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Old 16th January 2009, 02:36 AM   #1
rtill is offline rtill  United States
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Default positive rail fuse blowing

What would cause only the positive rail fuse to blow? I've replaced all transistors 3 times.
The only difference between my circuit and this schematic is that I substituted bc558's for the mpsa56's.
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Old 16th January 2009, 03:19 AM   #2
rtill is offline rtill  United States
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replaced the bc558's with the called for mpsa56's, works fine now, but it blows up 8 ohm speakers. What is going on?
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Old 16th January 2009, 03:30 AM   #3
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Have you done some measurements ?
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Old 16th January 2009, 05:08 AM   #4
rtill is offline rtill  United States
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no, what should I measure?
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Old 16th January 2009, 07:00 AM   #5
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How about measure the output with a Volt Meter before connecting your speakers......

That may save you a lot speakers
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Old 16th January 2009, 07:11 AM   #6
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Default then ....

50 !!!! volts rails for this schematic ..... without any form of protection like VI limiters or zeners looks kind of too much i thing....

also how do you bias this amplifier ...... is the diodes attached to the heatsink ????? you might allso have thermal drifting.....

also looking at this thread that is also yours...reducing voltage

it seems to me that you must be an ammature ...... nothing wrong with that each and every on of us has been there sometime ....

a couple of tips .....

-------dont jump from one circuit to another ..... construct one ( not importand if good or not ) tune it listen to it , evaluate , reconstruct properly , learn from your mistakes , and then when equiped with more construction knowledge go ahead for next project .....

------ miracle amplifier doesnt exist ..... with such low part count expect power from 50-60 watts increasing rail voltage to get more power will make all your circuits unstable .....

------ as about safe start ups you might find some information here vintage amplifier repair/upgrade manual

------ finally before constructing you have to be familiar with a few basic things ....like wiring voltage psu cooling idle and so on

----- posting the pictures of what you ve done so far might help a bit since many forum members will see and spot mistakes .....
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Old 16th January 2009, 07:05 PM   #7
rtill is offline rtill  United States
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I am an amatuer, still learning, will always be learning!

My take on learning is - make it, learn what you did wrong, find out the fix. This way I learn what each section of the amp does and can troubleshoot other builds with what I've taken in from previous builds. (sounds like a relationship )

You are correct, I have 3 builds going on right now. I jump between them, because after troubleshooting for a week, I'll get bored- must be an A.D.D. thing. I appreciate your criticism.

So....for future reference for me, let's get back to the thread....what causes a speaker to blow up in smoke?
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Old 16th January 2009, 07:36 PM   #8
jaycee is offline jaycee  United Kingdom
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DC voltage on the output of the amp. The speaker coil windings can't handle the continuous power, so they overheat and burn out.

It's extremely foolish to just connect a speaker and hope for the best. Test with a multimeter.

On the circuit you've posted -have you got the BC558 the right way around? The pinout is the reverse of the MPSA56.

Same goes for all of the transistors, are the pinouts right?

Are you using the right type of MOSFET at the output ? If they are not lateral MOSFETs, the circuit will not work properly.

What about PCB? Have you made it yourself ? Photographs?
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Old 16th January 2009, 07:56 PM   #9
lineup is offline lineup  Sweden
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PCB can be an issue.

Especially the ground rails should be done properly.

For example, 10R 2W and 100N link should be be placed across the output terminals.
That is be connected to POWER ground point.
Connecting that output filter close to input ground rail, can cause troubles.

Otherwise, I should check all components values,
and of course Transistor pinnings.

The two 470 Ohm resistors, should be put close to MOSFETs Gate pins.
Should not be put at the PCB, if wires are used to connect MOSFETs.
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Old 16th January 2009, 08:24 PM   #10
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Check the 10R on the input to ground.
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