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Old 21st February 2009, 03:43 PM   #141
Bakmeel is offline Bakmeel  Netherlands
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Default The Culprit was found

Last Friday afternoon I spent an hour or two in the lab trying to find out what caused the amp to fail. Again, it meant to desolder some important components. (which I don't like, and neither does the PCB. some pads have been seriously damaged by the repeated soldering / desoldering).

However, the culprit was found!

Z5 failed in parameter drift. The curve tracer showed that for lower zener currents, the voltage drop was much less than specified, and varied a lot with temperature. It also showed some hysteresis effect (probably causing the noise).

This caused the diff pair to enter current starvation, and hence, operation ended up far away from the properly balanced point.

I replaced the zener and Amp no. 2 is again playing in my workshop

/me happy now....
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Old 22nd February 2009, 10:32 PM   #142
amp_guy is offline amp_guy  United States
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Ah that brings up a point I was going to mention, I have found quite a bit of variance in zener diodes and have had peoblems with this in circuits I have designed for use in my current profession. Maybe this is another item we should test select in the aleph amps.
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Old 22nd February 2009, 10:44 PM   #143
Zen Mod is offline Zen Mod  Serbia
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Default Re: The Culprit was found

Quote:
Originally posted by Bakmeel
....

However, the culprit was found!

.........

I'm glad ..........

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Old 23rd February 2009, 02:30 AM   #144
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Default Re: The Culprit was found

Quote:
Originally posted by Bakmeel
However, the culprit was found!
When the amp doesn't work, it's always something real.

No voodoo here...

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Old 24th February 2009, 08:58 AM   #145
Bakmeel is offline Bakmeel  Netherlands
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Default Component Failure Modes

Because component failure modes are also of interest to my work environment, I took some time to test the failed zener diode. Perhaps the result is of interest to this community as well...

I put the failed zener on a curve tracer (an invaluable, but also unaffordable piece of equipment when building solid state amplifiers). Attached is two photographs of the display. (This device is probably older than I am).

I am using 1N4696 zener diodes (datasheet here) for the constant current source feeding the diff pair.

The horizontal axis shows voltage, 1V/div. The vertical axis shows current, 1mA/div.

The healthy diode shows a nearly square behaviour and is very stable (as it should be according to the datasheet, minimum zener current is in the order of microamps).

The faulty diode is almost like a resistor until there's about 9mA flowing through it. At 4.5mA, the nominal current in the A2, zener voltage is around 5V. Almost half of what it is supposed to be. Furthermore, the resistive part of the "failed" curve was behaving very noisy and even had a hysteresis effect. Combined with a resistor bias, the voltage at the CCS mosfet gate must have been extremely noisy and instable. (perhaps explaining the hissing noise I heard.)

As an effect, the diff pair went in starvation, and barely had enough current to achieve proper regulation.

I guess Zen Mod had a good feeling from the start
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Old 24th February 2009, 11:06 AM   #146
Zen Mod is offline Zen Mod  Serbia
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Default Re: Component Failure Modes

Quote:
Originally posted by Bakmeel
.........

I guess Zen Mod had a good feeling from the start

when ?

when I wrote "Fugly!" ?

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Old 24th February 2009, 01:01 PM   #147
Bakmeel is offline Bakmeel  Netherlands
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Quote:
Originally posted by Zen Mod
are you using matched devices for input LTP ?

remove temporary Z1,Z2,Z3,Z4

replace both Q3 and Z5 ....... 15mA isn't enough through CCS ;

I have found lately several specimens of zenners to not have proper voltage
MythBusters say: Myth Confirmed....
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Old 24th February 2009, 01:30 PM   #148
Zen Mod is offline Zen Mod  Serbia
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Quote:
Originally posted by Bakmeel


MythBusters say: Myth Confirmed....

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Old 25th February 2009, 10:49 PM   #149
mosfets is offline mosfets  Canada
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This thread reads like a "cliff hanger". Very good.

From the wear and tear on the solder pads of your printed circuit boards, will you continue to deploy them or is it time to place new PCB's in the "twin Towers"?

All the best!
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Old 25th February 2009, 10:56 PM   #150
Manu is offline Manu  Europe
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Default Re: Component Failure Modes

Quote:
Originally posted by Bakmeel

I guess Zen Mod had a good feeling from the start
My master



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