Amplifier bad, 37 VDC at the speaker output

[bernat]

Hello, I bought this amplifier from Ebay in "perfect working shape", is a MAcrom Synthesis 1000, when arrived at home I opened the case and looked inside , the layout looked good with no visible burned components .... well ,that seems good I was thinking, then I connected to a power supply and the control led turns green, Perfect !!! now starts my stupidity, connect to my expensive home loudspeakers for test and ..... nothing ... well , yes ... a maravellous smell a fried voice coils ! I take the multimeter and check 37 V at both speakers outputs, all the rail voltage of the power supply goes directly to the speakers, seems the amplifier dont have DC output offset protection circuit. I've tried to repair but my knowledge of electronic is minimal.
After inspection of the bottom side I can see the amplifier was tried to repair before, the mosfets in the PS are replaced and the board nea them is burned.
I have been searching a lot about DC current output and I was thinking in shorted output transistors , pre-drivers and drivers. I REPLACED ALL OF THEM, INCLUSE THE MOSFETS for the same model of transistors installed before and I'm still having 37VDC in the outputs. Now I dont know what more to change.

Can some one give me help?

Thanks

 

[Perry Babin]

What's the DC voltage on the center legs of the TIP35s?

What's the DC voltage on the center legs of the TIP36s?

What's the DC voltage on pin 8 of the 8-pin op-amps (all should be very nearly the same - confirm)?

What's the DC voltage on pin 4 of the 8-pin op-amps (all should be very nearly the same - confirm)?

 

[bernat]

Thanks Perry for the fast reply,

tip 35 35v

tip 36 36.8v

and when I was disposed to measure the op amps the amplifier started to go into protection, I believe some thing more is now broken, I will check the mosfets now

 

[Perry Babin]

IS the voltage on the TIP36s a negative voltage?

Are the two in the center op-amps? I'd suspect that they're opto-couplers.

There appear to be op-amps on the vertical boards. Check those and any others in the amp.

 

[bernat]

Now is imposible to take measures, the protection led is flashing every second green-red-green-red ... the two opamps in the center has 6 legs and the inscription says 4N36 836ATK63

 

[Perry Babin]

Yes. Those are opto-couplers.

Did an output transistor fail (short)? Check them all.

 

[bernat]

I replaced the 20 transistors with new ones of the same batch, I noticed some of them was replaced before (different brands mixed, mospec and ST).
Maybe they are broken again, I will desolder them and check

 

[Perry Babin]

You can make a quick check without removing them.

 

[bernat]

Perry, your web site is fantastic, thanks for share your knowledge.

Tomorrow I will check the tip36-36 and the mosfets following your tutorials