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Old 20th December 2006, 06:22 PM   #1
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Default Stupidity

Let's hope this exercise in frustration can be of some use to someone. It certainly is to me.

Recently, after some opamp discusssions in another thread i decided to try the AD815 as a line amp. I figured it will take me two hours max to get it up and burning in. Was i wrong?

Of course, no point making pcbs for a quick try out. My 815s came in soic so i had to cut a nice piece of copper foil to serve as a heatsink. All the other parts were positioned on a protoboard with a decent, if not perfect, 3-d grounding and decoupling arrangement.

Power on. Seems to work. About 30mV offset without compensation. Scope on. Yup, works. Only at very low volumes on the pot it starts oscillating. Looking at CarlosFM circuit i notice the 1k resistor at input, which would limit a very low resistance on the pot is missing. With the resistor in there is no oscillation at all pot positions.

Now, lets check the other channel. Dead. Full supply voltage at output. These things don't happen to me often. Last time i blew an opamp was two years ago. Could i possibly have shorted the output to Vcc? Not likely.

Chip No 2. It all starts nicely again with exactly the same ending. One channel dead.

Now it gets really frustrating. Chip No3 is the last i have and i ain't taking no chances. All my connections look fine but apparently the whole setup is cursed. I start afresh - new chip on a virgin board. Extra cautious with static.

Finally my work is rewarded. It works! Let's leave it on the bench just a bit longer and run a few tests.

Uh-oh. One channel is dead.

At my age i have almost forgotten what it feels being a newby and blowing things up for no reason, not even for fun. It's not a nice feeling. An hour of staring at the dead chip brings little in the way of closure.

I start desoldering and suddenly feel a slight pricking in my elbow. It's resting on the smooth Vampire input jack and there is no obvious reason for the sensation. In my other hand is the grounded soldering iron. Quick check reveals 118v between sodering iron and input jack. Not good.

Suddenly the penny drops. It's only the scope ground connected to circuit. The scope is off. But two weeks ago i came back from Europe and brought a nice 10-way extension for all my 2-prong appliances. The scope was the first to taste the new power. Only it seems the new extension never got connected to safety earth. The scope (pos) transformer leaked 118 out of the 220 volts into my circuit ground. The rest is sadly history.
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Old 20th December 2006, 06:37 PM   #2
lineup is offline lineup  Sweden
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good story
and a few things to notice and learn

.... and it all follows the golden rule:
when all looks fine & alright
and there is still another little chance something else can go wrong
- then it will go wrong



you found the source of malfunction, in the end
..... a happy ending, we can say


lineup
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Old 20th December 2006, 06:49 PM   #3
KBK is offline KBK  Canada
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And, when someone says their AD815 is dead, You ask, what channel? and then you know for sure what went on, and that the unit is for sure --dead.

The bright side, is you know the failure mode of a AD815. That is-if the same channel went every time. That alone would be interesting in terms of understanding the overall characteristics of the 815.
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Old 20th December 2006, 07:06 PM   #4
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Quote:
Originally posted by KBK
And, when someone says their AD815 is dead, You ask, what channel? and then you know for sure what went on, and that the unit is for sure --dead.

The bright side, is you know the failure mode of a AD815. That is-if the same channel went every time. That alone would be interesting in terms of understanding the overall characteristics of the 815.


Not quite. Anything would have died in its place anyway. The ground refence of the prototype was at 118v with the scope ground connected. The soldering iron, otoh, was at 0v. Anytime i soldered with the scope gnd hooked something would potentially get zapped.
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Old 20th December 2006, 07:19 PM   #5
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Thanks for sharing. It makes me glad that all my power strips are grounded...
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Old 23rd December 2006, 02:14 PM   #6
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Quote:
I start desoldering and suddenly feel a slight pricking in my elbow. It's resting on the smooth Vampire input jack and there is no obvious reason for the sensation.
*ding!*
It was obvious to me the second I read that sentence!

I feel for ya...
Been there,done that...too many times to count.
That "prickling" feeling has tipped me off more than once..

I've "forgotten" that my scope was earth grounded once or twice too while working with higher currents..
*POOF*
Atleast the jumper wire for the front-panel GND terminal is easy to replace.
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Old 23rd December 2006, 02:47 PM   #7
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A good story to learn from
(I have been there )
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Old 27th December 2006, 09:26 PM   #8
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Default No grounds..

Down here grounds seem to have been forgotten(220vac),
when I came down here with all my 110 gear I had to get various 220-110 transformers to run all my stuff..The 'old' plugs used here are two prong round with no grounds..I still get tickled with leaking voltages all through most my gear, I don't have any ground loop problems to speak of...One of the first things I shall do when wiring my newly built Shed/lab is to drive a meter long copper rod into the ground & wire it PROPERLY!
___________________________________Rick.........
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Old 27th December 2006, 11:37 PM   #9
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Default Re: No grounds..

Quote:
Originally posted by Richard Ellis
drive a meter long copper rod into the ground & wire it PROPERLY!
I used to live in Brazil and had the same problem. Believe me, a 1 meter rod is not nearly enough. I built ground networks for myself and friends of mine and used 3 meter copper rods which I could find at electrical supplies stores. In the best case scenario which was the second one I built for myself, I buried 4 rods spaced 3 meters from each other in a flower bed and achieved 0.4 Ohm. The worst case, a friend of mine in a rural area, it took 7 rods spaced 3 meters from each other to achieve 0.5 Ohm, which was the maximum we were willing to accept. Keep that in mind when building yours.
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Old 28th December 2006, 08:46 AM   #10
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Default hot grounds

when i was a teenager i did a lot of repairs on radios, tv's and guitar amps. some of them had one side of the line cord directly conected to the chassis. with the guitar amps, there was sometimes a ground reverse switch (most of these were tube type, but i have seen a few solid state designs with s hot chassis).
i had 2 tv's side by side, one had a fried flyback, but had a uhf tuner, the other worked fine, but had no uhf tuner (but had provision for one, an if input plug and an open spot on the tuner switch), so to watch uhf, i would run them in tandem with an IF cable running between them. problem was, they were both "hot chassis" designs, so if i plugged one in backwards, it would pop the fuse downstairs.

some antique radios used "hot" speaker frames, because the B+ was filtered through the speaker field coil, and i got bit bad more than once trying to adjust the angle of the speaker, while i was turning the volume control shaft with the other hand.

guitar amp ground reverse switches usually tie one side of the line or the other to the chassis through a 1M resistor and a cap, but if your ground switch is in the wrong position, and you're playing guitar, and you sing into a mic, and touch the mic, you get bit pretty good, so i always had a meter packed with my amp, to make sure everybody's amps were properly grounded on a gig.

with o-scopes, i always make sure the scope chassis is grounded, and if the amp or other device under test has either a hot chassis or a hot rectifier section (like the primary side of switching supplies) i use an isolation transformer. all the isolation transformer does is to allow the secondary to float, so if you connect a scope ground, the "hot" chassis assumes ground potential.

here in the US, one side of the line is "neutral". meaning it's supposed to be at or near ground potential, but that isn't always the case, so it's always a good idea to use cords and outlets with a ground pin with everything that is supposed to be grounded, even soldering irons. and also check your outlets to make sure ground is actually grounded, that neutral is at or near ground, and your "hot" is on the correct side of the outlet. you never know, the wiring in your house might have been botched, and nobody ever really noticed because the outlet you just plugged your scope into was always used for table lamps or some other appliance with a 2 prong cord.
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