Electric noise from RCA plug

[JHZR2]

While this is a consumer electronics issue (car amp), it is also an electronics repair, so I'll post it in here.

I have a two-channel car amp. One day a while back, I hit a big bump in the road, and immediately I started getting noise in my speakers. It was a high frequency noise that varied with engine speed.

My wife and I diagnosed it, and it is from one of the RCA plugs that inputs to the amp. If I push it hard upwards, the sound goes away. If I let go, it comes back.

So I'd imagine it is a bad ground on the RCA connector, right?

Is it likely possible to fix by opening the case (it is an NB quart amp, fwiw), and then trying to resolder or add a supplementary ground path?

Is this worth the time or likely to fail?

Thanks!

 

[Scorch]

Can't really tell without opening up the amp, but give it a shot and see.

 

[eljefino]

Does sound like a solder thing.

 

[Papa Bear]

Originally Posted By: eljefino
Does sound like a solder thing.


+1

 

[3311]

I agree it's a solder issue especially if it's manufactured a silver solider(ROHS Compliant). They are typically much more susceptible to heat or I should say heating/cooling cycles. Typically with use you will develop micro fractures or tiny cracks over time and the bump gave the final nudge.
If you resolder try to use a non lead free soldier if you can, but at least desolder then resolder. If you simply reheat the joint it will only be a temporary fix.
I see it a lot with the boards in our charging equipment. The lead free stuff is garbage when heat cycled.

 

[doitmyself]

So, is it a bad solder joint in the amp, or in the male rca connector?

People argue ad nauseum about soldered vs. crimped connections in instances where there is vibration. The conclusion that I reached is that both work well when done correctly. Both can fail when done improperly.

Tell us what you find out.

 

[JHZR2]

I tried doing resistance real quick but my fluke was reading 15 ohm with the leads touching, so looks like it cannot be trusted...

Need to investigate more. All looks solid inside.

Any recommendations of from what to what to test? I have the amp open.

 

[3311]

Originally Posted By: doitmyself
So, is it a bad solder joint in the amp, or in the male rca connector?

People argue ad nauseum about soldered vs. crimped connections in instances where there is vibration. The conclusion that I reached is that both work well when done correctly. Both can fail when done improperly.

Tell us what you find out.
I took it to mean a bad solder joint where the RCA plug is soldiered to the board.

I agree about the two types of connections working well when done properly.

 

[3311]

Originally Posted By: JHZR2
I tried doing resistance real quick but my fluke was reading 15 ohm with the leads touching, so looks like it cannot be trusted...

Need to investigate more. All looks solid inside.

Any recommendations of from what to what to test? I have the amp open.
Look at the RCA solder joint with a magnifying glass. You may not be able to "test anything". I would just resolder it and see if that fixes your problem

Good luck with it.

 

[JHZR2]

The connection is on the right behind the capacitor. Nearly impossible to access. How do I resolder without burning the board or putting too much heat on?

I tried probing last night.

There is no continuity between the case and the 12v ground. What surprised me was that there was no continuity between the RCA ground and the case or the chassis. How does one determine potential on that?

There was no difference that I could find, continuity or otherwise between the RCA plugs of interest and the rest of the amp... Including continuity between the plug and points on the board past the solder joint of interest.

Very frustrating.

 

[Win]

Originally Posted By: JHZR2
....

There is no continuity between the case and the 12v ground. What surprised me was that there was no continuity between the RCA ground and the case or the chassis. How does one determine potential on that?

....


There should not be any potential along that path.

Generally, in my personal experience, when doing low level audio builds, you want as few grounds as possible, ideally a single piont ground conection, otherwise you get potential differences between the points of ground which invariably leads to all kinds of noise issues. Hum, if it's on an ac line, alternator noise, like you now have, if in a vehicular application.

You are assuming you have broken impaired a ground connection, but it is equally possible an unwanted ground conection has been created and now you are dealing with the dreaded ground loop.

Search "ground loop" and consider this possibility while you are troubleshooting.

I doubt those little 1/4 to 1/8 watt components have enough inertia to break a solder connection no matter how bad the bump, unless it was built really, really, really, badly.

 

[Papa Bear]

Originally Posted By: JHZR2
The connection is on the right behind the capacitor. Nearly impossible to access. How do I resolder without burning the board or putting too much heat on?

I tried probing last night.

There is no continuity between the case and the 12v ground. What surprised me was that there was no continuity between the RCA ground and the case or the chassis. How does one determine potential on that?

There was no difference that I could find, continuity or otherwise between the RCA plugs of interest and the rest of the amp... Including continuity between the plug and points on the board past the solder joint of interest.

Very frustrating.


Take it to your local electronics repair depot and have a tech fix it for you.
When I was a tech I saw some awful botched repairs that would have been simple had I got them first.
Your "best efforts" may turn a $10.00 repair into a $100.00 repair. My $0.02

 

[sleddriver]

If all was fine before, and moving the plug eliminates the whine, then I'd replace the plug with one that has a tighter fit. Much depends upon the quality of the line-level cable-set & the connectors at each end. Furthermore, connections are always a weak spot.

I'd swap cables and see if the noise persists.

BTW, there is a difference between power ground & audio ground. Most(probably ALL nowdays)car amps use switching power supplies and that noise has to be kept out of the signal path.

 

[Smokescreen]

I agree with sled, start with the cheaper fixes first, then work your way up.

 

[JHZR2]

Originally Posted By: sleddriver
If all was fine before, and moving the plug eliminates the whine, then I'd replace the plug with one that has a tighter fit. Much depends upon the quality of the line-level cable-set & the connectors at each end. Furthermore, connections are always a weak spot.

I'd swap cables and see if the noise persists.

BTW, there is a difference between power ground & audio ground. Most(probably ALL nowdays)car amps use switching power supplies and that noise has to be kept out of the signal path.


Sorry, I dont think I noted it. I did swap the original RCAs with others, thinking it was a bad connection in the wire. No fix.

I changed amps and the noise is gone. It is indeed internal to the amp.

So the issue is that I cannot find where the ground path is that creates the noise.

 

[sleddriver]

Troubleshooting this all depends on your skill and having the necessary equipment, such as an oscilloscope.

 

[JHZR2]

Which I do not have.