through the signal path?

Exactly. First check at spkr posts so you know what the distortion looks like. General technique is to divide the amp in half and test then divide in half again.

 

Test at spkr post then test at the pre-out/main-in links, this should confirm a power amp issue. Generally test at the base of the power amp transistors, if in doubt compare with the other channel.

 

2 hours ago, ampaddict said:

Will begin with an audio probe as this seems less complicated

Nah... go with the scope, it will give you more detailed info like clipping... A scope is essentially a fast volt meter with a few bells and whistles. Feed in the 1 khz sine wave, set scope to AC, 1mS... will try and find a shot of your scope.

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Power up scope
Connect probe to CH1
Press the "OFF" button between "position" and "Scale" vertical knobs.
Need to select CH1 the adjust "position" to centre line and
adjust scale to 1-2V.
Next hit the menu button between "Position" and "Scale" horizontal knobs.
Set scale to 1-2mS.

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Legend, thanks Mike. Will let you know how I get on ...

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Posted (edited)

Traced it back to one of the decoupling caps I added to the underside of the board. I've simply removed them both and hey presto, all is good. Are they really necessary?

 

Thanks everyone for your input.

 

I have another 555A that I'm going to do a real number on to include a nickel plated chassis and upgrades similar to this: Leestereo's Restoration/Upgrade of a Sansui AU-555A | Audiokarma Home Audio Stereo Discussion Forums

 

Cheers,

Brad

 

Edited by ampaddict

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