Weconic amp in protection

[Mote]

I have this Weconic amp which lights up both LEDs power and protection.
Voltage on FETs 1: 0 2: 12,38V 3: 0, voltage on output transistors: 1: 0,18V 2: 0,18 (0,42 on some of them) 3: 0,42.
I removed rectifier diodes. Then protection LED did not light up.
Voltage on FETs 1: 3.64 2: 11,80 3: 0
Reinstalled rectifiers checked that they are OK, put them back in right places, they are identical (not -><- and <--> but both are ->->)
Voltages on power supply chip (tl494cn):
01:0.02
02:4.7
03:4.68
04:0,18
05:1,41
06:3,30
07:0
08:12,4
09:0
10:0
11:12,40
12:11,68
13:4,87
14:4,87
15:4,87
16:8,69
I read from Dr. Zeus's Hifonics Zeus ZXI6406 thread (Hifonics Zeus ZXI6406) that pin 16 8V means that something triggers overcurrent protection.
I tried to trace what is connected to pin 16 - opamp KIA4558 near the TL494CN through diodes (see attached photo)
I lift one leg of each diode and now I can switch the amp on without it going into protection.
Voltages on FETs 1: 3,30 2: 11,10 3: 0, output transistors 1: 0,02 2: 25 3: 0,02
All four channels produce good audio. Amp does not draw current (I am using 12V lamp as a current limiter, lamp does not even glow).
Nothing gets hot. Only weird thing was that sometimes after switching off the amp, there were some sparks near one of the rectifiers.
Made some big resolution photos of the amps guts and tries to photoshop them together, that did not go so well but the photo is here:
http://mote.planet.ee/weconic/weconic_vx-4260.jpg

At the moment I dont know what to do next.

Found german site which has photos of the same amps guts:

I see that someone has tried to repair my amp before and I see that output transistors are different, on german site photo they have metal back but my amp has plastic back transistors (2SD998, 2SB778). Using wrong replacement could cause something like this?

[Perry Babin]

Post the DC voltage on all of the pins of the 4558.


Pin 1:
Pin 2:
Pin 3:
Pin 4:
Pin 5:
Pin 6:
Pin 7:
Pin 8:

[ppia600]

Check the solder joints of the rectifiers you mentioned sparking, you will need to resolder those.

[Mote]

Pin 1: 1.54
Pin 2: 9.86
Pin 3: 5.58
Pin 4: 0
Pin 5: 9.26
Pin 6: 4.87
Pin 7: 9.88
Pin 8:10.53

Isnt pin 4 supposed to have -10.53V on it?

[Perry Babin]

Pin 4 is likely on the primary side of the power supply where there is no negative voltage. Pin 4 would be directly connected to the amp's main ground terminal.

Follow the circuit back from pin 5. What's driving that pin?

Does it come from a thermistor or from the audio amplifier section of the amp?

[Mote]

Pin 5 goes to Q2 (A1266) center leg, first leg of that transistor is connected to leg 12 of TL494 and to the same KIA4558 8th pin. Third leg of Q2 is connected through diode and resistors to audio sections Q221 (C3198L) center leg.

[Perry Babin]

Do Q121, Q221, Q321 and Q421 have their collectors connected together?

If so, reconnect the diodes and anything else that you've disconnected and remove the transistors listed above one at a time to see which channel is causing the amp to go into protect mode.

[Mote]

Their collectors are connected and base and emitter are connected to emitter resistors.
Ended up removing them all and still protection mode triggers.

[Perry Babin]

A transistor in the location of Q2 rarely fails but it may have here. Try removing it. If it still goes into protect ode, post the voltages on the 4558 again.

[Mote]

Removed Q2, amp does not go into protection but the transistor seems fine. hfe=150, resistances between legs are also like they should be.
What about Q1? It is also connected to Q2.