plate amp dead?

[yappari]

hey guys, i have a polk plate amp, PSW200, i got it off ebay from someone who said it was referbished. when i got it, it said 'bias modified' on it. worked fine for 5months, but yesterday it did something weird..

i forgot it on for a few hours w/o a signal, and then i found it playing a signal tone, and a strange smell coming from it. i tried unplugging the audio cables from it and it didn't stop. i opened it up and it looks like nothing was fried and everything is in one piece. i'm not sure if its still repairable or not, i have some knowledge with electronics, soldering, ect, but not enough to figure otu the problem.

if anyone can give me some tips or input, that would be great

[westend]

Can you take a picture of the different parts of the amp?

[eezip]

Bias is the continuous current running through a circuit when no signal is being reproduced - it's the static or quiescent current. Adding bias to a class B amp makes it class AB, and adding even more bias will make it class A. Because bias is directly related to losses and heating, a misadjusted bias can lead to overheating and poor audio performance if there's too much bias or poor audio performance if there's too much bias. It's a good guess the bias was either adjusted or some circuit change was made when the amp was refurbished which is normal - whenever an amp is serviced bias should always be checked and adjusted if necessary.

Now down to business, picture would be good. Also, if you can get the part numbers of the power devices that will help nail down what type of amp is it.

If you put a signal in, does it reproduce the signal?

I'm afraid what may have happened was the 'signal tone' you mentioned is an oscillation and the smell was something damaged. Electronics can make a lot of different smells but after 5 months of use, smells are a bad thing. We'll try to get her going again if we can.

[yappari]

i'll take a picture later today.
this morning i disconnected the amp from the speaker and just turned on the amp to test the signal coming out with a multimeter. as soon as i turned it on, it fried..
gray smoke came out. when i opened it, the fuse was blown, but everything seemed to look okey, my uncle (an electric eng.) said it smells like the soleniod, but it still seems to look like its fine. i tested the soleniod with a multimeter to see if it was what was doing the shorting. it came out to 5.6ohms.. so 120/5.6=~21amps??!?! i donno, does it sound like its the soleniod??
btw, thanks for the help guys!

[eezip]

I'd be really surprised if there's a solenoid in your amp. I'd imagine he meant a relay. What exactly do solenoids smell like??

On the plus side, a big fault that lets out smoke is oten a major fault that damages a few parts and leaves the rest unscathed. So, it's likely a few new parts and everything will be good to go - as opposed to a difficult and lengthy troubleshooting session. Looks aren't everything - even though it looks ok it's obviously not.

You'd have to tell me what you were measuring on the relay, or whatever it is, for the resistance to be of any help. Take several pictures close up and at least one of the whole tamale.

[westend]

I don't have any hands on with Polk amps but if they are similar to any of the other usual ones, components can be added to the audio board to boost frequencies. If thats the case, a picture of both sides of that board (or a close inspection) might lead to the failure. You might need to detach this board to see both sides.

[yappari]

did i say soleniod? sorry i meant tolenoid (the big coil thing that transform the the 120v ac) my uncle thinks it shorted and since its reading 5.6ohm (the fuse is rated at 1.6amps), i think hes right, but the tolenoid doesn't show any kind of damage on the outside so i'm not sure.. the electronics on the board (not sure about hte mosfets though, they're covered by a heatsink) seem to be fine.

sorry i'm having a hard time getting my hands on a digital camera :-/ lent it out, so i have to wait until tomorrow

[westend]

tolenoid=transformer? You can measure the outputs if you think it's bad. Usually, the transformer is not the culprit.

[eezip]

No, a 5.6ohm reading is probably fine for a transformer. A transformer should be shorted - it's a coil of wire! For a 150-180VA consumer transformer 5.6ohm is an ok resistance. Was the 5.6ohm on the primary or secondary? You need to measure both and any additional secondary windings.

I get it now, transformers do have a pretty disctinct smell Many transformers have a fuse inside. Sometimes it's resettable and sometimes it isn't - in which case you're hosed.

Is the fuse opened?

If you can get the heatsink off that can often be a big clue as to what happened.

[yappari]

thats good to hear, i got 5.6ohm when i measured the black/white wire going to the coil (power going into the coil) i tried to measure the red/yellow wires (the transformed power going to the pcb) and couldn't get a reading.
i can unscrew the toleniod and check it out, so if there is a fuse it should be inside it?
do u guys think a bad toleniod would have made the amp do what it did (lol)?