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Old 16th July 2007, 04:54 PM   #1
KP11520 is offline KP11520  United States
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Angry OK, This is where it hurts the most (newbie that hurt his op amp)!

Hi All,

So I have been playing with the idea in Carlosfm's "The OPA627 really sings" of biasing the newly installed OPA627AP op amps in my Rotel RCD 965BX. Zei helped me understand how to measure the voltage to figure out the resistors I need.

I am looking for someplace local to buy the two 1.8K ohm resistors. I have some time to kill so let me understand what he (Carlos) was saying about the desired 5-10ma for this mod. So I take out my trusty DMM and plug in the probes to the Amps connectors, set to milliamps and touch the red to the +Vs and the black to the Ground/common on the screw to the chassis. Big Spark!

Uh oh! What did I just do? I'll bet it isn't good!

Back to the trusty DMM but I an NOT using the ma setting. Back to the DC volts setting! Now on the Sparking op amp, 3.9 volts on the 6 pin (output) and back to the un-sparked pin 6 and got 6 volts!

YIKES! What did I hurt, my op amp or something around it?

The op amp is my first mod and it wasn't easy to get under the PCB due to the disassembly required. I think I gonna CRY now.

HELP! I feel like such an idiot! Thanks!

Regards//Keith
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Old 16th July 2007, 06:41 PM   #2
zei is offline zei  Sweden
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Hi

Lets first hope that you did,nt fry something,,
Then.. i did,nt understand fully how and what you messured.. so is it possible to explain abit more? ( a picture perhaps?

Also,, take a look at this link.. it might give you some more insight in what you are trying to achive..

And last, I schematic of you amp is always helpful in trying to figure out stuff..



Lets works this out

/Zhttp://tangentsoft.net/audio/opamp-bias.html
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Old 16th July 2007, 06:46 PM   #3
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A meter on current measure is effectively a short circuit (very low impedance.) You have just shorted +Vs to ground and probably destroyed something.

Bad move

I think you are going to need some close hand-holding to sort this out as you apparently do not really understand voltages and currents and how they relate.

Do you have a friend / neighbour who can help you out?
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Old 16th July 2007, 07:11 PM   #4
r221b is offline r221b  United States
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I'm not familiar with your circuit, but the OPA627 uses a +/- supply to obtain a "balanced" output. Normally, if you ground the positive supply pin (which you did through your DMM when set to measure current) of a single supply chip, the chip most likely won't be damaged. However, with a dual-supply chip like the OPA627, you severely unbalanced the internal biasing of the chip. I'm afraid to say that you most likely did damage it. If you need (or want) a data sheet of that chip, you can get it here:

http://www.datasheetcatalog.com/data...OPA627AP.shtml

Good luck with your project.

sherlock
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Old 16th July 2007, 07:19 PM   #5
KP11520 is offline KP11520  United States
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Hi zei and cliff,

zei, I am glad you are still around! My turn for some wine!

cliff, you aren't kidding about a "Bad Move". This is where the learning part for new guys s*^ks! You guys and DIYAudio are my friends and neighbors when it comes to this! I don't even know of a decent local audio/elecrtonics repair guy that is good with mods and creativity!

Here is the hyperlink to the TI specifications pages for the OPA627AP: http://focus.ti.com/lit/ds/symlink/opa627.pdf

Here is a schematic of the op amp PCB area of the Rotel CD Player, the OPA627s replaced the XR5534s:
http://www.diyaudio.com/forums/attac...amp=1184457241

The schematic shows all the components around the op amps. I think I fried one or more of the components in the op amp! I have two more if necessary to replace the one. I wasn't sure how I was going to add the resistor to the two pins of the op amps without taking them out and rutting the resistorpin and the leg of the op amp in the same holes and soldering. Now it looks like I have been shown the way! Yikes! Be careful what you ask for!

Hi sherlock, just noticed that you posted as I was writing this! I was afraid it was something like that. I would think that an IC is a self contained PCB with many components inside and at least one of those components got fried on the + side.

So gentlmen, does it look like time for surgery? Thanks for your help and support, now where did I put that Prozac?

Regards//Keith
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Old 16th July 2007, 07:39 PM   #6
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Your best move may be to take out the op-amps and measure the DC voltage (20V DMM range!) between each pin and ground.

Report the values back here and we may be able to track it down.
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Old 16th July 2007, 11:05 PM   #7
KP11520 is offline KP11520  United States
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I will remove the op amp either late tonight or tomorrow late morning! Do I measure the DC voltage at the PCB where the +Vs and the -Vs pins would go from one to the other, or one or the other to groung/common? The +Vs to -Vs is 30 volts with the op amps in and the +Vs to common and -Vs to common are both 15 volts with the op amps in. These voltages are the same before my name was changed to "Sparky" and after! The only thing that changed was the output voltage on pin 6 in the one op amp that made a spark! The un sparked op amp remained constant at all values. I will bring the news!

Like Arnold says," I'll be back!"

Regards//Keith
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Old 16th July 2007, 11:07 PM   #8
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It looks as though you have zapped an amplifier, then.
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Old 16th July 2007, 11:15 PM   #9
Arx is offline Arx  Canada
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Yeah, sounds like your power supply is probably still okay, but your opamp is probably blown.

The easy way to get a current measurement is to measure the voltage across a resistor in the path and then use ohms law.

To measure current directly with a multimeter you need to disconnect a component, and put the meter in series with your circuit, not in parallel.

Often there's a resistor you can measure to get the same result without all the messing around.

-Nick
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Old 16th July 2007, 11:31 PM   #10
KP11520 is offline KP11520  United States
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Gentlemen,

Thank you for your expertise! I guess this can get ugly sometimes! But there is a power supply Guardian Angel out there somewhere! Nick, now that you mention it, I read that somewhere about using a resistor in line. NOW I won't forget it!

Thank G^d I have two more in the house! Well now I can add the resistor to make it bias to Class A easily by sticking the two ends in the same holes as the proper pins and solder it up!

Thanks to all my teachers on this lesson! (Can't wait to graduate already!)

I owe you all at least a drink! Zei, cliff, sherlock and Nick and The PS Guardian Angel too!

Regards//Keith
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