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Sansui AU-719: how does the protection work?

PE9ZZ

PE9ZZ

Event Horizon
Subscriber
I have this amp on my workbench and it went into protect after some time. As per complaint. The thyristor SCR601 was bad. I should have replaced it with a replacement (MCR100) but since this was not present in my tray and there is time pressure I replaced it with a DIY thyristor consisting of an NPN (BC547B) and a PNP (BC557B) transistor (image Wackypedia):
Thyristor.svg.png

While this seems to have solved the problem (still need to do the soak test to confirm) I cannot figure out how this circuit is supposed to work. Very helpfully Sansui included this block diagram but failed to provide an explanation:
3.png

Question is: how does it work? Is it sensing the driver going into clipping, detecting high frequency components associated with clipping or something I am overlooking? In other words: what is TR17 detecting?
 
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It is even worse: it is fundamentally different. The 517/717 uses a sense transistor (TR09) across an emitter resistor of the output stage (R44). The 719 does its mystery detecting in the driver stage and is capacitively coupled.
 
petehall347 said:
its not the same .
Click to expand...
I am a novice at electronics. And your right it is not the same, with the same idea I meant : detecting DC; detecting excessive current from the driver board. Because you where asking about clipping detection.
But you're in good hands because @PE9ZZ is a professional. So I am going to keep following your thread and hope he can give an explanation that I as a novice can understand.
 
PE9ZZ said:
and is capacitively coupled.
Click to expand...
If you look at the schematic rather than the block diagram, you will see that its input at least, is DC coupled - with a 100KΩ resistor. The capacitive coupling at the output (C24, 1µF) must mean that a sharp voltage change (when TR17 turns on), passes through the capacitor as a 'spike' - and latches the Thyristor on, which after a series of events opens the protection relay. I also think TR17 is 'detecting' the B-E voltage of TR701, and the voltage drop across R52 (0.33Ω).
 
Hyperion said:
If you look at the schematic rather than the block diagram, you will see that its input at least, is DC coupled - with a 100KΩ resistor.
Click to expand...
You're right. It only works if the output is shorted to ground. The voltage developed across R52 lifts the B-E junction of TR701, forward biasing TR17's B-E junction, turning it on. The resulting spike triggers the thyristor and this locks the amp into protect. A regular clipping doesn't do that because TR17 will be blocked by D05. Also D03 will bleed base drive away to ground.

f-2926.png

See PDF for bigger pic.

My thyristor kludge is a bit too sensitive and sometimes triggers at poweron. Hmmmm...
 

Attachments

Got it sorted. Reduced R610 from 1k to 820R and this solved the go-into-protect-at-poweron issue. The final test, the short circuit test, was PASS. I am always a bit nervous doing this test...
 
This is the DIY thyristor schematic:
diy_thyristor_sch.jpg

This is how it turned out:
scr601-1.jpg

scr601-2.jpg

The turquoise resistor beneath the DIY thyristor is R610, lowered in value from 1k to 820R.
 
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