Sansui AU-999 Protection board F-1334 resistor smoking

[ReisDaddy]

Hello i just get Sansui AU-999, and as usual i use DBT 60w, and first check dc offset and bias.
But when i turn it on, the bulb bright just 1-2 seconds, then it off, then i smell burnt plastic. Then i turn off, its late night.
So next day i turn it on again i see smoke, but im not sure from where, and i turn it off again. I check all the board, i found that 3 board from the left is not fully plug F-1209, F-1209, F-1208, is it related? After checking any suspicious component, i found R907 burnt. In service manual its 150 ohm,. i replace it, then it smoking again. DBT is not glowing all the time, just glow at first half second like a normal amplifier.
I thought it was TR9001, but it is still good? hfe 44.

From what i read is that we need to remove those protection board F-1334. But im afraid of losing final transistor. So what should i do now?
This my first amplifier that doesnt have relay protection. Please guide me, where to start.

Can you imagine how i feel that the legend already in front of me, but cant hear how it sound...

Any help would be appreciate.

 

[ReisDaddy]

Here is the protection block in service manual

I disconnect the cable (black dot) A and B that they connected to speaker fuse.
Then after power up, the DBT full bright now.
Where to start to what cause the problem?

 

[Hyperion]

This might help

AU-999 Restore - (2SC875/2SA532) replacements on F-1334

Hey all, Doing a full restoration and have seen multiple threads on replacements for this transistor pair. Can someone clear the air ? I have 1220/2690's in stock. I have also seen threads where ZTX695 are used ???? Many thanks in advance.

audiokarma.org

 

[ReisDaddy]

I found this thread is close to my problem, the dbt is like an emergency lamp. Bright, off, brifght , off.. so on by tobyj17

Starting my Au-999 Recap

Now that Cobblefest is over, and my speaker project is near complete.. I've got some time to get my cap list together, and place an order. Here's my Worklog so far: F-1159 boards (quantities for two boards) 16v 47uf (bi-polar) (x2) 25v 100uf (x2) Flying saucers that need to be replaced...

audiokarma.org

I check that one of the bias transistor TR809 (F-1159 Left board) is off. After i solder them, did check that it has hfe reading.
I did replace all TR801 + TR803 differential pair with A992, dc offset is stable now 0.001Vdc, also bias to make it safe i turn it fully counter clockwise.
It is singing! But, right channel only. If i swap the board, right to left, left channel is good, so it is left channel F-1159 board is in problem right?

I did check the voltage in (compare to right board (looks good)
pin 1 = 45vdc,
pin 14 =48v
pin 6 = -37v
pin 9 = -47v

I did test with sinewave 1 KHz using REW (from bluetooth to AUX)
Here is comparation between left and right board in Vac mode:

F-1159 - PIN	13	11	7	5	12	10
LEFT	0.434 Vac	0.425	0.02	0.02	0.02	0.02
RIGHT (Working)	0.57 Vac	0.57	0.070	0.048	0.280	0.330

And also did check all transistor CDC 8002-1 and CDC 9002-1 has hfe reading.

EDIT: All resistor in this board F-1159 (left channel) also checked. All good


Please help, any idea in which part should i check?

 

[ReisDaddy]

Yeahhh success guys!
8 days waiting...

Too many problem piled so many error. Still lucky its just on board F-1159 left channel.
From strange fuse slightly smaller (looks same) for power rail, it not hold tight, not steady.

MY SOLUTION:
- That old trimpot leg not connecting inside, so fragile replace all dc VR801 and bias trimpot VR803 both channel Bourns single turn.
- Strange bias transistor, no name replace with nos 2SC281 (lucky to get one).
- replace unknown transistor at TR824 with MJE350 PNP.


Very fragile, even i can desolder it, but when trying to clip them the leg off. Thats why we cannot measure them correctly right?
When trying to adjust dc in -2mv, while it is close to +, it is easy to get 0, but cannot go +1 or even +5mv.



All power rail in both F-1159
pin 1 = 43v
pin 6 = -41v
pin 9 = -45v
pin 14 = 45v
All close to the manuals.

Now i can set bias and dc offset easily.
Both channel sing!

I did not replace other transistor and recap yet, to compare old vs new.
Cannot wait to test it tonight with my main system chain.


Hope this help future user that has Sansui AU-999 !

 

[ReisDaddy]

Whoaa.. did not realized testing it 1.5 hours non stop, until i see a clock..
It is nice sounding..
But honestly, from my heart, subjectively:

Sansui AU-999 (non mod, non recap)
vs
Sansui AU-7500 (non recap , except signal path only)

Their sound signature is equal? sound same?
And yes the bass more round in AU-7500
So far my favorite is Sansui AU-7500

Cant wait to recap then do mods homework...

 

[ReisDaddy]

This time, i do the Kevzep preamp mod, as usual i test only the right channel, to compare before and after. But after i put the board, and test the sound, i feel that the right sound become less gain? So i feel the vocal position more left instead of more right?



I did check and recheck what was changed to the resistors.

 

[ReisDaddy]

UPDATE:

Nevermind, i think my ears need adaptation. After full recap, and i understand now what is "will get more headroom". After just a view days the sound is amazing. I think its my end game (hope). I feel very-very lucky to acquire this amp. Never regret. Thankyou for the mod @kevzep .

Im in audionirvana ...

 

[bolero]

Agree, kevzep deserves an Academy Award for the circuit tweaks he figured out, and shared with everyone.

My AU-999 was totally lackluster, stock. Now it is an endgame amp for me. And I haven't done the CCS mod either.