Author Topic: Why do I get this on the scope?  (Read 1495 times)

Offline d0dja

Why do I get this on the scope?
« on: September 12, 2013, 03:13:21 PM »
Was sorting out my SRPP, where I used to get some crosstalk from other channels, and relays wouldn't select all the time. I didn't connect a ground as I thought it unnecessary -- I hooked the grounds of the RCA ins to each other and to the signal ground on the volume pot board, and the hot pins of the RCAs to selector board and then to the volume board -- didn't think the selector board also needed ground as it only switched the hot pins.

So as it turns out the ground was needed -- but what I don't understand is why I get this fierce ripple from the selector board -- when nothing is plugged into it, and it is plugged into nothing. Basically, the board is hooked up to the RCAs, which are empty, and there's no power to the board. But if I put the probe to the loose signal wire coming off the board, i get his crazy ripple.

Sure, I get it that high impedance floating connections can do weird things -- but what can it be picking up? It's a 50Hz (mains I assume) fundamental with jaggies all over - this is reduced by turning off lights, etc near it (12V halogen lamp with a txformer).

But I'm curious -- it's 100-200mV, not inconsiderable. And I get nothing like it if I touch probe anywhere else on amp.

Offline Schalk

Re: Why do I get this on the scope?
« Reply #1 on: September 12, 2013, 09:24:13 PM »
Any floating, unreferenced object connected to your scope will act like an antenna, and your mains frequency pollution is probably the highest of most available "stations" buzzing around our environment in a house / office. And the scope IS an amp with a face that shows all. It speaks with pictures where normal audio amps speak with speakers.

Offline ludo

Re: Why do I get this on the scope?
« Reply #2 on: September 12, 2013, 09:42:32 PM »
Schalk beat me to it. Here's the long version anyway ;D

The scope has 1 Meg input impedance. Usually a JFet input stage. Sustaining a voltage on that is easy. So it shows you what's in the air. Mostly it's the mains voltage. You are seeing capacitive coupling from mains wires to the probe tip.

If you put your finger on the probe, the noise gets worse as the probe tip is "coupled" to the mains voltage better, due to the increase in surface area (more capacitance.) Anything even slightly conductive will have the same effect when connected to the probe tip. I see 20V peak to peak on the chassis of an unconnected piece of equipment here. It also looks a bit "cleaner" than the noise from the probe on its own. As the reactance changes (due to larger capacitance) the low freq content of the mains becomes more obvious.

When working with JFets (or valves) you have to be rather careful with capacitive coupling to the gates if you want to avoid crosstalk. You need only more or less nothing current-wise to change the voltage on a JFet gate.
If it 's going vrot, put it on the buffet ! - Fats

Offline GearSlave

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Re: Why do I get this on the scope?
« Reply #3 on: September 12, 2013, 10:28:42 PM »
You're all wrong. It's an extraterrestrial signal, probably from Alpha Centaury if I had to guess from the sequence of ripples.
Resistance is not futile; it is voltage divided by current (R=V/I)

Offline Kent Kassler

Re: Why do I get this on the scope?
« Reply #4 on: September 12, 2013, 11:19:56 PM »
Nonsense!...Dodja my friend,someone from your past with the initials MW is trying to make contact.....somethings coming through about a terrible tragedy that took place,,,, and that you are forgiven?Hope this helps.
Audiophile Sound Sommelier Extraordinaire....aka Manic Depressive Temporary Void Filler Deluxe.

Offline Steerpike

Re: Why do I get this on the scope?
« Reply #5 on: September 12, 2013, 11:49:06 PM »

Offline d0dja

Re: Why do I get this on the scope?
« Reply #6 on: September 20, 2013, 07:46:26 AM »
So basically using a 'scope can really get you intro trouble if you don't know the black arts? You can be chasing down a problem that was really just the scope interacting with the stuff it's near...

Just a bit surprised that _only_ the selector board did that -- but perhaps the shape of it made it a perfect antenna.

Would it be an idea to AC couple the selector board to the chassis ground through a small capacitor? Or is this one of the black art things where you have to suck it and see?

I a while when I have a mo' I'm going to post some pics of a scope trace I get when I put my (old valve) signal generator through a rectifier bridge. It will be an amazing game of 'WFT is going on there'. It has me baffled...

But then again, I baffle easy.

Offline GECO

Re: Why do I get this on the scope?
« Reply #7 on: September 20, 2013, 08:23:53 AM »
you can AC couple it. would work
i use two diodes back to back with a 100nf cap. works a charm.