Troubleshooting Sanusi AU-X901

[tone2222]

Hello,
I have Sansui AU-X901 and made the mistake of hooking it up to two pairs of 6 ohm speakers, but did not realize that the switch on the back was set at 8-12 ohms. The amplifier played fine for about 15 minutes at 40% volume and wasn't running hot then all of the sudden the volume went way down and now it just sounds very muffled and distorted.

I disconnected the speaker cables and tried the headphones but the same muffled distroted music came out. I tried various other sources (cd, tape etc.) in different inputs but the same distorted/muffled sound came out. I finally opened the unit to see if any transistors or resistors looked bad, but all looks good - even the internal fuse was good.

Can anyone gove me an opinion on what the problem could be before I take it to the service shop?

Thanks for any advice on this one.

[Leolabs]

Do you have a multimeter???

[tone2222]

Yes I do, and tried testing the transistors (onboard) but I could not get a reading from any of them.

[anatech]

Hi tone2222,
With any luck, a resistor went open. Either an emitter resistor or a resistor leading to an output transistor's base.

If you are slightly less lucky, an output transistor shorted and the emitter resistor opened. Any combination of the above could occur.

Do you have a schematic of the amplifier stage?

Also, I'd suggest setting your multimeter to the diode check range. Unplug the unit and allow the supplies to discharge. Then measure the transistor junctions and low value resistors. That should give you a good idea of what is going on.

-Chris

[tone2222]

Thanks Chris - that is encouraging. I was worried I might have a bigger problem (like transformer or main capacitors). I ordered the schematics last Wednesday.

Do you think this was actually cause by hooking up my speakers so that the amp saw 3 ohms per channel?

In the same configuaration I ran the amp at 60% full volume for an hour without a shutdown or overheating so I wasn't sure if this breakdown was related to my speaker connections.

[anatech]

Hi tone2222,
It's hard to make a guess, but most consumer amplifiers built past the early 80's had inadequate power transformers and heat sinks. If the thermal had gone on the transformer you would not have any power. I'm going to bet you were close to opening the transformer before this event.

Knowing this, not many consumer amps would survive a three ohm load. It's not wise to do even with a well made amplifier. Also, having experience with amplifiers, I know your heat sink was hot. Even when you had one pair driving it as loud as you were. The transformer takes a while to build up heat, but then it stays hot for a while.

Unplug your amplifier now. Test the parts tomorrow and take your time. Write down each result as you test. Then relax until you get your schematic.

-Chris

[tone2222]

Okay, each transistor has been carefully tested and all of them have almost identical readings. After careful inspection of every resistor and cap etc. I still cannot find anything that looks out of the ordinary.

Any other thoughts before I take it in?

[ilimzn]

Edit - sorry, duplicate post

[ilimzn]

And how exactly do the output transistors measure?

[tone2222]

Fixed!

I took the amp in for service and found 2 transistors needed replacing - Cost = $100 Canadian. I'm good with that.

So now I'm being really careful and runnning my speakers in series for a 12 ohm load, but now have to turn it up to the 55% point to get the same volume as I did at 40% running at 3 ohms.

Does anyone know where I can get a copy of the manual for the Sansui AU-X901?

Thanks