frog

Not a mechanical problem for a change Sorry for the long post

Right, I had (see later) an alpine head unit (TDA-7556R), and I have a soundstream reference 414s in the boot, sub and components up front.

A few days ago, the sound from the head unit was somewhat garbled, then it died altogether...
So, I took it apart (as you do) and found some of the PCB tracks were burnt out
Repaired them, put it back in the car, worked for a few seconds and started smelling, er, hot... Took head unit apart again, and more tracks have burnt out.

I've just bought another head unit cos mine is now beyond repair, but, I don't really want to plug it back in without making sure there isn't a problem somewhere that would make it blow itself to pieces too

I've traced and tested the wiring, all appears ok The remote turn on wire draws 8 mA which sounds ok, however... the RCA leads show a 10V difference with the car and amp's grounds... Both on the ground and signal (tip) of the RCA.

Is that normal ?

Just to check, I disconnected everything off the amp and used the amp's power lead to feed the remote turn on, and it's the same, 10V between amp ground and RCA ground.

Any ideas most welcome, I am not sure I want to tempt my new head unit just yet

Mark 3D

iTrader: (29)

surely aslong as you have a fuse inline u should be ok to test the new unit!!

it seems to me your old head unit was fubar'ed rather than a problem with your wiring.

frog

There is a fuse on the old unit, and that hasn't suffered or melted in any way

It's only a Ł130 gamble at the end of the day , not even double or quits though

Mark 3D

iTrader: (29)

that would suggest your old head unit didnt receive any sort of power surge!!

Im now even more convinced it was a problem with your old unit!!

Eagle

iTrader: (2)

were the burnt tracks near to the ouotput stages (speaker connections) might show a short in the speaker wires somewhere...

frog

I must admit I didn't check for a short on the speakers, will do tomorrow

The burnt tracks are in the middle of the circuit board, can't trace them easily, it's a double sided board and packed full with electronics (As you would expect really )

biglee

usually if you have a short in the speaker system there is a idiot diode that will blow first and you wont have an output at all untill it is replaced, a sort of fail safe.. if you have burnt tracks then it is likely to be(depending on the size of them) your output stage has gone DC which basicly means the output transistors bolted to the side of the chassis usually on a heatsink or some sort of larger lump of metal have gone tits up.. probably by over heating of being driven into the wrong load.. with car audio it really is touch and go... if you are 1 ohm over on your loading capacity this would probably mean it would work for a while an eventually give up... do yourself a favor... look at the head uints spec.. see if its a max of 4 ohms per channel maximum load...most usually are, then check each speaker channel and make sure that none are under 3.5ohms if they are then you will kill your next one.. or buy a 2ohm stable headunit.. oh its all getting a bit technical now... but also a crap power source can stuff things up... make sure you have extremely good power connections.. if your running your headunit hard and it cant draw enuf current then it will start to burn things out.. check for "dimming" under hard loads ect...

Eagle

iTrader: (2)

burnt tracks are normally a fatal sign of excessive current draw, be that from the output stages or something internal thats given up the ghost.
output stages are normally easy to trace as they are bolted to the rear of the headunit on a nice thick bit of ally plate.
to be honest i still am thinking short circuit on the output stages. i cant see an rca causing this although some of the headunits have quite a high output voltage and yours is a 4 volt one so might be a short on the rca leads, only way to check to be honest is start removing panels etc and look at the cable, dont suppose its gonna be an easy task but might have got trapped under a seat or what ever and just worn through...

biglee

Eagle
most head units are designed with idiots in mind and usually wont simply burn tracks out if a masive load is used... as i said they use idiot diodes on the o/p that usually give up... i think excesive draw is the problem here due to a low impedence or just a crap power source..

now then lookin at the diagrams for alpine 4v line level o/p... there is NO circit short protection.. so this could be the case.trapped cable as you say.. some alpine are 13v line level o/p... dont know why on earth you would need all that tho..
ive had some cases of mosfet amplifiyers going DC-ground to line level aswell... so your amp could send +12v up your line cables.. oh dear.. pop goes your head unit...again somthing to check simply with a multimeter... its all Kirchov's fault...one for the theorists

HI EGALE, long time no talk.... im currently in barcelona on the Maximo Park FOH engineer spot... primavara sound main stage...

frog

Thanks guys,

biglee
My head unit is only connected to the rear speakers, the fronts are driven by the soundstream amp.
I normally have the fader all the way up front (I don't like sound coming from the rear), they are only wired in so that if I have passengers in the back, they can get a little back fill ().

So I am effectively never over-driving the head since the amp does all the work.

The tracks in question are pretty thick ones (2mm or so), so it would suggest they are there to carry high-ish currents.

I did check voltage on the RCA inputs of the amp in the boot with nothing put power connected to the amp, and I am reading 10V between the amp's ground and the RCA grounds and power, could that be what you mean ?

Where did you get the diagrams ?

Thanks again for the knowledgeable help guys, much appreciated

biglee

if you stick a prob up the inside conector of the RCA input and one to the ground you should see no more than 0.9v float...ish plus or minus 0.5v well definately not over 1 volt, if you see more than that then you have mosfet/transistor leakage... not too good for your head unit.. if the tracks on your board are thick ones then the will be the power sorce to the OP transistors.. if you have a blown one it will cause a short.. and take the tracks out...
check your amp tho and make sure our float voltage is under 1volt then you will be ok putting another headunit on it...

frog

Saw 10V last time I measured at the amp end

Going to speak to my local car audio shop for advice before I proceed methinks

Eagle

iTrader: (2)

i hate you now love barcelona

biglee

today im at the nurburgring... in the middle of it doing a festival called ROCK AM RING lol.... just saw a few really good bands play today.. working too hard tho...haha yea rite....