Sansui 3000a protect light

The Fuxtor

Addicted Member
So I finally pulled out my Sansui 3000a with original packing box and some literature ! Unit is is very nice condition and I was pleased to see the service recall issue had been taken care of with factory issued mounts! Been listening to it's excellent tuner for a couple weeks but recently the unit shuts down and has the red protect light on....
Question, I haven't looked at the service manual yet, but I assume the issue is heat related as it plays normally for a period of time until the protect comes on? Thinking about pulling the outputs and reapplying some fresh thermal grease ? I will check the bias while it's open too. Anything else to check??
 
Ok, well checking the dc offset shows about 1.2 dcv, but it appears to be a transformer coupled amp? So really it should read 0 I believe? Not sure on this... The safety mod has been done which added some coupling caps as well?? Either way I was able to adjust the bias to 25mv at the emitter resistors, the manual shows pulling the fuses but my meter has a dud fuse for measuring the 80ma there...
At this point I am telling myself I need to recap this one, but what's the forum think??
Charles
 
Good to hear bias is staying fine, it still goes into protect however? Is that 1.2 volts you're seeing on the output? Or mV? It is transformer coupled, but not to the speakers, just the input to driver & output stage.
 
Good to hear bias is staying fine, it still goes into protect however? Is that 1.2 volts you're seeing on the output? Or mV? It is transformer coupled, but not to the speakers, just the input to driver & output stage.
Interesting setup for sure....
1.2 dcv at output ( speaker inputs terminals)..... After 2-3 hours of use , protect light comes on and unit becomes silent!
 
have been looking at the protect circuit trying to see how it works ..
looks like the scr turns the light on and mutes the inputs to the channels .. cant figure out what triggers it yet but it looks to be adjustable . i wouldn't mess with adjusting it as we dont know how its supposed to be set
suppose you could change tr901 to see if it fixes it . doing this however may change the triggering point .
 
The amplifier is by nature incredibly unstable. With that sort of DC on the output it obviously has not had the capacitor coupled mod done to the output.
You need to go through, replace all the trimmers and then set the amp up again and see if its stable.
If not you'll have to dig deeper.

Forget about the protection circuit for now, the amplifier is where the problem is.
 
Forget about the protection circuit for now, the amplifier is where the problem is.
i would like to understand the protect circuit some more . as it helps me learn as to where the fault lies .or close proximity . what actually triggers it to be on ? like what is it monitoring ? yes i see there is dc at the output but cant see why it would trigger protect . i do see d901 d902 .....
sorry if i am late to the party with this unit i am just keen to learn and help as per.
 
Screenshot_20180828-180640.png
The amplifier is by nature incredibly unstable. With that sort of DC on the output it obviously has not had the capacitor coupled mod done to the output.
You need to go through, replace all the trimmers and then set the amp up again and see if its stable.
If not you'll have to dig deeper.

Forget about the protection circuit for now, the amplifier is where the problem is.
The unit has the safety mods done, but looking at it , was probably done 45+ years ago. All factory clamps and same era caps used. I'll grab a pic next time I have it opened up...
I grabbed a pic that looks similar in setup, but the the caps in my unit are the old grey caps identical to the power supply caps, the ones in the pic look more modern...
 
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There is a lot more to the mod than just those caps. There are several diodes to change or add and also some resistors.
file:///C:/Users/Patrick/Downloads/CE007A_3000_3000A.pdf
 

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There is a lot more to the mod than just those caps. There are several diodes to change or add and also some resistors.
file:///C:/Users/Patrick/Downloads/CE007A_3000_3000A.pdf

We realise this, my point was how on earth is the DC getting through the output caps.....
 
We realise this, my point was how on earth is the DC getting through the output caps.....
Ok, well to further expand, I tested at speaker terminals while the unit was under a load... I had a pair of dummy loads connected... Manual asks for 10 ohm 1watt resistor but just used my 8 ohm 100 watt resistors..
Anyone have any guidance on what/how to test more?? I can retest with another meter to double check as well .
Oh, I just to clarify, the dummy loads were connected for biasing purposes....
not specifically for DC offset testing
 
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might be the test equipment .. on another note what is vr803 an vr804 for ?
They are the trimpots for the left channel.... There are 4 trimmers on this unit. Biasing calls for dialing in 80ma at each fuse holder, 2 for each channel...
I found that as one test point was adjusted, it would pull the other test point on the channel down. I had to use 2 dmm to equal them.
The bottom 2 pics are the DC offset measurements . I retested using 2 different dmm's
 

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Yeah, biasing these amps is tricky because you are adjusting each individual rail, and depending on how patient and particular you are, when you eventually get the balance right, then the DC offset will be in range.
Best to use a pair of meters as you have found out, one channel affects the other...
 
Yeah, biasing these amps is tricky because you are adjusting each individual rail, and depending on how patient and particular you are, when you eventually get the balance right, then the DC offset will be in range.
Best to use a pair of meters as you have found out, one channel affects the other...
On this note, do you think 25mv is a decent target? Measuring across emitter resistors for this....
 
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