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pborlace 11th October 2006 11:09 PM

Quad 405-2 Problem
My amp recently blew FS2 and I'm now reading 50v DC across the fuse holder. I've checked all the resistor values, checked TR1, TR3, TR4. I've replace blown TR10 and checked all other transistors. I've replaced C10, C19, C18 and C17 as a matter of course as I'd already bought the components to upgrade the amp. I'm still getting 50V at the fuse holder, any ideas please?


Wavebourn 12th October 2006 05:27 AM

Can you post schematics, please?

Snorkeltjes 12th October 2006 09:22 AM

What do you read across FS1 without fuse?
I think it is normal cause there isn'd any current without the fuse.

RogerGustavsson 12th October 2006 10:18 AM

FS1 and FS2 are in series with the normal suppy lines. Should it not be about 50V at all times?

pborlace 12th October 2006 10:29 AM

The circuit diagram is at the following link. I haven't copied it in case of copyright. My boards are M12565 issue 6. which is page seven of the diagrams.


pborlace 12th October 2006 10:29 AM

I've just tested it again and now I get 0.0012v across bothe fs1 and fs2 but when I insert the fuses fs2 immediately blows.

AndrewT 12th October 2006 10:36 AM

assemble a mains light bulb into a wander lead. The light bulb is wired in series with the LIVE wire only.

It will allow you to plug in the faulty amp without the risk of blowing the whole thing up.

When operating normally the bulb flashes briefly at switch on and then either goes off or glows very dimly.
If it glows bright and you are using an amp with a very high quiescent dissipation then choose a higher wattage bulb (40W to 150W).

If the amp is faulty and tries to draw too much current, the bulb lights up normally and the amp sees only a few volts across the mains transformer. This can allow some measurements to be taken to help locate the fault.
Power dissipation with the bulb inline is very low and nothing gets hot, although some components might just get warm.

edl 12th October 2006 10:38 AM

Is Tr8 alive?

pborlace 12th October 2006 12:10 PM

Just to add, I've tested the voltage across fs1 and fs2 on the working board and now I'm getting the 0.0012v across both. Only problem is as I've said, the bad board keeps blowing the fusae at FS2.


pborlace 12th October 2006 01:04 PM

TR8 tested and ok.


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