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Old 4th April 2008, 07:49 AM   #111
cdswift is offline cdswift  United Kingdom
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Default just blew up two of these

How did you blow them up - was it whilst setting the bias? My recommendation if you can get your hands on one is to power them (one at a time) via a variac (variable mains transformer) and slowly increase the mains voltage whilst watching & adjusting the bias current - keep it down at 5ma until full mains voltage is reached and then increase the bias up to the desired level.

Whilst I was doing this, I noticed that at a certain point the bias current shot up off the scale - if I had not been controlling the voltage on the output transistors, they would probably have gone up in smoke. In the event, all I did was wound the mains voltage back down a bit and the re-adjusted the bias current downwards before starting to increase it again.
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Old 4th April 2008, 08:46 AM   #112
ttan98 is offline ttan98  Australia
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Default Re: just blew up two of these

Quote:
Originally posted by cdswift
How did you blow them up - was it whilst setting the bias? My recommendation if you can get your hands on one is to power them (one at a time) via a variac (variable mains transformer) and slowly increase the mains voltage whilst watching & adjusting the bias current - keep it down at 5ma until full mains voltage is reached and then increase the bias up to the desired level.

Whilst I was doing this, I noticed that at a certain point the bias current shot up off the scale - if I had not been controlling the voltage on the output transistors, they would probably have gone up in smoke. In the event, all I did was wound the mains voltage back down a bit and the re-adjusted the bias current downwards before starting to increase it again.

Can you set the pot to the lowest setting before powering up? If my memory serves me right, there is a bias pot.

Once full power is applied, wait for a while eg half to one hour for the power transistor't's temp to settle down(stabilise) then adjust the pot to the reqd. bias current. Is is possible?
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Old 4th April 2008, 02:08 PM   #113
hoka is offline hoka  Netherlands
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I did not even have the time to get a reading on the bias, everything blew in a split second. My PSU is ok, so I have no idea what went wrong I measured every resistor before soldering and took extreme care in putting the thing together.

Is there anyone who has got the "new and improved version" working?

I am trying to figure out what went wrong and what the extend of the damage is, how do I get pictures uploaded? then someone might be able to diagnose the boards.

[IMG]Users/mini/Desktop/clone.jpg[/IMG]
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Old 4th April 2008, 02:22 PM   #114
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Were Q1 and Q2 in the wrong way around, i.e the correct way since they are marked wrong wrong on the layout.

Click the image to open in full size.

if you look at that you can see that Q1 and Q2 need to be reversed compared to the outlines on the boards. Also the Gnd connectoins appears to be marked wrong as someone else has said. The SG, signal gnd has another hole net to it which would probably be the best place to take your gnd back to the psu. And the GND should probably be taken to that point as well. Its a bit weird. Maybe look at the chinese thread for these boards will help:-

http://bbs.hifidiy.net/viewthread.ph...&extra=&page=8

Phil
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Old 5th April 2008, 06:52 AM   #115
hoka is offline hoka  Netherlands
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here is a picture of the board the yellow wire is "signal ground"
and the psu wires have been removed.

Q 1 & 2 are aligned correctly and there is no short anywhere.

Click the image to open in full size.
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Old 5th April 2008, 01:30 PM   #116
john65b is offline john65b  United States
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Deleted - answered my own question...

Looks like you two have completely different boards. Is there a rev on the boards? Which is the new board blue or white?
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Old 5th April 2008, 01:48 PM   #117
brianuk is offline brianuk  United Kingdom
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There are two revs, not sure which is which now as these look the same but just a different colour?

Anyway Hoka, you've made a damn fine job of blowing that up, right through those components to the output.

You sure nothing was shorted?!

Its a real shame that you have done both boards but might help you fix it as it must obviously be a common fault you have made.

Not just a wiring fault is it?
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Old 5th April 2008, 01:52 PM   #118
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Hoka did you set the bias pot to minimum resistance before powering up? Also did you actually have a heatsink attached or not?
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Old 5th April 2008, 01:56 PM   #119
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I am about 95% sure the white and blue pictured above (long boards) are the same. The ealier smaller boards are a very different layout and there are more like a NCC200 rather than a Nap140. I have just bought 4 of these boards of a chap on another forum. Not sure what to do now since is seems people are having issues with these newer ones.
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Old 5th April 2008, 02:01 PM   #120
john65b is offline john65b  United States
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Richie00boy -

I think I have seen amp boards that attain max bias current at min resistance on Bias pot, so should this be identified first?

Since I do not have a variac, I always try to significantly drop voltage (a big monster series resistor ) on rails and then check for smoke, Then turn the bias pot to see which direction the bias moves and reduce as much as possible before putting full voltage on rails.
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