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ampaddict

Sansui AU-101 issue after rebuild

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Hi and Happy New Year folks.

 

I've attempted my first AU-101 rebuild and have hit a wall. So confident I'd get it right first time too! 

 

Have replaced all electrolytic caps , small signal transistors and power transistors on the main amp board and getting 26V (not 10mV!) across the emitter resistors. I have even reinstalled the original transistors 1 at a time to see if if any of them was the issue. Tested all resistors for open circuit.  Just can't get any bias voltage close to spec and the VR's make not difference. Such a high DC voltage.

 

Have taken extreme care to watch pinouts on trans and even used a semiconductor component analyser to ensure BCE correctly identified.

 

Have also done a recap and transistor replacement on the tone board.

 

All caps like for like bar 1uF on the tone board replaced with WIMA MKS2 film caps - these don't appear to be polarised?

 

Also, upgraded the main filter cap to 2200uF and the 2 output caps to 3300uF but even switching these back to the original Elna caps made no difference to the bias issue.

 

The amp was working perfectly, only a little warm sounding, until I got my hands on it.

I should not have recapped and replaced trans at the same time and should have taken a single component>test approach. Lesson learned.

 

I'm sure I'm missing something blatantly obvious but can't put my finger on it. It does perhaps point to a power amp problem but where to look??

 

Any help would be greatly appreciated.

 

 

 

 

20200102_212801.jpg

20200102_212741.jpg

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GLW finding the issue...here's hoping it's a lil thing some expert on here could point you in the right direction with fixing...

It's a sweet titch of an amp with a rewarding un-fatiguing warm sound...have one myself!

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The AU-101 is cap coupled. Measuring DC offset at speaker posts requires a dummy load or spkrs to be connected. The voltage (to GND) at the speaker line/emitter resistor eg, R835 C817 node should be about half rails voltage ie, about 21Vdc. However measuring 26Vdc across the emitter resistor (R835?) would cause that resistor (0.5ohms 2W) to fail.

 

Resistance check all 4 emitter resistors.

Problem same on both channels?

 

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Thanks for that.

 

I'm just used to measuring across the emitter resistors. Had no idea it would cause them to fail on this particular amp. The service manual mentions nothing about adjusting the VRs. 

 

I will check the 4  emitter resistors. I do have a dummy load so once I've replaced the resistors, I'll connect the dummy loads and measure DC at the speaker terminals, using the VRs to adjust. 

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This is probably not going to solve your problem but the first thing you should have replaced are those carbon trim pots for bias adjustment. Put some new 20turn pots of equivalent value and recheck your soldering and basic component orientation before moving forward. 

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Good advice. I had replaced them with multi-turn  trimpots but reverted to the original as part of the troubleshooting. So on this amp, the only points to measure bias is the speaker terminals?

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Problem is same on both channels. Amp works but is slightly distorted on both channels, very noticeable when applying base. 

 

Resistance in circuit as follows:

 

R837 0.5ohms

R835 0.5ohms but seemed to fluctuate up to 1ohm before settling on 0.5 - could be just a dirty connection

R836 0.5ohms

R838 0.5ohms

 

To confirm, do the VR's adjust DC offset or BIAS voltage and where exactly to measure? Just the speaker posts as MBZ pointed out? If DC offset, am I aiming for a reading as close to zero as possible?

 

On other Sansui's like AU-2900, I've measured bias voltage across the pair of emitter resistors to get a low, mV reading and used the VRs to bring it to spec. I assume the 2900 is not a cap-coupled amp like the 101?

 

Thanks for your patience and help

 

 

 

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The AU-101 has no DC offset or centre voltage adjustment pots - the trimpots are for bias adjustment only.

 

 

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8 minutes ago, pete_mac said:

The AU-101 has no DC offset or centre voltage adjustment pots - the trimpots are for bias adjustment only.

 

 

am I correct to measure the bias across the pairs of emitter resistors (R837/R835) (R836/R838)? What's the target voltage please?

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Sorry, I haven't got time to research this today (I'm at work) so I'd suggest having a look at audiokarma.org as there are numerous AU-101 threads there.

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Hang on a sec... I have a few spare mins.

 

There are two options:

 

  • 5-7mV across the emitter resistor; or 
  • 10-14mA if you remove the fuses F002 and F003 and use a multimeter to measure DC milliamps in place of the fuse.

 

Make sure you use the correct setting on your meter, as if you don't, you can blow the fuse and this will cause screwy measurements.

 

 

 

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Thanks Pete. I was measuring across the pairs of resistors and getting 26V but when I measure across one of the resistors, I can adjust the bias to spec. Seems I was simply measuring it incorrectly!

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Great news!! Woo hoo!!!!

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Posted (edited)

... hangs his head in shame but happy all that work wasn't for nothing!

 

So used to measuring across a pair of resistors, I didn't think to change my approach.

 

It sounds bright and detailed with the WIMA and Nichicon FG series and all new trans.

 

Thanks everyone!

Edited by ampaddict

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A head which has solved a problem should never be hung in shame... particularly as this might affect your perception of stereo imaging and sound staging!! ;)

 

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Just now, pete_mac said:

A head which has solved a problem should never be hung in shame... particularly as this might affect your perception of stereo imaging and sound staging!! ;)

 

HAHA, very true - all the best Pete

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That's better. Multi-turn trimpots reinstalled. Bias good. It sounds amazing and I'm assuming will only get better once the caps bed in ...

20200103_112355.jpg

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Another one hooked.

 

Yes give it a couple of hours.

l started with a couple of 101's, then traded up to AU505's, AU717 and AU70.

Most get daily use.

 

regards Bruce

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Have some work ahead of me ...

IMG_8742.JPG

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ahhh, too late for you.

 

regards Bruce

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