Smoke, and gain control not working. What to look for?

[chuey_316]

Hello everyone. I have a vibe 1800d and tonight I decided to hook up a remote gain control that I had sitting in my basement from another amp. I hooked it up and left for the store across town. There was no change in output regardless of position with the knob. A minute or two after leaving, I unplugged the wire from the knob, and noticed that the output didn't change, and wasn't as loud as usual.

As I was pulling in the parking lot I started to smell the smoke. I quickly stopped and shut everything down. Now whether the gain is all the way down, or all the way up makes no difference, and if the gain is up past about 1/3 there is sound from the subs, even when I have the sub off via my deck. Seems odd that the subs will play even when the amp output is off on the deck when all the smoke was from the amp.

I've taken the amp apart and don't see anything visually burnt, or any caps bulging. When I 1st got the amp, I had to replace the fets. I checked them and they all look fine as well. Where should I begin to look?

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[ThatChevyGuy 5,000+ posts]

that amp has a defect, theres a reason they made a 1801d

 

[chuey_316]

Can you elaborate more on the defect? Never had a problem with the one I had before, nor with any of the 3 that my cousin has owned. This one was fine until I plugged in the gain control from a different amp. Not sure if the control itself has a problem or is not compatible, or if it was the cable I used, but it definitely happened due to plugging that in. Not saying that I'm a big lanzar fan, just that the 1800d had always worked well for me.

 

[Louisiana_CRX 5,000+ posts]

the amp remotes run off potentiometer...that adjust the gain off resistance measured in ohms.......if using the wrong value you can fry the amps internals...and cause serious problems....

 

[chuey_316]

Yeah, that's exactly what happened. Problem is, I don't know where to look to fix it. I have a schematic for the board, but reading it is beyond me. If I knew what parts needed replaced I could probably repair it, but it's finding the problem that I have trouble with. I've fixed a couple amps in the past, but those were mainly replacing fets, and a couple other parts that were noticeably damaged. After a 2nd closer look I do see that one of the 2300uf capacitors, while no bulging or leaking, does has some of the outer cover melted on one side.

 

[Louisiana_CRX 5,000+ posts]

im sure you tried this...but did you test it with the gain unplugged after using it ?

 

[chuey_316]

Yes. I unplugged it as soon and I smelled the smoke. There is some output, but very little, and if I turn the deck up more than a 1/4 or so the output drops to almost nothing.

 

[chuey_316]

Just checked radio shack, and they don't carry an 80v 2300uf cap, only a 2200, and a 2700.

 

[Louisiana_CRX 5,000+ posts]

Yes. I unplugged it as soon and I smelled the smoke. There is some output, but very little, and if I turn the deck up more than a 1/4 or so the output drops to almost nothing.

Sounds like something in the preamp section...and being caused by the remote gain makes it even more possible...

 

[chuey_316]

I'll be honest, I don't know what areas of the amp are what, but here is a pic with the areas I see.

View attachment 26550555

 

[chuey_316]

Nothing on the preamp looks to be burnt, but I know that doesn't necessarily mean anything. Is there any way of testing it?

 

[chuey_316]

Ok, so I got it working today. Seems to be doing alright, although maybe not full power but very close if not. Still have the problem where I turn the sub off via the deck, and there is still bass, just at a lower level like I had it on with the gain down. There's also some noise when music isn't playing, but whenI had just one rca hooked up, and touch the jack with my finger it stops, even after i stop touching it until I plugged in the other rca. I'm thinking it screwed up the preamp on the deck.

 

[Louisiana_CRX 5,000+ posts]

is it a Pioneer deck ?

 

[chuey_316]

Yes it is. It's a DEH-P7800MP, I'm thinking it may have popped the output transistors, or pico fuse. Never had to fix any of them. Are they fairly easy to replace? I'd rather fix it right than to just ground the rca's shields.

 

[chuey_316]

Did some searching and found a pic of the decks internals. Doesn't look hard to fix the fuse. I'll just solder in an small inline fuse and run it out the back of the case. That way If it ever pops again, it'll be quick to replace.