Blown output on Sony ta-1150d

[Command8]

I did all those measurements, everything was going fine, measured and set idle current to 25mv, right side was spot and and left side jumping a bit tp 24.2 to 25mv.
Let the amp run without the DBT, it was fine for about an hour after that BLAM.

All output transistors but one shorted and blown.

(these measurements were before the output transistors getting blown)
q309 Vb 0.59, Ve 0.002, vbe 0.6
q307 vb 0.6, ve 0.002, vbe 0.6
q305 vbe 0.569
r313 0.0026 voltage drop
q301 base -0.085,ve -0.7
q302 base 0.002, ve -0.7
r302 measures 51kohm

I changed them all again with my last pair of MJEs, and now another short from the DBT. I've identified and changed diode D556, something else is still shorted.
The TDA7293 option is seeming more and more viable with all the heartache this PA is giving me.

 

[orbanp]

That is rather unfortunate!
Only thing I can think of is the Vbe multiplier stage failing somehow.
I would replace VT304, also would modify the VR301 pot position. Do check for cold solder joints, possible PCB trace brakes in the Vbe multiplier circuit. Do touch up the solderings around that circuit.
Also would remove the protection circuit temporarily, the diodes around VT552 and VT553.

I might be missing something, but what does "DBT" stand for?

Regards, Peter

 

[Command8]

I still have a shorted component somewhere. All solder joints were refreshed. I replaced all the differential input transistors for both channels as Q301 or Q302 was shorted.
Both VR301/401 have been reset to the middle position around 500ohm. VT304/404 has been checked again, I don't have the replacement for this one so i'll leave this one in.
DBT stands for Dim Bulb Tester.

I don't yet know what component is shorted yet from the supply voltage, at r312/r412 we are only drawing 1.37v.
The good news is at the very least changing the differential input of both channels has fixed the Q301 base voltage from -0.08 down to -0.014v.

I tested the driver transistors and they are thankfully still good. Ill test q303 once more (all other transistors tested fine out of circuit on the lcr-t4), though I believe that transistor is what pulls a high voltage load from the power supply.

 

[orbanp]

I would change out the Vbe multiplier transistor!
Your failure was after half an hour of use, so a momentary test of that transistor most probably would not show anything amiss.
The type of that transistor is not critical at all, any small signal Si transistor is OK in that position, e.g. a 2N3904, or anything similar you might have in parts drawer.
Watch out for the order of the pins!

As mentioned earlier, I would disconnect the protection circuit, it would eliminate one possible source of the problems.
In any event, in my opinion that circuit is next to useless.
It is supposed to limit the output voltage if something fails in the amplifier circuit, and that protects the speaker from burning down.
The problem is that it relies on the proper operation of the final driver and output transistors, and most often a DC output to the speaker is the result of the failure of one of those transistors. So it does not work when really needed.
When properly done, such a protection circuit controls a relay and that disconnects the speaker.

Regards, Peter