Need advise on replacement transistors for AU-555

[datafone]

I have this issue that when I try to read the DC off set I get a reading that doesn't settle, bounces around constantly in a range of -28mv to +28mv, according to EW the transistors are responsible :thmbsp:

A friend replaced all the caps in the power and driver stage for me, I'll be bypassing all the tone controls & switches on the front soon, and that will address a constant white noise that remains.

But I now need to know what transistors to get to replace the existing ones on the driver board?

Schematic:

[westend]

Do you have numbers for those transistors?

[datafone]

I'll have a close look at the schematic and get back to you on that :)

[catrafter]

Data,
I'm no Sansui expert but replacing the 2SC458 transistors in the tone amp with something better might get rid of the noise. Get the ones in the phono amp too. 2SC2240 work well. Be sure to check the basing.
See this thread if you haven't already.
http://audiokarma.org/forums/showthread.php?t=170301

Steve at Consolidated probably has the parts for the power amps as well.
http://www.ceitron.com/

Tom

[datafone]

Thanks catrafter :)

The ones on the main amp board are as follows:

2 of 2SC281
4 of 2SC734
2 of 2SA561
1 of 2SF656

Head amp board are:

4 of 2SC458LG

Control amp board are:

2 of 2SC458L
1 of 2SC458LG

Hoping to get them locally here in Australia if possible, as for the adjustment we set the bias current ok but doing the first part using the oscilloscope is very confusing, I outlined this in my thread in the Sansui section.

[EchoWars]

Thanks catrafter :)

The ones on the main amp board are as follows:

2 of 2SC281
4 of 2SC734
2 of 2SA561
1 of 2SF656

Head amp board are:

4 of 2SC458LG

Control amp board are:

2 of 2SC458L
1 of 2SC458LG

Hoping to get them locally here in Australia if possible, as for the adjustment we set the bias current ok but doing the first part using the oscilloscope is very confusing, I outlined this in my thread in the Sansui section.This amp uses a capacitor-coupled output, therefore DC offset is not really a valid adjustment, since if the output capacitor is functioning properly there can be no DC at the speaker terminals (on a capacitor-coupled amp, the adjustment is more properly called a 'centering' adjustment). This doesn't mean that this cap isn't bad...it's pretty long in the tooth by now, and is likely the reason you see issues at some frequencies with a signal gen and a scope. But measuring DC offset wth a meter at the output terminals of the amp is a waste of time (which I would have told you before if I knew that the amp was cap-coupled).

I digress...the purpose of connecting a scope is so you can adjust the audio output of the amp so that you 'clip' (maximum output) evenly on the top and the bottom of the waveform. The proper procedure would be to use a 8 ohm dummy load, capable of dissipating 30W or more, and feeding a 1000Hz sine wave to the input, and watching the output with a scope. Increase the output level till you see clipping, and while doing this, adjust the centering trimpot so that the clipping occurs evenly on both the positive and negative peaks of the output waveform. It's as simple as that.

I do not know if I would recommend a wholesale replacement of semiconductors just yet, except perhaps the 2SC458's. Since you are Australia, I don't know what to recommend. If you'll tell me what major suppliers you have there (Mouser, Farnell, Digikey, whatever), I can perhaps help.

[datafone]

Oh excellent :D

I wish the manual explained it as well as you did EchoWars :thmbsp:

As far as suppliers go, we have to my knowledge:

http://australia.rs-online.com/web/

They are just down the road from where I live.

http://au.farnell.com/

Also, is it worth replacing the trim pots?

On a side note, I recapped my AU-505 too and it is sounding awesome also, and quiet as :D

I appreciate all this help guys.

[Tony75]

If you get no joy with either Farnell or RS, give WES a try. They are only in Sydney, but i get all my orders couriered from them at very reasonable prices.
They are at www.wescomponents.com
and they carry lots and lots of stuff, all at pretty good prices.
This reminds me that I should get around to fixing my sick AU-555 tthat lives under my workbench waiting patiently for a rainy day.
Cheers
Tony

[datafone]

Thanks Tony, will check them out :)

My amp is sounding very nice and setting the bias output current to 16mv gives it a nice dynamic bass, hope this inspires you to drag yours out ;)

[datafone]

A by-pass of only the controls after the source selector was a strange affair so we just went past everything at the front apart from the power switch.

A switch on the back allows full circuit mode or when switched a cut down mode where the input is feed directly into the main amp board with a volume pot in between.

Incredible sound when switched to the main amp feed, higher res, fuller more realistic and accurate sound stage, has taken this little beast to a new level of sound quality :D