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Sansui AU-919 Protection

ivandezande

ivandezande

AK Subscriber
Subscriber
I made the mistake of restoring a Sansui AU-919 for a friend. Never again...

Anyways, this one when turned on sometimes comes out of protection for a few seconds. It did this before restoration as well.

The amp section is not spitting out DC, and will bias fine. I replaced the caps and relays on the protection board, but that's it. Shotgun the transistors? Anything strange about this model I haven't found? Did a lot of reading to no avail.
 
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ivandezande said:
I made the mistake of restoring a Sansui AU-919 for a friend. Never again...
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Oh dear, what was it about the AU-919 that make you say that?
ivandezande said:
Anyways, this one when turned on sometimes comes out of protection for a few seconds. It did this before restoration as well.
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Can you clarify this, 'comes out of protection for a few seconds' - so, the LED stops flashing and the speaker relay closes - is that a good description?
ivandezande said:
The amp section is not spitting out DC, and will bias fine. I replaced the caps and relays on the protection board, but that's it. Shotgun the transistors? Anything strange about this model I haven't found? Did a lot of reading to no avail.
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Nothing that I am aware of, other than the protection relay driver transistor gets very tired (running in saturation all the time), a replacement is a good idea. And the thyristor (2SF656) in the protection circuit has given problems in this and other models where it is used. The trigger circuit for this is measuring the voltage (or current, effectively) through one of the OP transistor emitter resistors, so if the thyristor is faulty, I think it will cause issues, just not sure if it will cause the issue you are seeing. There is a modern equivalent for this, the type number escapes me at present - but I can find it if needed. ;)
 
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Hyperion said:
Oh dear, what was it about the AU-919 that make you say that?
Can you clarify this, 'comes out of protection for a few seconds' - so, the LED stops flashing and the speaker relay closes - is that a good description?
Nothing that I am aware of, other than the protection relay driver transistor gets very tired (running in saturation all the time), a replacement is a good idea. And the thyristor (2SF656) in the protection circuit has given problems in other models where it is used. The trigger circuit for this is measuring the voltage (or current, effectively) through one of the OP transistor emitter resistors, so if the thyristor is faulty, I think it will cause issues, just not sure if it will cause the issue you are seeing. There is a modern equivalent for this, the type number escapes me at present - but I can find it if needed. ;)
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It’s just a really packed amp that doesn’t come apart nearly as easy as say a 717.
Yup, that’s exactly what it does.
Cool, I’ll replace the drive transistor and do some research on the thyristor.
 
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If it were me (god help us all), I'd set out to replace the relay and check/replace components in the relay/protection circuit... Assume you've done caps already so maybe check resistors/diodes/transistors
 
AUD101 said:
Possible 2SF656 Thyristor replacement's are: (Please read carefully all of the following posts)
MCR100-6G - https://www.audiokarma.org/forums/index.php?threads/au-519-project-protection.603908/page-4 - (pins need to be addressed)
X0402MF - https://audiokarma.org/forums/index...sui-au-719-protection-mode-help.759837/page-3 - (Pin for pin replacement)

Edit:
Also see here: https://audiokarma.org/forums/index.php?threads/small-job-on-au999-big-job-on-au-x1.726770/page-7
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Is there something I can toss in it's place (wire bridge, normal diode) until I order the part to see if that's the issue?
 
Side note, would love to hear how you think the 919 compares to a 717 both being restored. Other than the power difference :)
 
Over here there are still some 03p1m (100V 0.47A) and 03p2m (200V 0.47A) available. Another option could be the 03p4m. These are all TO-92 NEC parts with pin order 1-Gate 2-Anode 3-Cathode.
 
I bought some of these - just in case - I haven't used any of them yet. ;)
https://uk.rs-online.com/web/p/thyristors/7142647/?sra=pstk

The original thyristor 2SF656 pin order is G A K (leads down and flat facing you). The one I suggest here has pin order K G A - rated at 400V and 0.8A, which is plenty, not a big deal to reposition the leads to suit. ;) (I bought 400V rated devices, so I can use them elsewhere if I need to).
 
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Actually digikey currently stocs the littelfuse S8X5ECS TO-92 with GAK pin order, 800V 500mA, and the BT169D which comes in 400V. 8A,in TO-92 but the pin order of this part is AGK. Mouser also carries the bt169, so at this noment there are plenty of options.
 
So, it's still not 100% happy.

It played for a few hours, sounding great, then started humming/hissing in the left channel decently loud.

I turned it off again, plugged in just headphones, and set the pre/main switch to disconnected. No hum!
Then I slid the switch over, and it started the noise again, but putting it back to disconnected didn't stop the hum/hiss.
Turning it off and on again, still the hum/hiss.
 
ivandezande said:
So, it's still not 100% happy.

It played for a few hours, sounding great, then started humming/hissing in the left channel decently loud.

I turned it off again, plugged in just headphones, and set the pre/main switch to disconnected. No hum!
Then I slid the switch over, and it started the noise again, but putting it back to disconnected didn't stop the hum/hiss.
Turning it off and on again, still the hum/hiss.
Click to expand...

Have you tried cleaning the separated/connected switch itself? I've read that it can cause issues. :dunno:
 
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