405 Fuse blows

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26-Jun-2019 10:08 AM

lool

lool

Posts: 11

Due to a problem with my pre-amp (to high voltage on the output channel), the left channel of my 405 blew it's fuse #1. After replacing it, the fuse blew again. So probably there's more broken on the board. Any idea where to look ?
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26-Jun-2019 01:13 PM

Stefaan

Stefaan

Posts: 613


It looks like one or more of the power-transistors in the faulty channel is blown.

I would check all the transistors with a multimeter.

Replacement-transistors are here: http://www.dadaelectronics.eu/FAQ.htm

You can also send the board to Dada for repair.

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26-Jun-2019 02:23 PM

lool

lool

Posts: 11

And with power transistors you mean TR7/8 and TR9/10 ?
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26-Jun-2019 03:42 PM

Stefaan

Stefaan

Posts: 613


TR9/10 are the power transistors and TR7/8 are the drivers.

Best is to measure them all.

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26-Jun-2019 04:15 PM

lool

lool

Posts: 11

Thank you for the info Stefaan. Could I replace the power transistors with a pair of 2N3055 I still have laying around ?
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26-Jun-2019 05:02 PM

Stefaan

Stefaan

Posts: 613


Hello,

No, 2N3055 does not have enough power. You should use MJ15003.

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28-Jun-2019 09:58 AM

lool

lool

Posts: 11

Looks like one of the MJ15003 got fried. Just ordered a new one. Thx Stefaan.
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01-Aug-2019 10:29 AM

lool

lool

Posts: 11

I replaced TR10 and the amp works fine now, but the transistor runs very hot in a short time it's in use. I measured the TR8 before replacing TR10 and it measured fine, so I didn't replace it. TR9 also measued just fine.
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02-Aug-2019 02:39 AM

EJP

EJP

Posts: 1644

It should run warm to the touch on the front heatsink, but not hot. Does it sound OK at higher volumes? If not, check D5 and D6, and D13 if present.
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08-Aug-2019 09:12 AM

lool

lool

Posts: 11

The sound is just great. I checked D5 and D6 and they are fine, no D13 in my version of the board.

So TR9 was replaced after blowing up, I didn't replace TR7 and TR8, but after checking the diodes I checked TR10. It gave me some strange readings while in the board with small voltages in all directions between BCE.

Could it be that TR10 is not functioning as it should and TR9 is now doing the heavy lifting ?

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08-Aug-2019 11:42 AM

lool

lool

Posts: 11

I found this very interesting thread perhaps related to my issues (open loop gain):

http://www.diyaudio.com/forums/solid-state/314072-quad-405-2-overheating-channel-4.html

Post of 5-2-2018 --> Apparently Joost of Dada had a similar case. I'll check what exact version TR10 is and replace TR7 and TR8.

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09-Aug-2019 03:40 AM

EJP

EJP

Posts: 1644

Tr10 should have 0V on the collector, the full negative rail voltage of about -52V on the emitter, and the base should be about 0.55V higher than the emitter. If you diode-check Tr10 you will get a low b/e reading because of R33.

If Tr9 blows you will almost certainly have to replace Tr7, and similarly for Tr10 and Tr8, because the driver transistor will probably go crazy trying to drive its failed power transistor in each case.

I've never had a problem with MJ15003Gs, but then I've never used MJ15031s either.

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09-Aug-2019 09:56 AM

lool

lool

Posts: 11

I just replaced TR7 with a BD242c and everything works fine now. Although the old RCA driver was measuring correct, it either doesn't work well with the MJ15003 or took a hit when TR9 blew up.

Thanks for all your support EJP ! Great site and huge knowledge on Quad stuff. Recommended to everyone.

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