Screen intereference - Subwoofer/bass related

**Pabs** · 10-17-2005, 01:02 PM

Peeps

I am hoping someone can help - i have searched but didnt come up with another similar problem....

Basically, my carPC was all going really well till my carnetix psu went pop. I repaired it and all is well with the PC again.
However, my jbl amp and sound was really messed up. However, i put this down to dodgy wiring and so i ripped it all out and started again - this time its 90% better and sound is no longer a problem.

However, I have, somehow, created a problem with screen intereference. I am gettting horizontal lines across the screen, but this is something I can fix. The big problem is, i believe, power related. Now, when i have music on, I get severe dimming of the screen in time with the music, so when the subwoofers are on the screen "pulses" with the music. The louder I put it, the worse it gets. At one point it even turned the screen off for a second, then back on again. This i can assume is the voltage dropping too low across the screen power input.

Now, what could be the problem? Seeing as it was all working really well before my issue, I am a bit confused. It seems as though the amp is drawing much more power/current than before.

I will,when i get time, pull off each subwoofer in turn and see if its one of the 2 subs, i will take both off and see if the problem goes. But the amp/sub works fine indoors off a battery.

Alternator has been checked and is fine - charges battery.
Battery voltage is fine - sits at 12.5V when car is off, when running I get around 14.5V.
Screen is wired into car power, not from carnetix or PC. Nothing else in the car dims other than the screen (ign switched power)

Any ideas, solutions, or links would be great. Many thanks!

**Will Albers** · 10-17-2005, 01:29 PM

Bad ground. Re-do them. When a section of power flow is bad, and voltage drop occurs, components will tend to draw more amperage to compensate. This is usually caused by a bad ground. I have seen many a fuse block melted from this type of thing.

Your powerflow could be too thin.

Let me give you a hand with trouble shooting. Everything was fine until you did X. Solve for X.

**Vinister** · 10-17-2005, 01:55 PM

Obviously your screens' input voltage is dropping heavily.

put a multimeter set on voltage across your battery terminals, and pound your bass, see if it drops below 12V.

1) If it does, check your power wire and connections to the amp. check the connection at the battery, check the fuse and connections, check to make sure your cable is big enough, check the connection at the amp, check the ground cable (big enough? too long?), check the ground point at the chassis. Make sure its bolted onto the main chassis, not a thin peice of metal thats attached to the chassis. Make sure you scrape the paint off under the ground, so there is direct metal to metal contact. Test this point with a multimeter set on 'ohms' against the negative terminal of the battery. You should see a very low number, ie 0.02 ish.

2) If it doesnt, then it might be your screen power or ground location. These should be on the input or output of your cpu power supply (your choice).

3) If all your connections test ok then you must have changed the impedance load that you're driving with your amp, causing it to draw much more current than before. did you previously have something wired in series and change it to parallel? Also there could be a minor short to ground on one of the speaker leads, or one of the speakers may be shorting its voice coil. Measure the impedance of all the speaker connections - measure them at the amplifier, but you must disconnect them from the amp first. If you have 4 regular 4 ohm speakers and one of them is measuring much less than the others, then that speaker wire might be shorting on something or the speaker might be going bad. If one of your subs measures much less than the other, then it may be going bad. If you have wired your subs so that the total impedance is too low for your amplifier, then you need to wire them in series instead.

4) If everything still appears ok, then you should load-test your battery. Make sure it is fully charged before the test. Disconnect your center coil wire and ground it to the engine. Put your multimeter across the battery terminals and measure the voltage while cranking the engine. The voltage should not drop below 9-10 volts. If it does, then looks like you need a new battery. Constant pounding bass can kill a battery even if the alternator is good. What you need in this case is a capacitor, which will act as a buffer and help the battery survive longer.

5) if your battery survives the load test, then go back to step 1 and check everything again. If everything is still good, I'd consider a larger alternator. Calculate how much current you think you're drawing, and see if the alternator is even capable of outputting that much. If it should, then have it tested and/or replaced. If it's too small - buy a bigger one or have yours re-wound for higher current output. Either way - I'd still recommend a capacitor!

let me know if it helps...

**Pabs** · 10-17-2005, 02:37 PM

wicked answers there - thanks a lot. Gives me a lot to look at/think about.

The main reason for posting was because not much has changed. The cables are all very large, think they are 4gauge, so nice and thick. The amp is a 6 channel JBL gto amp (550w max) powering 4 standard door speakers and 2 200w subs.

The only thing i think i have changed is a second subwoofer, and tidying the wiring. I originally had 2 subs, but one of them had a dodgy connection on the speaker chassis which i didnt know about for a long time, so i might only have been running on 1 sub for quite a while.

I use an autometer 4way digital distributer (fused) box in the boot, 1 20A fuse (quite large i know) for the PC and a 60A fuse for the amp (apparently amp draws up to 53A) The distributor has been working fine, and an led for each connected fuse lights. However, just recently when i put the amp fuse in all led's go out, inc. the car PC fuse, although all are connected. This is with everything connected but switched off. I would assume that too much current is being drawn, but the fuses do not blow - they are still working fine.

All was working fine until i decided to remove some unwanted cable length (had a lot of slack) and a few neons from the setup.

The ground wire is again large gauge, goes directly to the factory grounding point in the boot along with other ground cables. This ground goes straight to the amp ground connection, then the carPC ground is connected to that as well.

I shall have a look next weekend (i hope) at the wiring and see if i can find the fault. In the meantime if the above helps locate the problem more please let me know!

**Will Albers** · 10-17-2005, 07:10 PM

I bet you have the subs wired incorrectly, dropping the amp to an impedence it cant really handle... How did you wire the subs?

**Pabs** · 10-18-2005, 03:25 AM

as its a 6 channel amp so each sub has its own channel....

thinking about it i am sure there is a switch for the number of channels on the amp... think i might have knocked that to an incorrect setting :S

Cheers!