Restoration/Upgrade Of Scribby31's Sansui AU-9500

[Chrizzle]

Update,

I have used the 9500 pre out into the 9090db and it sounds great. So the problem lies after the pre amp stage of the 9500 methinks.

anyone got any ideas what to look at first?

 

[Leestereo]

...i had two local decoupling caps for both the driver and output stages round the wrong way, reversed the polarity on them and and changed two of the fuses on the back and now i have sound on both channels.
however, after some listening sessions the bass and treble and highs are great but the midrange is quite quiet in comparison and vocals just seem a bit dampened and no longer crisp and loud...

Were the two decoupling capacitors that were incorrectly installed replaced with new ones?

Update,

I have used the 9500 pre out into the 9090db and it sounds great. So the problem lies after the pre amp stage of the 9500 methinks.

anyone got any ideas what to look at first?

Agreed, problem lies downstream from the preamplifier stage; post a picture of your driver boards.

 

[Chrizzle]

Here are some pictures of the driver boards. I’m now going to replace the potentially shorted caps for new ones and I shall report back.

the pics are just of one driver board, as they are identical.

please let me know if anything obvious is visible.

regards

 

[Chrizzle]

Problem solved, replaced the caps that I put in the wrong way round. Only had 4.7uf ones to hand which were what the amp originally had in .
amp absolutely sings now, been listening for the last hour or so and it’s definitely up there if not above the 9090db.
Will have to give it a few more hours listening to decide,

I’ll definitely check and double check polarity next time. Will save a couple of sleepless nights.

 

[Chrizzle]

Were the two decoupling capacitors that were incorrectly installed replaced with new ones?



Agreed, problem lies downstream from the preamplifier stage; post a picture of your driver boards.

hello again, sorry to drag this thread up again, after many hours of listening to the amp again after rectifying the reverse polarity decoupling caps, it’s struggling on the highs and mids and I was wondering if I may have damaged some components on the driver boards downstream of the reversed caps.

would running it with the caps reversed for a minute or so could have damaged something on the driver boards or the output transistors?
Thanks in advance

 

[Leestereo]

...it’s struggling on the highs and mids and I was wondering if I may have damaged some components on the driver boards downstream of the reversed caps...

It is possible that there was damage to the components downstream from the improperly installed decoupling capacitor, but IMO, it is not that likely. Have you replaced these capacitors with new ones? In your previous post of Oct. 28th, you state that the AU-9500 performance is "...definitely up there if not above the 9090db.", has the AU-9500 performance degraded since then?

 

[Chrizzle]

It is possible that there was damage to the components downstream from the improperly installed decoupling capacitor, but IMO, it is not that likely. Have you replaced these capacitors with new ones? In your previous post of Oct. 28th, you state that the AU-9500 performance is "...definitely up there if not above the 9090db.", has the AU-9500 performance degraded since then?

yes I replaced them with new capacitors and it does sound ok, I think that because i went from terrible sound to sound my mind thought it was better than it was.

I’ve also been working away for a couple of months so I’ve no had time to listen to it.
Upon some more listening time it sounds quite flat and very quiet in the mids mainly, with vocals being slightly muted. Bass and treble s are good and it’s incredibly clear with zero distortion, just lacking a lot in loudness in the mids.

I can compensate somewhat with having the loudness on and the midrange turned up to the max but I know this is not the answer.

im wondering if I have set the bias wrong, could this have such an effect on the output stages?
I did it per the manual and it is not getting hot. In fact even after playing it loud for quite some time it doesn’t even get warm

 

[Leestereo]

...Upon some more listening time it sounds quite flat and very quiet in the mids mainly, with vocals being slightly muted. Bass and treble s are good and it’s incredibly clear with zero distortion, just lacking a lot in loudness in the mids.

I can compensate somewhat with having the loudness on and the midrange turned up to the max but I know this is not the answer...

Interestingly, your sound description of the AU-9500 matches that described for the unit prior to its restoration/upgrade; see post no. 15 which details pre and post restoration/upgrade sound impressions. To date, all three AU-9500s that have passed through my bench have sounded similar following their restoration/upgrade. May I ask what are the source and speakers that are being used with the AU-9500? How does the AU-9500 sound with headphones?

im wondering if I have set the bias wrong, could this have such an effect on the output stages?
I did it per the manual and it is not getting hot. In fact even after playing it loud for quite some time it doesn’t even get warm

IIRC, the bias was set to ~20mA by adjusting for 20mV across an emitter resistor pair, e.g., R893 + R895.

 

[Chrizzle]

Interestingly, your sound description of the AU-9500 matches that described for the unit prior to its restoration/upgrade; see post no. 15 which details pre and post restoration/upgrade sound impressions. To date, all three AU-9500s that have passed through my bench have sounded similar following their restoration/upgrade. May I ask what are the source and speakers that are being used with the AU-9500? How does the AU-9500 sound with headphones?



IIRC, the bias was set to ~20mA by adjusting for 20mV across an emitter resistor pair, e.g., R893 + R895.

Haha yes just read post 15, I’d does somewhat fit the description.

Interestingly, your sound description of the AU-9500 matches that described for the unit prior to its restoration/upgrade; see post no. 15 which details pre and post restoration/upgrade sound impressions. To date, all three AU-9500s that have passed through my bench have sounded similar following their restoration/upgrade. May I ask what are the source and speakers that are being used with the AU-9500? How does the AU-9500 sound with headphones?



IIRC, the bias was set to ~20mA by adjusting for 20mV across an emitter resistor pair, e.g., R893 + R895.

Currently using a chord qutest DAC into the aux input and for speakers I’ve got a pair of Rogers studio 1a’s. Not overly demanding speakers to run.

I haven’t tried the headphone output yet as I can’t find my headphones atm, although I have used the 9500 as a pre amp and there is no issue with the pre amplifier stage. I’m just wondering what to look for on the driver boards that could be at fault. Where would you be looking first, transistors? Power supply? Driver boards?

I could really do with another 9500 and swap the driver boards over, don’t know anyone near me though

do I need to find someone local that’s handy with an oscilloscope to trace the issue? Or is there a simpler way?

thanks

 

[sweatyk]

Awesome thread!!!!
Your attention to details and the before and after pictures was a great read. I came across this thread because I had someone bring me the same amp for some service. I'm a novice guy with electronics who has some skills and do this as a hobby.

Leestereo......do you offer a restoration kit of parts needed for us DIY guys? I saw a kit on Ebay from Hifi Audio but wanted to get some thoughts and advice from others before proceeding.

 

[Leestereo]

@sweatyk: thanks for the kind words, much appreciated.