Onkyo M-504 Amplifier blows fuse F803 and power transistors

Jock Strap

jacksonsinliss
I have an Onkyo M-504 amp that blows fuse F803 on the left channel when switched on. I found the main power transistors were bad so I replaced those but I could not find any other components bad, which does not avoid the fact that something else is wrong in there to cause them to blow. Any tips on what it might be would be helpful. The small transistors all checked out OK, as did the diodes, the big caps and the small caps. I am about to tear it down again to check out the other components.

The failure is probably my mistake?:
a) I replaced the meter lamps with LED, which work perfectly and are powered from the right channel, anyhow.
b) I replaced one set of speaker binding posts with 4mm banana sockets: I disturbed some mainboard components (C453 & C454, Q427-430) removing the speaker binding assembly so this was the most likely part where I introduced a problem but I checked it out and all looks good.
c) I cleaned up a previous repair to the mainboard where the big caps are mounted. Looks like it had a big problem in the past, see pics. The damage appears to have been caused by a screw shorting, there is a piece missing from the screw! The white wires were the original repair and I replaced them with blue wires after cleaning the area. I had to remove the copper plate connectors to test the big caps and one thing led to another.

I am using a Variac and 60W lamp to start up to avoid further damage: the right channel works fine, with F803 removed, and the new LED meter lamps are excellent. When I replace F803 then the meter lamps are not lit and the lamp glows... I quickly switch off the mains and nothing is damaged. I have tested all caps with my Mastech ms6013 and transistors using diode setting on my Mastech ms8200g.

Any ideas are welcome,

regards,
Roger
 

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I’d certainly look at bias adjustment procedure

Once any change made in output stage, bias should be set to manuals spec... - generally they say set to minimum and after warmup adjust.

I recommend replacing all output transistors on the affected channel... collapse of one can upset others... make sure drive transistors are fine (and coupling resistors from base of each output transistor)

Do check for inadvertent shorts and dry joints... seen these culprits rain havoc.

Lucky you... I’d love to get a M-504 someday... they look stellar and are a good amp... if it were mine, I’d have gone to MT200 output transistors (A1494/C3858) the bigger brothers of the orig A1492/C3856
 
Just here to say good luck and good on you for saving that amp. Holy smokes that thing's been abused! :yikes:

What's going on in that big damaged area where there aren't any repairs done in either pic? (Middle left?)

Cheers,
Nathan
 
Just here to say good luck and good on you for saving that amp. Holy smokes that thing's been abused! :yikes:

What's going on in that big damaged area where there aren't any repairs done in either pic? (Middle left?)

Cheers,
Nathan
That damaged area appears to have been vaporized by the screw head shorting to the solder blob right next to it, that was part of the prior repair? It is possible that my repair was incorrect so I am trying to match the board to the schematic... not easy!
 
I removed all of the white wires and used blue when I rebuilt it after cleaning the area to see what was going on. I keep checking the jumpers and they always appear to be correct?
 
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