Finding an earth track on pcb...

[Pentium]

I need to add some 0.1uF caps on the PSU rails (+/-) to earth, of some opamps, some already have them (ceramic) and want to understand the methods people use to find earth rails in an audio circuit, I do have a schematic but am not super duper with them yet.If I went it alone I would have to painstakingly follow traces to a known earth point on chassis / balanced I/O and this might be fraught with encountering components which mean if it's too distant from the opamp I might not have enough lead length on the earth side or it is not longer a reliable earth due to components in the circuit (I have have some polpropolene 0.1uF axial capacitors with 30mm leads each side) Any advice would be great, the unit will be in bits and not possible to connect to power, I do have a multimeter. thanks

 

[hobiesound]

find the GROUND trace (earth is for tree hugging whimps) at the power supply then put one probe on your multimeter set to diode test there, then start probing with the other probe till you meter either goes beep or shows a zero. But usually most pcb's have a ground pour so you can just use that. Do check in the schematic if the pcb has separate audio and power supply ground and use the power supply ground. Also it's possible to just put a 0.22uF capacitor right between V+ and V- to decouple. Old philips consoles with opa627 opamps use that instead of 2 separate 0,1uF caps to ground.

greetings,

Thomas

 

[JohnRoberts]

When adding HF decoupling caps, it is not only important to identify a trace that is connected to ground, but look at the path that trace takes back to PS common. At HF long narrow traces will have more impedance so affect the capacitors affect. At HF you could even see a compliance between the local and power supply ground that in extreme cases supports oscillation. I recall one large console in the main beam of an AM radio station transmitter that had significant 960 kHz singing between two grounds.

Also if the decoupling cap is connected to a clean signal ground, it can couple HF noise from the power supply into that local audio ground.

I don't mean to scare you off, just be aware of considerations and possible unintended effects.

JR 

 

[sodderboy]

When I am hunting for a trace I use a 2-3 cm piece of 14 GA speaker wire stripped back a cm as a probe broom.  I stick the other end on the other probe and sweep around testing continuity.

I agree with John but my British schemos use the term "zero volts"  0vl for the power supply common.  Use it for the other side of the cap.  Chassis or "ground" is usually, I said usually not on the PCB's.
Mike