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Sansui AU-919 Protection

Did you replace every single SA726 and SC1313? They're notoriously noisy. What happens when you disengage the flat amp by using the jump switch?
 
lbcgav said:
Did you replace every single SA726 and SC1313? They're notoriously noisy. What happens when you disengage the flat amp by using the jump switch?
Click to expand...
Yup, but it's not noise, it's for sure an oscillation of some sort because it's triggered somehow. With jump engaged at startup the noise is gone, and I can't seem to make the noise start by switching the main in switch like I could before.

With the Jump switch engaged, I still can't make the noise come back. Hmm.
 
Last edited:
ivandezande said:
Yup, but it's not noise, it's for sure an oscillation of some sort because it's triggered somehow. With jump engaged at startup the noise is gone, and I can't seem to make the noise start by switching the main in switch like I could before.

With the Jump switch engaged, I still can't make the noise come back. Hmm.
Click to expand...
Could it be the FETs on the flat amp? Did you change the MV12s on the flat amp as well?
 
ivandezande said:
For sure could be... Yup, all varistors were replaced.
I'll keep trying to get the noise to occur.
Click to expand...

Hi, flying above all that is related to 919 as one awaits for me to dive in and recap it and one very subject that I dig out nowadays concerns these varistors.
I found few hints here and there and ended up with the following (you to tell me if you did samely)
the MV 12 can be replace by 2 x 1N4148 in serie
the MV103 or SV03 can be replace by 3 x 1N4150 in serie
Usage of Series types multi-chip silicon diodes under SOT-26 case as in HERE could do the job with values almost as close as the genuine product.

What was your pick to replace these Varistors ?

Thanks, Yves
 
le_syd1961 said:
Hi, flying above all that is related to 919 as one awaits for me to dive in and recap it and one very subject that I dig out nowadays concerns these varistors.
I found few hints here and there and ended up with the following (you to tell me if you did samely)
the MV 12 can be replace by 2 x 1N4148 in serie
the MV103 or SV03 can be replace by 3 x 1N4150 in serie
Usage of Series types multi-chip silicon diodes under SOT-26 case as in HERE could do the job with values almost as close as the genuine product.

What was your pick to replace these Varistors ?

Thanks, Yves
Click to expand...
Yes to the MV12.
I left the triple diodes as there was no clear consensus on replacement. Didn't want to ruin anything.
 
I thought I'd just add another cause for protection kicking in on this model - not that it's a weak link but it had me scratching my head for a while after a full re-cap (the problem was there before the re-cap but lots were in need of replacement anyway so I got all that out of the way first.)
Bias and DC offset were set up after repair and was all perfectly fine and in spec.

Anyway .... at anything up to about -42 on the volume it all worked well, sounded sweet, but turn the volume above that and protection kicked in.
Turn the volume down and it would subsequently come out of protection after a second or two.


A bit of detective work ensued:

I then confirmed it wasn't simply DC protection kicking in by simple checks at pin 1&2 (DC protection) of the protection board (F2840) , I then checked pin 20 (over current prot) and there it was.
So as this protection signal is summed on the power amp board (F2839) from both left and right channels, it's easier to just pull the rca plug and just drive the amp with left or right .... with both right and left the amp faulted as described, with just the right rca plug the amp worked fine, with just the left connected it quickly pinpointed that the problem was just on the left channel.

Double checked that DC wasn't entering the power amp board (F2839) at pin 1 & 2 - nothing there... so I'd narrowed it down to one channel and one area (something on F2839 :eek:)

After a lot of voltage checks and a few component confirmations, it turns out it was TR33 (leaky) a lowly 2SC1845 (part of the protection signalling) , simply wiggling the body of it would trip the amp .... spraying a tiny bit of freezer on it double confirmed it was the culprit....:rockon:

A case of not a actual protection fault, but rather the protection signalling system at fault :beerchug:
 
Hello!

Sorry to resurrect an old thread, but i think it's a useful contribution which might help someone in the future.

This thread was enormously helpful in helping me fix my Au-919. It was stuck in protection mode, but no shorts or other perceivable problems present. I replaced the thyristor on the protection board with a X0402MF. The old one was faulty according to my component tester and sure enough - the unit started working and leaving protection mode after a few seconds on start. However - bringing the volume up anything past around -26 caused it to re-enter protection mode. At this point - ZaphodBB's latest post saved me a lot of time. I checked that DC wasn't entering the power amp board as well - it wasn't. So, I went straight to TR33 and gave it a few taps with my black spudger. Tapping it made no difference, so I checked the corresponding 2SC1845 on the other channel. To my delight - tapping this one (TR34) caused the amp to enter protection mode straight away. I replaced it and now the amp works absolutely perfectly.

I suspect the thyristor + TR33/34 faults are common on this unit and hopefully this thread helps lots of people in the future. Obviously anyone tackling this model should perform their own checks to confirm they have the same faults before blindly replacing, but it saves a lot of time knowing where to check first. I gotta say - I absolutely HATED working on this amp. It's very impressively constructed, but replacing anything on any of the boards comes with a great deal of disassembly and hard-wired connections to deal with. In the process of replacing the thyristor, some cold solder joints on the fuse PCB were further stressed and stopped working. This resulted in no power light at all - it took me quite a while to discover the cause of this, eventually realising it was cold solder joints on a large resistor on the fuse board.

Best of luck to anyone tackling this model!
 
ZaphodBB said:
I thought I'd just add another cause for protection kicking in on this model - not that it's a weak link but it had me scratching my head for a while after a full re-cap (the problem was there before the re-cap but lots were in need of replacement anyway so I got all that out of the way first.)
Bias and DC offset were set up after repair and was all perfectly fine and in spec.

Anyway .... at anything up to about -42 on the volume it all worked well, sounded sweet, but turn the volume above that and protection kicked in.
Turn the volume down and it would subsequently come out of protection after a second or two.


A bit of detective work ensued:

I then confirmed it wasn't simply DC protection kicking in by simple checks at pin 1&2 (DC protection) of the protection board (F2840) , I then checked pin 20 (over current prot) and there it was.
So as this protection signal is summed on the power amp board (F2839) from both left and right channels, it's easier to just pull the rca plug and just drive the amp with left or right .... with both right and left the amp faulted as described, with just the right rca plug the amp worked fine, with just the left connected it quickly pinpointed that the problem was just on the left channel.

Double checked that DC wasn't entering the power amp board (F2839) at pin 1 & 2 - nothing there... so I'd narrowed it down to one channel and one area (something on F2839 :eek:)

After a lot of voltage checks and a few component confirmations, it turns out it was TR33 (leaky) a lowly 2SC1845 (part of the protection signalling) , simply wiggling the body of it would trip the amp .... spraying a tiny bit of freezer on it double confirmed it was the culprit....:rockon:

A case of not a actual protection fault, but rather the protection signalling system at fault :beerchug:
Click to expand...

Richard Leo said:
Hello!

Sorry to resurrect an old thread, but i think it's a useful contribution which might help someone in the future.

This thread was enormously helpful in helping me fix my Au-919. It was stuck in protection mode, but no shorts or other perceivable problems present. I replaced the thyristor on the protection board with a X0402MF. The old one was faulty according to my component tester and sure enough - the unit started working and leaving protection mode after a few seconds on start. However - bringing the volume up anything past around -26 caused it to re-enter protection mode. At this point - ZaphodBB's latest post saved me a lot of time. I checked that DC wasn't entering the power amp board as well - it wasn't. So, I went straight to TR33 and gave it a few taps with my black spudger. Tapping it made no difference, so I checked the corresponding 2SC1845 on the other channel. To my delight - tapping this one (TR34) caused the amp to enter protection mode straight away. I replaced it and now the amp works absolutely perfectly.

I suspect the thyristor + TR33/34 faults are common on this unit and hopefully this thread helps lots of people in the future. Obviously anyone tackling this model should perform their own checks to confirm they have the same faults before blindly replacing, but it saves a lot of time knowing where to check first. I gotta say - I absolutely HATED working on this amp. It's very impressively constructed, but replacing anything on any of the boards comes with a great deal of disassembly and hard-wired connections to deal with. In the process of replacing the thyristor, some cold solder joints on the fuse PCB were further stressed and stopped working. This resulted in no power light at all - it took me quite a while to discover the cause of this, eventually realising it was cold solder joints on a large resistor on the fuse board.

Best of luck to anyone tackling this model!
Click to expand...

Thanks above for sharing the problem and solution to solve the protection problem. Not going to create another thread about my D907 going in to protection mode.
My D907 went into protection mode intermittently.I track down the problem to the left channel by pulling the 3 pin plug coming from F-2845 into the F-2839.The right channel alone don't go into protection.
When not in protection mode, when volume turn up to -28 will trigger the protection.
After replacing the TR-33 with a new KSC1845, My unit is now happily playing.

Here the culprit which test good.
20230409_112129.jpg

For future ref:
Left Channel TR-33 BAD
B = 1.29V
C = 1.69V
E = 0.67V

Right Channel TR-34 GOOD
B = 0.59V
C = 27.94V
E = 0.23V

Hope above information will help other too

Peter
 
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