AF4 - RFI - grounding? damaged IC?

[quickshift]

Hello.

This relates to the AF4 boards from the group buy but I've spammed the GB thread with questions too much already. Since it's kind of chip amp related, I thought I'd pop the thread in this forum.

I'm having a problem with RFI on one channel of a 3 way active filter I've just finished. I think the problem is with the input buffer circuit because if I touch the non inverting input leg of the IC (LM6172) with a metallic object, providing I'm not insulated from the object, the noise stops completely.

I've checked all the solder joints etc, there are 3 other boards, 2 identical pairs but this is the only channel with any problems. The grounding etc is the same for all boards. I've gone over every 0v "perimeter" connection not already grounded with a wire grounded at one end and couldn't get rid of the noise.

I've got 1 pair of RCAs connected to 4 inputs - might this have anything to go with it? One set of inputs is fine though.

Oh, I also may have accidentally connected the power the wrong way round although this would just kill the IC if it was going to do anything wouldn't it?

The RFI noise is quite loud when I switch the unit on but reduces over about 10 seconds.

Any advice or ideas appreciated

[BrianDonegan]

A link to the schematic would be helpful.

It's either a cooked component (perhaps a blown cap), or a ground loop. Perhaps an RCA shell is shorted to the chassis?

[quickshift]

This is the schematic. http://www.delta-audio.com/Filter_DI...io.com.sch.pdf

It's touching pin 5 on the IC3 (IC3B) or the side of R12 which is connected to pin 2 of IC3 (IC3A) which makes the RFI noise stop.

I've checked the RCAs and they aren't touching the chassis.

[Minion]

Maybe a Bad solder Joint on the Non-inverting input??


Just a Thouight.....

[quickshift]

Yes, I thought it may have been as I was soldering quite "fast" to minimise any risk from heat damage but I've run over all the joints on the board again and got no improvement.

[Nico Ras]

Originally posted by quickshift
This is the schematic. http://www.delta-audio.com/Filter_DI...io.com.sch.pdf

It's touching pin 5 on the IC3 (IC3B) or the side of R12 which is connected to pin 2 of IC3 (IC3A) which makes the RFI noise stop.

I've checked the RCAs and they aren't touching the chassis.

You are relying totally on IC3 A to provide the virtual ground reference for IC 3B, which could be causeing the problem and the stray capacitance and high impedance could make it oscillate. By placing your finger ar R12 you are giving some shunt to the oscillator that makes it stop,

I will try to put a resistor to ground between R16 input side with the same value as R12. This will reference the inverting input to groud also.

Nico

[quickshift]

Originally posted by Nico Ras


You are relying totally on IC3 A to provide the virtual ground reference for IC 3B, which could be causeing the problem and the stray capacitance and high impedance could make it oscillate. By placing your finger ar R12 you are giving some shunt to the oscillator that makes it stop,

I will try to put a resistor to ground between R16 input side with the same value as R12. This will reference the inverting input to groud also.

Nico

Many thanks for your reply

I should have also explained that there is a jumper between pins 1 and 2 on the in- pads. Sorry I'm quite inexperienced in this area so tend to miss out vital details!

Would you please show me on the attached diagram where I should place the resistor? I'm quite likely to put it in the wrong place otherwise...

I've swapped R12, R16 and C10, the noise is a bit quieter now.

I think it picked up a minicab transmission earlier

[quickshift]

Diagram.

I can't even do attachments today!