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Old 1st December 2006, 04:14 AM   #1
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Default Why main fuse and not rail fuse?

So I just picked up another 555mkII on the cheap. It blows the main fuse, but not the rail fuses for the affected channel.
I put a smaller fuse back in the main, took out the rail fuses, powered through a 40 watt bulb, and put the fuses back in the rails. The right channel lights up the light nice, the left channel just a dim glow, maybe it is sinking 10 watts.
So I can assume that I need at least some output trannys, and maybe a driver for that side. I will take it apart further when I get the other 555 done!
Anyway, my question, why would the main fuse blow in this and leave the rail fuses alone? Main fuse is a 12 amp, and the rails are 7. Seems like that something is engineered wrong, or is my thinking skewed?

Peter
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Old 1st December 2006, 04:48 PM   #2
anatech is offline anatech  Canada
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Hi Peter,
Check the rectifiers and filter caps.

-Chris
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Old 2nd December 2006, 01:12 AM   #3
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An important tool for amp testing is a SERIES light bulb. Yes, wire up a standard light bulb socket (120V or 240V) in series with a line cord. It works wonders in keeping the fuse from blowing, yet passing some current to the amp. Often a good amp will actually work OK at idle with this bulb in series, if it is large enough in wattage. Start with a small wattage bulb and build up.
When troubleshooting, you can more easily find what is really wrong, because you can measure the amp when it has some voltage running into it.
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Old 2nd December 2006, 01:58 AM   #4
anatech is offline anatech  Canada
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Hi John,
I have far better luck with a variac.

I mostly use a 2A model, but I have a 15A model in reserve for those big amps. CJ Premier 1 comes to mind.

By far, the most indispensable tool to have is a mind (in working order).

-Chris
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Old 2nd December 2006, 03:35 AM   #5
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I am getting the same voltage on both sets of rails, so would that eliminate the rectifiers/caps - or could I still have a failure there?
I would hit it with a scope to check for a/c, except that my scope is a handheld velleman, and only goes to 30 volts. I have yet to get a attenuating probe.
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Old 2nd December 2006, 04:11 AM   #6
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Default Main Fuse Blows

I've seen similar problems when the bias circuit is bad/out of adjustment (open pot). This causes excess conduction of the outputs which draws equally from both rails effectively doubling the current capacity of a single rail fuse. This could explain why one channel is okay and one kills the fuse.
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Old 2nd December 2006, 05:55 PM   #7
anatech is offline anatech  Canada
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Hi Peter,
Quote:
Anyway, my question, why would the main fuse blow in this and leave the rail fuses alone? Main fuse is a 12 amp, and the rails are 7.
I think I just "got" your question, correct me if I'm wrong. Why is the main fuse blowing and not the rail fuses? Right?

Sometimes the main fuse needs to be a slow blow type to handle the charging surge for the caps. It also sees the current from both channels plus the charging surge.

Remember that the primary fuse is intended to protect the power cord and primary circuit. Some secondary side faults will blow the primary fuse. Also, a fuse blowing is a timed thermal event. It just depends sometimes which fuse reaches that limit first.

-Chris
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Old 2nd December 2006, 06:08 PM   #8
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Well, I have not put much time in on this one yet, just a quick "look see". I am replacing trannys on a different 555 right now.
But here is what happened in my quick exam : The main fuse blows if all the fuses are present (rail fuses). Remove the rail fuses and the mains fuse is fine, the power supply runs up fine with the proper voltages. Install the rail fuses one at a time, and the right channel starts loading up enough to NOT pop the rail fuse; but pop the main fuse. I have not gone any further than that - ran out of the 12 amp ceramic fuses. I was using a 6 amp fuse and the 40 watt light bulb to continue with my quick exam. But I just found it odd that (assuming the outputs are shorted) that the rail fuse did not blow - (with all that stored energy in the ps caps!) and the main did.
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Old 3rd December 2006, 02:11 PM   #9
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Quote:
Originally posted by john curl
An important tool for amp testing is a SERIES light bulb. Yes, wire up a standard light bulb socket (120V or 240V) in series with a line cord. It works wonders in keeping the fuse from blowing, yet passing some current to the amp. Often a good amp will actually work OK at idle with this bulb in series, if it is large enough in wattage. Start with a small wattage bulb and build up.
When troubleshooting, you can more easily find what is really wrong, because you can measure the amp when it has some voltage running into it.

Has anybody ever figured out the relationship between a bulbs rated wattage and how much current they will let pass?

IE: if i know i need x amps at idle, what size bulb should be choosen?? etc.

Zc
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Old 3rd December 2006, 03:16 PM   #10
anatech is offline anatech  Canada
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Hi Zero Cool,
I imagine it's a balance between what the equipment normally draws and the lamp filament characteristic.

I'm lazy, I use a variac.

-Chris
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