Help needed - Sansui AU-9900 transistor substitution

[mubs13]

Hi all,

I am in need of a bit of advice with transistor substitutions in AU-9900. Mine is in need of repair, having issues with unstable bias and slight distortion in one channel. I was thinking of replacing the usual suspect transistors on the driver board (F-2583) as it still has original transistors there which might be giving up their ghosts now...

I have prepared a list of transistor replacements primary based on this extrmely helpful substitution list here:
http://audiokarma.org/forums/index.php?threads/once-more-sansui-transistor-substitutions.583009/

But I have also noticed some alternative transistor substitutes being mentioned here and there, so I just wanted to get it reviewed by the experts and experienced people here to make sure I have got the replacements right (e.g. based on the location and usage on the particular board)

For the F-2583 driver board, I have the following:


Tr01, Tr02, Tr03, Tr04 2SA726 KSA992FA
Tr05, Tr06 2SC1628 KSC3503
Tr07, T708 2SA818 KSA1381
Tr11, Tr12 2SC735 BC33716
Tr13, Tr14 2SA562 BC32716
Tr15, Tr16 2SC1124 KSC2690AYSTU
Tr17, Tr18 2SA706 KSA1381ESTU

Tr19, Tr20 2SD382 MJE15030G
Tr21, Tr22 2SB537 MJE15031G

Does this list look Ok? For easy reference, I have marked the replacements based on the position on the board, Please see this image here:

I am also a bit confused with the Transistor list given in the service manual for F-2583, in particular for Tr05/06 and Tr07/08. The list has 2 different transistors printed for both set of positions, grouped by a big curly brace e.g for Tr05/06 both 2SC1124 and 2SC1628 are given (see image below). However, on the schematics Tr05/06 are given as 2SC1628. Similarly, Tr07/08 are given as 2SA818 only.

And lastly, I may also need replacement Molex socket connectors for the driver board. Whoever serviced it before decided to solder the wires directly into the pins! I have searched and found this post here with a suggestion - http://audiokarma.org/forums/index.php?threads/au-9900-pcb-connector-blocks.398717/#post-9626896

Is the pitch value of 5.08mm right for F-2583 as well? If anyone has replaced or fixed the sockets on this board, any suggestions or tips would be very helpful.

Thanks guys. Really appreciate the help.

 

[Hyperion]

Is the pitch value of 5.08mm right for F-2583 as well?

That is correct.

2SA562 normally replaced with KSA733 or BC327.
2SC735 normally replaced with 2N3904 or BC337.

 

[mubs13]

That is correct.

Thank you!

 

[ManyMoonsAudio]

Have you replaced the fuse resistors?

 

[Hyperion]

Have you replaced the fuse resistors?

I think you will find there aren't any in this model Paul.

 

[LBPete]

The AU-9900 is a great amp.

Please post the serial number for the database. Click on the link in my signature block.

- Pete

 

[mubs13]

Have you replaced the fuse resistors?

I think you will find there aren't any in this model Paul.

Great, one less thing to worry about

I do plan to check and replace other components too as needed. I will also be recapping the driver board as well as the power supply boards. Finding the resistor and capacitor replacements are easy, its the transistor replacement I needed some help with. Just want to make sure I have the right substitutes. I really like this amp and would love to see it working well again.


Any advice regarding the Tr05/06 and Tr07/08 transistors parts given in the list in service manual? Are they equivalent? Does that mean either of those can be used in those positions? Or should I just go by schematics and use the ones given in there?

Cheers.

 

[ManyMoonsAudio]

I think you will find there aren't any in this model Paul.

Oh, right you are... It is the "A" that has them.

I am a bit surprised to see this amp having an issue though. These things are bullet proof for the most part, unless the 2SA818 and 2SC1628 are present in your amp. Some came with these, some did not.

 

[ManyMoonsAudio]

While replacing all the transistors is not too bad of an idea, sometimes, I would rather try and find the best replacement for the actual offending parts.

That said, if your amp has the 2SA818 and the 2SC1628, those are where I would point blame first. The rest of the transistors should be fine really.

Unfortunately, there are not allot of options to replace these. KSC2690AYS KSA1220AYS or KSA1381ESTU is also good.

 

[mubs13]

Hey all,

Thanks for all the help. In between my kids and the job, I was finally able to get some time to work on my au-9900 this weekend. I replaced all the transistors on the F-2583 driver board as per the replacements given. I replaced them all because I do not have a proper transistor checker so I couldn't have confidently checked each of them. Also, there were mixed transistors on the board, some replacements and some originals. So I decided to just replace them all for consistency and reliability.

Good news is that the amp powered up fine and the original issue of distorted channel seems to be fixed now. It played fine on both channel using my test speaker. Bias looks stable and responds well to adjustment.

However, there is a small issue. DC offset is fluctuating on one of the channel. On first power on, I set the bias and DC offset on both channels and played some test music and it was all good. I turned the amp on again after an hour or so and this time protection kicked in after about half a minute. The amp does come out of protection on power on but eventually goes into protection after a short arbitrary amount of time. I tried to measure the offset before the protection kicks in and I can see it swinging between 2 digit to 3 digit number on my digital multimeter. Adjusting the DC offset VR seems to have some effect but I can not get the offset voltage to stabilize.

Any clues what could it be please? Any particular area or bunch of components I should test and focus on?

Thanks.

 

[mubs13]

Did some more investigation as below.

Tried to measure DC output on faulty channel before the relay, after the protection kicks in. Is the circled point in the diagram below correct to measure dc offset before relay? The 'brown' point had 38V DC measured with negative to the ground . Orange measured only around 4mv DC.

Also, R channel Tr06 (replaced with KSC3503) on the driver board gets really hot within 10 seconds of turning on the amp. Too hot to touch comfortably. The Tr05 on L channel remains normal.

Is this looking serious? I really hope its not anything major. Please, any help will be much appreciated.

 

[Hyperion]

The KSC3503 - is it installed correctly, no pin mixups? - if it's getting hot it's probably being turned 'hard on' by a fault with another semiconductor, and if it's getting as hot as you say it won't last long before it too goes faulty.

 

[mubs13]

The KSC3503 - is it installed correctly, no pin mixups? - if it's getting hot it's probably being turned 'hard on' by a fault with another semiconductor, and if it's getting as hot as you say it won't last long before it too goes faulty.

Hi John, I have double and triple checked the pins, it's installed correctly. Also, the same transistor Tr05 on the other channel is installed same and is working fine. I feel it's probably something else as you have suggested. I am also worried that if I leave the amp on for too long, it might kill the heating transistor. So I do quick measurements and turn off the amp within a minute. This is making a bit difficult to diagnose too.

I'll keep looking in the mean while. If I could get some clues as to where to look, that could help me a lot.

Cheers.

 

[Hyperion]

So I do quick measurements and turn off the amp within a minute. This is making a bit difficult to diagnose too.

That's what a DBT is for, allowing you to keep the unit on for a bit longer so you can take voltage readings and troubleshoot.

 

[mubs13]

That's what a DBT is for, allowing you to keep the unit on for a bit longer so you can take voltage readings and troubleshoot.

I guess it's time for me to get one

I'll head to my local hardware store to get the parts and make one asap.

Thanks for the tip. Would a 100watt bulb do?

 

[Hyperion]

Would a 100watt bulb do?

I would go lower - 60watt might be better. - try to get both wattages - then you can see for yourself.

 

[mubs13]

Hi all,

Apologies for the late update on this. I had been sidetracked due to job and other family priorities. Progress is slow but I have not given up on my amp yet

I was able to take some measurements before as well as after the protection kicks in. Here is what I found:

Before protection kicks in, B,C & E on Tr06 measure in the range of 1.8 - 2 v. This is similar to readings on Tr05 on good channel.

After protection kicks in, voltage on BCE on Tr06 goes high to ~39v. Same voltage as what I can measure on output before relay. Voltage on EBC on Tr10 also goes up to ~39-40v.

Also, voltage on B & E on driver pair (Tr22 & Tr20) goes high to same ~39v on both transistors. Where as the voltage on these pins on working channel measure around 0.6v (E) and 1.8v (B).

Please note that all the voltage readings are against common ground. Also, some of the above may be -ve in polarity but not indicated.

I am really confused and not really able to figure out what could be causing this rise in voltage. Could it be any of the diodes? Electrolytic capacitors are all new and recently replaced.

 

[Hyperion]

I am sorry I can't help at the moment, tied up with something else at present. I can look again and give some help in about a week or so.

Anybody else, don't be shy, please help if you can.

 

[mubs13]

I am sorry I can't help at the moment, tied up with something else at present. I can look again and give some help in about a week or so.

Anybody else, don't be shy, please help if you can.

Thank you sir, I am in no hurry, so please feel free to respond when you can

 

[Overundr1]

Verify TR02 solder pads do not have any micro breaks, in fact I would inspect the pads with all solder and the transistor removed. A quick continuity check may not indicate a broken (temperature sensitive solder joint).
Also check orientation of C18 and C20 and did you replace D06 with a paired 4148 diode?
Ask if you are not sure what a paired 4148 is
-Lee