• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

SANSUI AU 7900, HELP!

diodes in lots of cases go short . maybe it has a bad connection ? it needs some simple testing to see if its open or shorted . a bad ground would make for this condition .
 
RELAY IS WORKIG!!!!!!!!!!!
when i conected the "2" terminal the relay just Click!!!!
Is this normal??? I'm worried.....
iOC2oZe.jpg
 
I just did some searching and found a couple threads where EW states that the 15015,16 are “slow” transistors.
 
Yep, that would be normal. This is the input to the power amp from the pre-amp, without this connected there is no ground reference at the front end of the power amp.
This will make the offset rise to almost full rail voltage. I did spot that terminal wasn't plugged in, but did realise thats how it was when you were powering the amplifier up.
jFF_Bulbo said:
RELAY IS WORKIG!!!!!!!!!!!
when i conected the "2" terminal the relay just Click!!!!
Is this normal??? I'm worried.....
iOC2oZe.jpg
Click to expand...
 
Hello again.
Recap is going well. Bias & dc offet are working properly.
Left channel sounds horrible... The suspect is a C1313 transistor.
I don't have KSC1845 (mouser, Newark & digikey ran out of stock...)
BUT i have KSC1815.
Are the 1815 good subs for the c1313???
Regards.
 
jFF_Bulbo said:
It sounds amazing.... I'm interested. I'll Pm You.
Regards
Click to expand...
Sure.
@kevzep, if you are interested I can give to you gerber file for my adapter you can go and register to www.jlcpcb.com, then upload the gerber and that is it. I just ordered 500 adapters for 11 bucks ( yes 500 for 11 USD) + shipping via FedEx and it came up to 21 USD. it is just 4 cents per adapter :)))))
 
Last edited:
There are no DC Offset adjustment trimmers on an AU-7900.
The KSA992 pair replacement worked well in my AU-7900. From what I’ve read, the DC Offset is largely governed by the matching of the differential input pairs.
Good that you got the protection relay working!
 
jFF_Bulbo said:
Hello again.
Recap is going well. Bias & dc offet are working properly.
Left channel sounds horrible... The suspect is a C1313 transistor.
I don't have KSC1845 (mouser, Newark & digikey ran out of stock...)
BUT i have KSC1815.
Are the 1815 good subs for the c1313???
Regards.
Click to expand...
The KSC1815 might not have enough gain. Depends on the application, if its available gain will be enough. Probably no harm in trying it out, though.
Sometimes, there are supplies of Fairchild transistors available on Bartertown. But I see you’re not a subscriber.
 
kevzep said:
I do not understand why you keep changing components which are not faulty.
Replacing a 2SA798 with 2 separate KSA992 is not a good substitute although it will work if you have no other option.
A handheld transistor tester does not give an accurate HFE reading, you need to do it properly with full operating voltage and a curve tracer.
The 2SA798 is thermally coupled together, this means the gain tracks perfectly as the heat changes. Two separate transistors cannot do this.
If the 2SA798 is not faulty, put it back in.

The MJ21193/94 as I have already said is not a good replacement for the original transistors, they have less than half the bandwidth (ft) then the originals, this does change the sound.
You want to use MJ15015G and MJ15016G, 6mhz and 18mhz respectively.
These will give closer performance to the original transistors.

The reason you see 44V on the relay coil is because the relay is not on, when the relay is turned on then there will be 24V on the coil.

You are seeing 30V on the output because there may be an imbalance in the power supply to the front end of the driver circuit which is too great to equalise with the DC offset adjustment.

You need to print the schematic, get yourself a highlighter, and verify all voltages are correct. When you find something is not right, then you can investigate that part of the circuit.

You need to stop pulling components and swapping things because eventually you are going to damage the circuit board traces and you are going to then have bigger problems.

Let us know how you get on.
Click to expand...
Can you actually hear the difference from a transistor with fT 4Mhz and one with fT 8Mhz providing all other components are the same? Is the difference in sound good or bad?
 
Bimmerb5 said:
Can you actually hear the difference from a transistor with fT 4Mhz and one with fT 8Mhz providing all other components are the same? Is the difference in sound good or bad?
Click to expand...
Some people say they can hear a difference. For the AU-7900 and the relatively small fT difference I think most would be hard pressed to notice. However, amplifiers with much higher fT original fit OP transistors (and drivers for that matter), the difference is easier to hear. I would characterise the audible difference as a slight reduction in the rendering of micro detail, in other words affecting the higher frequencies - it is very subtle.

I believe every effort should be made to use components that are as close to the originals as possible, Sansui chose those parts for a reason.

It's up to the individual to decide a). if they can hear it, and b). does it matter.
 
Last edited:
You must log in or register to reply here.
Back
Top Bottom