Old School Fosgate amp problem?

lukeman269 · Post by **lukeman269** » Sat Oct 02, 2010 9:02 pm

I just picked up an older fosgate amp for 100 bucks. It is the bd500.1

So my problem is that when I shut the deck off in my car, the amp will send a pulse through my sub and It probably shouldn't do that. The pulse sounds like a loud thump basically. I'm kind of worried that my sub is going to get damaged due to this quick pulse. It does it slightly when changing songs on the cd player too. It's like the amp is getting a feedback signal or something. My previous amps did not do this with the same deck either. Any way to fix this problem?

Post by **Eric D** » Sun Oct 03, 2010 6:21 am

If it does it when you change songs on your deck, I am 95% certain it is an issue with the deck, or the wiring (RCAs most likely).

If it did it just when you shut the car off, but not when you changed songs I would guess the problem was in the amp.

marko · Post by **marko** » Sun Oct 03, 2010 6:36 am

does sound like a bad rca shield on the amp or even the headunit or cable like eric said.

lukeman269 · Post by **lukeman269** » Sun Oct 03, 2010 6:43 am

Eric D wrote:If it does it when you change songs on your deck, I am 95% certain it is an issue with the deck, or the wiring (RCAs most likely).

If it did it just when you shut the car off, but not when you changed songs I would guess the problem was in the amp.

Well It isn't as bad when changing songs, but it's pretty loud when I shut off the car. Should I try the amp in a different car and see if it does it in there? Or is there a way to fix the rca inputs on the amp?

marko · Post by **marko** » Sun Oct 03, 2010 6:45 am

have a look inside, sometime the rca connectors are physically broken or there's a dry joint..

Post by **Eric D** » Sun Oct 03, 2010 6:57 am

If it were a bad shield on the RCA on the amp I cannot see it doing it between song changes, it would be doing it randomly all the time.

Pops between songs and when the car turns off might be a bad muting circuit on the deck's output.

But, I would not rule the RCA jacks out until looking into it. This issue is bizarre enough to have a number of causes.

The Golden One · Post by **The Golden One** » Sun Oct 03, 2010 7:25 am

could be a bad output transistor or one on the way out that thump is caused by stored up voltage not making it's way trough the transistor until you shut the amp down then it jolt's the speaker then thump it's released into the speaker.

lukeman269 · Post by **lukeman269** » Sun Oct 03, 2010 8:28 am

The Golden One wrote:could be a bad output transistor or one on the way out that thump is caused by stored up voltage not making it's way trough the transistor until you shut the amp down then it jolt's the speaker then thump it's released into the speaker.

Well I just ruled out the rca cords. Tried some other ones I had and thats not the problem, It shouldnt be the inputs on the amp, I just opened up the amp and soldered them. They seemed to be a little loose. Still got the jolt when I turn the deck off. What should I try next?

Post by **Eric D** » Sun Oct 03, 2010 9:01 am

Try a portable CD player instead of the deck and see if it still does it between tracks. If it does, then the problem is probably inside the amp.

The Golden One · Post by **The Golden One** » Sun Oct 03, 2010 9:03 am

if you have the amp bridged now you could unbridge it and then run only one channel at a time then shut the amp down to see if the thump is still there in ether channel. if the noise is on one channel and not the other then it most likely would be a bad output transistor on that respective channel, if both channel's thump then it may be something else.

Post by **Eric D** » Sun Oct 03, 2010 9:16 am

The bd500.1 is a Class BD mono amp. No bridging.

The Golden One · Post by **The Golden One** » Sun Oct 03, 2010 9:25 am

Eric D wrote:The bd500.1 is a Class BD mono amp. No bridging.

then that rules out the oldschool easy way to pinpoint a bad transistor

lukeman269 · Post by **lukeman269** » Sun Oct 03, 2010 10:15 am

Eric D wrote:Try a portable CD player instead of the deck and see if it still does it between tracks. If it does, then the problem is probably inside the amp.

I think it is the amp. What would be wrong with it? Everything internally looks good no broken or loose connections. Clean as well. The amp works good and everything but it just annoys me when it does that quick thump when I shut my car off. Will it damage the sub at all? Are there any products out there that I can hook up through my rca's to get rid of the problem?

Post by **ttocs** » Sun Oct 03, 2010 10:23 am

try some technical taps. with everything powered up start with some light taps with just your finger tip on the radio/amp. Slowly increase the amount of force you use until you need to move up to your knuckle. See if you can get the system to recreat the problem with your taps to try and figure out if ou can isolate where the problem is. Fiddle with all the connections as well. Twist them back and forth to ensure that nothing is loose or that the noises do not continue.

The Golden One · Post by **The Golden One** » Sun Oct 03, 2010 10:28 am

i just though of another thing you could try and that is to disconnect the rca's and even the remote but leave the sub's hooked up then use a short wire to go from the + power connector to the remote turn on. then you can turn the amp on and off with the jumper to see if the thump is still there if it is then the problem could be a bad diode or output transistor.