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Coconuttopdrop 7th September 2012 02:43 PM

Test IC TL074 CN

I'm repairing a AMEK mixing board but i dont have an extensor to power the channel's that i'm fixing.
What i'm doing is comparing some readings on a good channel with the bad channel.

The EQ section of the board as 4 TL074CN IC's, and some are busted, but i dont have a way to see wich are ok and wich are busted because i cant power the circuit.

So i need to do some kind of circuit to test the IC's.
The TL074 brochure has a audio amp circuit (fig.25) wich i can try to replicate, but i have some questions.

Using a DC power supply, should i feed 12V to CC+ and hook up the ground to CC- and feed 6V to ground?


indianajo 7th September 2012 04:24 PM

1 Attachment(s)
See this schematic. I use an 18VDC car race wall transformer, and 2 each 1n5344 8v zeners to make +-8v, with the middle of the two zeners the signal ground. I used 22 ohm resistors since the zeners are 5 W rating. If you have a supply with 0, 6, and 12 you could try that but the performance at +-6v doesn't give you much headroom. In that case the 6v point would be the signal ground, you could call the 0 point the -6 v supply. Corners of the 8 pin DIP IC are plus (pin 8) and - (pin 4) voltage.

Coconuttopdrop 7th September 2012 04:38 PM

So i can power the OPs building this?
What values should i use for the caps in your schematic?
The brochure tells me the pins for power are 4(Vcc+) and 11(Vcc-), not pin 8.

indianajo 7th September 2012 06:34 PM

Obviously your op amps have more than 8 pins then. Go with your datasheet, I'm not going to download a TL074 datasheet. Filter caps can be between 220 uf and 3300 uf, 16 v rated or above, whatever you have laying around. Don't forget to also put .01uf to .47 uf ceramic disk caps between the power supplies and signal grounds near the op amp socket (to prevent oscillation). The plus of the filter caps goes towards the plus of the wall transformer, but if you are not familiar with zener diodes, the line end goes to the resistor connected to the plus of the DC producing transformer. ( When you use regular diodes as rectifiers, the other end goes to the transformer and the line becomes the plus out. If you can only find an AC wall transformer, you would use a rectifier like 1n4007 to the transformer, followed by another electrolytic filter cap to make the 18 VDC).

Mooly 9th September 2012 07:44 AM

Its very rare for an opamp to fail unless there has been a problem with its power supply.

That said, if you really believe these opamps have failed then they cost peanuts to replace :)

Before you do that though, do make sure that their power supplies are OK.

A basic test of the opamp can be done in situ by measuring carefully the DC voltages on the two input pins (per opamp) and its output pin. If one input is higher or lower than the other then the output pin will appear to be stuck high or low too. And that could indicate some external component problem.

Failed opamps come way down the list of suspects and if one (or more) has really popped then there will be some contributing factor.

Coconuttopdrop 6th November 2012 11:20 AM

Hi guys.
Thanks for the help.
So i changed all the opamps and the problem remains, so it's not a IC thing.

I don't have a way to power the circuit to measure the DC of the two inputs....

What kind of test's can i do on a unpowered circuit (comparing it to a good one) to find a busted component?

Mooly 6th November 2012 11:46 AM

I suspected as much re the opamps.

Unfortunately comparsion rarely works unless you stumble across an obvious issue such as a short or open circuit.

You need to power this up and do basic voltage tests at the very least. Voltages you can compare one channel to another and that stands a good chance of revealing the problem.

Maybe if you posted some clear pictures of the PCB top and bottom then that (might) help.

sofaspud 6th November 2012 11:51 AM

An external supply connected to the rails at the normal DC input points should work.
But are you saying that the fault is bad enough that you can't power up the mixer normally?
For an unpowered circuit, you're mostly limited to resistance checks.

Coconuttopdrop 7th November 2012 12:42 PM

Hi guys, thanks for the help.

I can power up the mixing board. It's a large studio AMEK mixing board with an external power suply unit.
The channels are detachable from the board, i have to take them out to look (or measure) the components.
Normally this type of desk comes with and extension cord, so that you can power the Ch. of the board and do maintenance, but i dont have one.

I'm doing resistance test and voltage drop test (diode test).
I managed to find a diode that was 200mv off, i changed it but the Equalization section of that ch still does not work.

I'm posting a picture of the EQ. section.
I'm shure the problem is in the eq section cuz I changed this section with one from another Ch, the problem migrated (the ch that was working OK stoped having eq, the ch that had the eq busted started to work properly)

Thanks for the help

Mooly 7th November 2012 12:56 PM

Add your pictures to the site,

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