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Sansui G7500 HELP.

More info....I desoldered the left channel outputs.....they tested ok. I only have 1 set of outputs left.....should I put in new outputs on the left channel anyway?
 
Well, well.....right channel 2sa1106 was suspect...........it measured funny. I will change this'un. All other main outputs tested fine. Now we are getting somewhere.

Now it is just a problem of concluding if that right tr was busted before or after the new board went in! I have not had full power on the new board yet....only 100watt dbt power..............IF there was a problem with the new board's right channel(I did test all tr's on the new board)....COULD it have smoked the main output from dbt power??

Lets see........................................
 
So, you have powered up the driver board without the Output transistors connected and it was fine right?

Next, before you fire it up again, you need to make sure the Positive DC supply from the big cap goes to the NPN output transistors, and the Negative DC supply goes to the PNP output transistors.....
You need to very clear that this is correct.....
Because if I understand this correctly, you can fire this thing up okay with the outputs disconnected......
The symptom of this kind of oscillation to me is transistors with the wrong polarity supply going to them.....
To add to the problem, you also have found a suspect OP transistor too....

Lets us know what happens next....
 
anderslober said:
hey guys...where are ya'? a lot of new info.......see also pic of the bad tr.
Click to expand...

Why do you think this one is bad?


You need to go Base> Collector (one way will show OL the other will show the voltage drop)
Then Base > Emitter do the same as above....

Then Emitter to collector should be OL both ways....

From those two pic's it looks like that transistor is OK so far.....
 
anderslober said:
2sa1106 is BCE. I did the 6-way test.........5 'OL''s.
Click to expand...

Oh yes sorry Anders, you are correct, I do apologize, I am asleep on the Job, well then its open circuit.....Bad, kaput!!
Weird, they normally short out....

That 'could' make the amp oscillate the way it was....
 
Oh and hey Anders, if you need more you can order up NJW1302 and NJW3281 as excellent substitutes for the output transistors....
 
Oh boy............................................I'll check everything again and attempt a dbt. Just one thing........could other things in the driver section have suffered during the earlier dbt test(pulsating)? Or would you think its ok to dbt? Does the driver section get juice even when the outputs are disconnected? The dbt went very dim during that test, so that would indicate that the driver section is ok??
 
anderslober said:
Oh boy............................................I'll check everything again and attempt a dbt. Just one thing........could other things in the driver section have suffered during the earlier dbt test(pulsating)? Or would you think its ok to dbt? Does the driver section get juice even when the outputs are disconnected? The dbt went very dim during that test, so that would indicate that the driver section is ok??
Click to expand...

The main DC supply does the Output section and the Driver (TO220) transistors, but the rest of the driver board is powered off the supply on the driver board....
So yes, this narrows the fault down to the OP section. The only thing to check would be the Driver transistors.....
Then if they are OK , I say fire it up.....

I have to go out for a couple of hours to do my National Grade Rally License exam, but I will be back online say in 2 hours......

I'm working on a McIntosh MC250 at the moment, that is some experience I can tell you!!! The thing was a basket case!!!!
 
Rally....as in cars? My dad had a stint in the East African Rally a long time ago. Non-pro section.

Don't worry....I am 'a basket case' after this Sansui!!

Anyway...I will test the 2 drivers and then DBT................
 
Anders - I have spotted some Brown and Blue wires connected to your board that are NOT the same as the pics provided by Rob.

In the top right corner of the image of your board it goes blue-brown-brown-blue and in the other image (Rob's) it goes brown-blue-brown-blue.

No idea what those wires are - or if it is significant, but you might want to check this out. It goes without saying DO NOT change these wires until you get some confirmation from Kev or Lee on this - after all I have never worked on a G-7500.

Rob's Pic
100_4492.jpg
 

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John...please check post #235. I agree with you.........my 'old' board was blue-brown-brown-blue. The new board was brown-blue-brown-blue. I rigged as my old board. It is the power supply for the MIC board...+/-33v.

Could this cause oscillation/pulsating???

Remember...I disconnected the outputs and the dbt was a succes.....bright and then very steady dim.................the wires to the MIC board were still hooked up.
 
anderslober said:
John...please check post #235. I agree with you.........my 'old' board was blue-brown-brown-blue. The new board was brown-blue-brown-blue. I rigged as my old board. It is the power supply for the MIC board...+/-33v.

Could this cause oscillation/pulsating???

Remember...I disconnected the outputs and the dbt was a succes.....bright and then very steady dim.................the wires to the MIC board were still hooked up.
Click to expand...

Possibly.... it would very likely have the effect of unbalancing that +/- 33v supply making the voltages assymetrical - so if those supplies go anywhere else it would affect those circuits too.

I can't see the connections for the other wires very well in the pic's - perhaps you could just double check that all of those are as they should be - verifying that if there are 2 reds (for example) that each red goes to the correct point on the PCB.

Only note any variance - and run it past Kev and or Lee - they are the "thread masters" at the moment. :thmbsp:
 
anderslober said:
Rally....as in cars? My dad had a stint in the East African Rally a long time ago. Non-pro section.

Don't worry....I am 'a basket case' after this Sansui!!

Anyway...I will test the 2 drivers and then DBT................
Click to expand...

Yes Anders, Rally Cars.....Its my adrenalin fix....Ive been competing for about 15 years.......I have a 400hp Toyota Celica GT4.....

anderslober said:
Just tried de-soldering those 2 wires(power for MIC board)........still pulsating dbt.
Click to expand...

Its totally got everything to do with the output section.
I think you have a revised board, and I can tell you I had problems like this with my revised board I used in an earlier G7700 (same amplifier assembly as the 7500).
I think you have to start at one of the drivers, and follow the circuit along through to the output stage, transistor by transistor......
Its painful and takes a long time, but you have to do it......This is where Sansui is a pain in the arse!!!
There are a lot of those blue/grey resistors which go open and high impedance on you, especially when the circuit has been stressed, I took out a LOT of resistors doing my 7700....
Check the following,
R41, feedback resistor
R53/65 Base resistors of the OP's
R51/53 Emitter resistors for the Drivers
R35/39 Emitter resistors for the Pre-drivers
R37, this can go open or high when the bias skyrockets, should be 10.5Ω.
Make sure TR15 the NPN driver 2SD381 goes to the NPN output 2SC1106, and that the are on the (+) supply (I know we have checked this, but we are doing it again).
Then work through the rest of it the same way.....
The schematic will be correct, and you have to print it off and use a pen to colour in the parts of the circuit you have traced with a highlighter marker pen......check it of bit by bit.
I am convinced somehow, and I dont know how, there are output/driver polarity issues.......

There is an error in there and we will find it....
 
Ouch! Ok.......I will check everything that you have listed. Reg. polarity issues....do you mean the main outputs...I have checked and re-checked them 100's of times. Please confirm how they should be mounted...........2sc2581-2sa1106-2sa1106-2sc2581. This is the original config. This is how mine are mounted. There is also another thread with a 7500 where they are mounted like this. This unit has worked with this config before. SO, I think it may be an open resistor or tr.....somewhere in the driver section.

Anyway,,,after changing the bad main tr....I dbt'ed.......pulsating......do you think I should check ALL the main outputs again? OR did the dbt save something?

I can't believe Sansui made goo like this. The qrx9001 never gave me this kind of attitude!
 
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