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ScarabParamit

Regards to all Audiophiles + help needed with Sansui AU-888

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Hello there, first of all sorry for my english.

I heve sign in to this forum seeking help. 3 months ago my Sansui AU-888 right chanel has burned...most of transistors went down.

I menaged to replace components with equals or similar replacements, still i stoped on bias mesurment/adjusting mA and mV.

I hoped member "skippy124" with his large experiance could help me...sending instructions where to plug in multi-meter to mesure mA, i poses shematic but "plug + to 2 and - to B1" is just black magic for me.

Things i have done so far:

Inserted right quick acting fuses

Replaced burned 1Kom resistor (same for left channel to keep them "same", as new resistors have +/-5% and old one have +/-10% tolerance)

Changed all caps to Phanasonic for Audio, nichicon fine gold/fine grade, mkt, Elna for Audio, same capacity exept for main filter caps, few are bipolars as schematics sugest etc.

Changed VR regulation pot's to new ones (i heared old's are preety bad)

Switched voltage selector from 220 to 230 (right value in my "country", i know voltage varies +/-10% but i wanted to be on the safe side)

As all right chanel transistors burned and infamous "flying sourcers" ware present i decided to change them all:

2SC281 -> 2N3904 ....(in service manual there is 2SC984 but mine had 2SC281)

SC1111 -> C1402

2SC680 -> 2SC5171

2SA566 -> 2SA1930

CDC 8002 -> 2SC5171

CDC 9002 -> 2SA1930

Are transistors right choices?

in the future, what to insert in place of XA-495G ?

transistors whare choosen from bunch to mach hFe, are mounted on silicon thermopads and most with handcrafted "radiators" ("to be on the safe side")

now i need specific photos and instructions other than from unclear (for me) manual where to plug multimeter to set proper mA (is it in place of pulled out fuse? F003/F004)

mV mesurment is clear.

also is "minimal counterclockwise" = "turn to the right as far as possible" ?

If no one help me i will be forced to give this amp to some spec...problem is i spend most of my money reserves already =(

Link to two photos, shematics, datasheet of transistors and service manual:

http://www.mediafire...iokncl6stku6cou

Regards and thanks in advance.

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Greetings will tell skip yo have a read

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Hi,

Welcome to the forum and sorry to hear about your troubles with your 888.

I will be able to provide some guidance on an alternative bias setting procedure without needing to remove the fuses and measure the current, but I have been busy tonight, and am busy tomorrow night, so it will be a couple of days before I can post this up. To answer one of your questions, counter clockwise is turn to the left.

One thing that you can advise in the mean time, how did you mount the 2SC281 alternative (2N3904).

Cheers

John

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Thanks djb and skippy124.

I used aluminium and brass tubes, but simplest way is to take old antena, and discard outside and inside tubes until it fits and fill clearness between oryginal mounting piece and transistor. Also if clearness will still be a problem You could also crush/use pressure to change tubes shape from circle to elipse...

http://www.mediafire.com/?envwage8gdnsnt4

As for test method i'm afraid transistors may burn one more time, can i also ask for method with fuses removed?

...also "clockwise" is right, "counter..." is left ...but "minimal...counter...clockwise" is ...right? This is just stupid, why they didn't just stated "turn left/right to the end" -.-

By regulating pot's are turned right to the end and i think thats "minimal counterclockwise" ...or "maximal clockwise".

B.T.W. What is "State: ACT/NSW/VIC" under avatar?

Regards.

Edited by ScarabParamit

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That looks an acceptable thermal result for the 2N3904.

If you haven't powered up the amp after the repair, then I strongly suggest that you make up a dim bulb tester to use when first powering up. There is a lot of info re the construction of a dim bulb tester in the net, just do a google search. There is a sketch I did for an Australian version here. Anything from a 60W to 100W globe will be fine.

http://www.audiokarm...05&postcount=31

If you set up the meter in place of the fuses to measure Bias current, and there is still a problem on that channel after the repairs, then you will blow the meter fuse, and also may cause more component damage. Make up the dim bulb tester, and them connect the amp via this tester (with the bias pots wound fully to the left). The bulb in the tester should glow briefly and then go dim/very dim. If it comes on brightly and stays that way, then turn off the amp immediately, you still have a problems that will need to be found.

Let me know how you go, if the amp powers up correctly you can then have a go at checking and setting the Bias current, and I can provide a couple of ways of doing this.

Cheers

John

Edited by skippy124

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B.T.W. What is "State: ACT/NSW/VIC" under avatar?

Regards.

They are the states of Australia...............obviously they don't apply to you!

Good luck with your amp.

Skippy is a top man.

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Well, with amount of components replaced we can say both chanels are "new"...and need to be tested.

Before i posted for help i powered amp after repairs but without fuses F003/F004...I pluged multimeter in place of removed fuse...with pots (VR801 and VR803) turned to the right almost to the end...i have ~1.51mA on both chanels, then i turned pot VR803 (responsible for mA adjusting) to the right to the end, after mesuring on fuse sockets i have exacly 1.12mA on both chanels.

So turning those pots to the right lowers current, are You realy shure i supposed to turn them to the left??? ...this will rise mA an mV as i understand

Also, i can add non-conductive thermal paste around 2N3904 and brass tube, or this is uneccesary?

Edit: Anyone? Can then i just ask for Bias adjust methods ...without this knowledge-portioning-policy...its realy pointless.

I want to finish this project before christmas.

Regards.

Edited by ScarabParamit

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Hi

Sorry for not getting back to you sooner.

The bias current measurement you have done does not sound like you are getting the correct readings.

Try this method - you will need to remove the bottom of the amp and stand it up on one end, and then connect a voltmeter to either of the output transistor emitter resistors and set for a reading of between 12-15mv. it doesn't matter which resistor you measure for each channel. Be careful when measuring, insulated "mini grabbers" or similar are recommended, a slip of a meter probe and output transistors can be blown.

Biascurrent_2.jpg

Biascurrent_1.jpg

Re the thermal paste for the 2N3904, it won't cause any problems and may help.

Hope this helps

Cheers

John

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Don't understand a word but following with interest

Assume the dim globe tester saves the lives of mine canaries?

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Don't understand a word but following with interest

Assume the dim globe tester saves the lives of mine canaries?

The dim bulb tester is the Mine canary only it doesn't die.

Edited by vintageaudio50

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Well, with amount of components replaced we can say both chanels are "new"...and need to be tested.

Before i posted for help i powered amp after repairs but without fuses F003/F004...I pluged multimeter in place of removed fuse...with pots (VR801 and VR803) turned to the right almost to the end...i have ~1.51mA on both chanels, then i turned pot VR803 (responsible for mA adjusting) to the right to the end, after mesuring on fuse sockets i have exacly 1.12mA on both chanels.

So turning those pots to the right lowers current, are You realy shure i supposed to turn them to the left??? ...this will rise mA an mV as i understand

Also, i can add non-conductive thermal paste around 2N3904 and brass tube, or this is uneccesary?

Edit: Anyone? Can then i just ask for Bias adjust methods ...without this knowledge-portioning-policy...its realy pointless.

I want to finish this project before christmas.

Regards.

Scarab are you based in the ACT not to take anything away from John but I know of a repairer who's very good with amplifiers in Canberra and he's very good and not overly expensive.

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Hello there, i hope anyone will still read this.

 

 

(there was small "collision" in opinion which side pots should be turned before meashuring ...here's a link to topic where someone states clearly:

"Well, my fuse kept blowing because the pot turned all the way counterclockwise was full on. All the way clockwise was 0. I think it was another example of a dyslexic Sansui."

http://www.audiokarma.org/forums/archive/index.php/t-191187.html

 

So "minimal couterclockwise" was "max to right" as i though.)

 

Because of time shortage i left my Sansui repairs...until recently, i set pots (VR801 and VR803) to the right to the end and with multimeter plug on removed fuse socket F003/F004 left/right i started to turn pots to the left...

from 1.12mA i was able to set 30mA with pots turned into middle position, but now i have problem:

 

"Bias drift" ???...if i set anything above ~10-15mA readings starts to rise on its own ...slowly 15 become 20...and those 20,25,30 and so on.

I have this on both channels, is this normal? is capacitors charging or transistors are starting to whip Up/awake ...or this is coused by thermal changes or some resistors need to be replaced to something sturdier?

Additional important things to know:

Rising occour only with multimeter pluged into fuse socket, when multimeter is unpluged and pluged again readings don't drop (only little like ~0.5mA), they just starts to rise from moment before unpluging.

Rising occour on both channels, but separatley, so if drift rises on one channel other one is not affected.

Rising will not occour if pot (VR801 and VR803) i set under ~10mA...it just "return" and stays on this reading, if set-up higer mA starts to rise again.

The higer mA is set the faster it rises on its own.

Rising occour constantly with speed of ~0.02-0.1mA every half second.

No interconnects, speakers or cables other than power cable are connected.

Speaker fuses (accesed from back side) whare not removed.

Nothing exploded, melted or burned =)

 

Oh...and i Forgot...there are 4 pots overall, two (left/right channel) for mA (VR803) and two (left/right channel) for mV (VR801)

I was turning only (VR803) ...(VR801) was set to "minimal" on both channels, maybe this has something to do with it?

 

Here i upload service manual with shematics and instructions of bias setup:

 

http://www.mediafire.com/view/8wcj9d5tqk6h8gc/hfe_sansui_au-888_op_service.pdf

 

I didn't check how high mA can rise...im affraid of it.

 

P.S. i also added nonconductive thermal paste for 2N3904 as adviced, and rebuild a little backside to add EIC type socket (as most audio cables use this standard) ...yes i know its might be a profanity but that was the right call.

Here are some pics (fuse sockets are next to change):

 

http://imgur.com/a/oFLxt#0

 

I hope someone will know what to do.

Also about this post:

 

"...

Try this method - you will need to remove the bottom of the amp and stand it up on one end, and then connect a voltmeter to either of the output transistor emitter resistors and set for a reading of between 12-15mv. it doesn't matter which resistor you measure for each channel. Be careful when measuring, insulated "mini grabbers" or similar are recommended, a slip of a meter probe and output transistors can be blown.

Biascurrent_2.jpg

Biascurrent_1.jpg

...Cheers

John"

I'm supposed to do this with or without F003/F004 fuses ??

 

 

Regards =)

Edited by ScarabParamit

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