Sansui AU-505 DC Offset Issue

[kidcoma]

Hi all,

I've got a Sansui AU-505 that I'm trying to service. The DC offset is measuring around 300mV on the speaker terminals of both channels!

Here's the schematic for reference:
http://www.hifiengine.com/manual_library/sansui/au-505.shtml

The service manual at hifiengine doesn't have any info on adjusting bias or DC offset. I did find a bias procedure here: http://sansui1944.websitetoolbox.com/post/au505biascurrent-2031558

I've got the symmetrical outputs set at 30V, and bias across the resistors mentioned in the above procedure at 10mV. The DC offset is still measuring around 300mV.

I tested caps C819 and C820, they are both around 1800mf (although labelled as 1500mf), and the ESR is good. Would too high of a capacitance cause an issue with the DC Offset? If not, does anyone have any ideas? Output transistors?

Your help is much appreciated.

Edit Additional Info:

The link I posted with the bias adjustment seems to refer to checking the DC Offset with a load - is this correct? If I apply a 10ohm dummy load across the speaker terminals, I then measure around 12mV at the terminals. However, everything I've read says that DC Offset should be measured without a load. Curious.

 

[kidcoma]

OK, just found a thread saying that DC offset cannot be measured on a capacitor coupled receiver. Lesson learned!

 

[Mirilaj]

Hi folks,
I also measured around 350mV DC output without load on my AU-555A. After connecting the speakers the DC dropped to 12mV. I realized that the DC comes from the feedback resistor R815.

 

[SergioSansui]

Hello,
I purchased a Sansui AU 505 in good condition. Connected to my speakers, it seems to sound good.
However, the offset on the left channel, measured without a load, fluctuates between 160 and 170 mV, while the right channel is stable at almost zero. When connecting the speakers, the left channel drops to 9 mV.
I believe the output capacitors have been replaced because they are 1500 uf 65 volt Nichicons.
Could it be the trimmers? You recommend trying to adjust them, even though they might be oxidized.
Thanks, Sergio

 

[Hyperion]

However, the offset on the left channel, measured without a load, fluctuates between 160 and 170 mV

This is not how to measure DC offset, you need to do it with a load attached, meaning either a speaker or a resistor of similar resistance. If you are assessing the leakage current of the output capacitors this is a rough and ready way to do it. The fact that you get different readings from L & R channels just means you have one output capacitor that has more leakage than the other.

Regarding output capacitors:

I believe the output capacitors have been replaced because they are 1500 uf 65 volt Nichicons

Described exactly like that and with reference to their size in the attached picture - this is a reason to think they may not have been changed, and are probably original.

Regarding trimmers:
Unless you are taking some measures to clean up the original trimmers before you try to adjust them, this could be a serious mistake leading to damaged parts on the main amplifier board. There are very many accounts on AK of people trying to adjust old trimmers, which has resulted in a damaged inoperative unit that was working before they touched it. Trimmers are what keep units adjusted to spec, so they need to be in tip-top condition from the word go.

There are rebuild threads (on AK) for the AU-505, but some especially good ones for the AU-555 which is very similar, and the advice given can translate (with careful thought) from one model to the other - like output capacitor replacement advice, including sensible value increases.

Finally.
Please do not tag your post on to an old thread, start your own thread, then you can use this as the record of your journey from un-restored (with issues) - to fully restored (issues dealt with), and sounding great. It is true that other people do this, but they are mostly wrong too!

I hope you enjoy your AU-505, it's a great little amplifier.

 

[SergioSansui]

Thanks for the reply.
So if with the speakers connected I measured 9mV on the right and 0mV on the right, does that mean the amplifier is fine?
I read that the original capacitors were 1000uf and 50V, so I assumed they weren't original.
I posted on this old thread because I think the problem was the same as the opener. Sorry, I'll start a new thread next time.
In any case, since I'm not an expert, I think I'll have a technician check the amplifier. I don't want to risk damaging my Klipsch Heresy!