Electrical Engineer's (or those who know amps), amp help please.

Discussion in 'Home Theater Projects' started by Dustin B, May 8, 2002.

  1. Dustin B

    Dustin B Producer

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    Amp is the PE 300-792. I modified the boost on this amp to provide 1dB of boost from 25-30hz and have an Fc of 17hz. You can see pictures of the solder job and the sub it powers on my website linked to in my sig.
    The problem is the amp doesn't seem to work anymore. The situation that lead to the amp not working is as follows. Finished setting up my older speakers at my parents place (the sub was there already and had been working fine). Calibrated with Avia to 10dB under reference. Proceeded to play Matrix for demo material after calibration. Played lobby scene, followed by chain gun, followed by helicopter explosion. Halfway through the lobby scene I turned the volume up a bit more (I'm confident no more than 5dB increase). When the big rumble part of the helicopter explosion was about to hit the bass quit.
    Turned the amp off and let it cool for a while, tried again nothing. Left the amp in the auto/on position over night, tried again this morning nothing (well almost nothing, when I turn the amp from off to on I get about half a second of muted bass).
    I pulled the plastic cover off and the, not sure of the correct term, the heavy solid black thing that the power from wall connects to (the transformer?) was still warm. So I put a fan on it and cooled it off. Put it back together and tried again, same as before. Finally figured out were the fuse was, checked it and it's fine.
    So then I pulled the amp apart again and took a closer look at the circuit boards. Didn't see anything that looked wrong to me on the preamp board, but something looks to be wrong on the amp board. How wrong I don't know. I snapped a few pics:
    http://dustin.bunnyhug.net/forum/board.jpg
    http://dustin.bunnyhug.net/forum/close1.jpg
    http://dustin.bunnyhug.net/forum/close2.jpg
    How bad is it?
     
  2. Patrick Sun

    Patrick Sun Studio Mogul

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    Your last name is Bunnyhug?

    Do you smell smoke? Any transistor or cap obviously blown up? Could be a voltage regulator that bit the dusty.
     
  3. Dustin B

    Dustin B Producer

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    Hehe, its a Saskatchewan thing. You know them as hoodies I think. For some reason my roommate thought that was funny and picked the domain name on that basis.

    Didn't smell any smoke. The only thing I could see after taking it appart was what I took pictures of. The discoloration of the board and those resistors that look red and discolored (are they not resistors/are they supposed to look like that?).
     
  4. Saurav

    Saurav Cinematographer

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    I think those are diodes, going by the symbol under them and the D1/D2 designation. The reddish discoloration on the board does look strange, but I don't see any obvious signs of something smoking.

    I would first suspect the changes you made, though it's entirely possible that something else failed. Have you turned the board(s) over and checked for shorts, stray threads of solder, things like that? There are more things that can go wrong on the underside of a PCB.
     
  5. Will Pomeroy

    Will Pomeroy Stunt Coordinator

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    I would deffinetly investigate this area... if you can't find anything else, try replacing the two, and see what happens...
    [​IMG]
     
  6. Greg_R

    Greg_R Screenwriter

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    D = diode and they are directional (as you can see from the print on the board). When you replace it make sure you put it in the right way!
     
  7. Ryan Schnacke

    Ryan Schnacke Supporting Actor

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    I can't see anything wrong in the close2.jpg pic. It looks like you were just worried about the diodes since they do look a bit different than resistors.
    I agree with Will. Replace that diode (D1) and resistor (R3). Once you've removed them you may want to test them to confirm that at least one of them is damaged. And as a note, its extremely seldom that a diode or resistor would fail under normal use. Much more likely that some other failure caused a very large amount of current to flow through them. By replacing the resistor and diode you may only be treating a symptom of the problem.
    Seems like somebody (maybe on the PE board, I can't remember) posted a full schematic for one of the PE amps. That sure would be handy now. Or maybe you could trace out what other components are directly connected to R3 or D1. Those components would be suspect. And as previously mentioned double check your previous handiwork for shorts.
     
  8. jeff lam

    jeff lam Screenwriter

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    Check for black burn marks on the components and board. Do you have a volt meter? this can be used to check the different stages and see where the problem lies. Be sure you know what you're doing though if you power the amp up while fiddling with it.
     
  9. Dustin B

    Dustin B Producer

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    Well I feel really stupid now, amp wasn't the problem. Dad came home and asked if I was still sure that amp was the problem. I said ya I'm pretty sure. I hooked up the 6th channel on the technics amp to the sub, not meant to drive 4ohm loads, but it was causing a little movement of the cone, when I tried to turn the volume up the amp went into protection mode. The PE amp would give muted output for a fraction of a second and then nothing. But dad suggested I should hook the amp up to the old piece of junk Technics sub under the end table not being used. I had completely forgotten about it. Hooked it up to the amp and it worked fine. Well damn.

    Pull the driver out of the enclosure. Hook one voice coil up to the amp, get a little movement. Hook the other voice coil up to the amp, get a little movement. Put the parallel wire back in and hook the driver up free air one more time. Get movement, turn the stereo up get lots of movement. Put the driver back in the enclosure and it works fine.

    Same thing that happened the first night the sub was completed.

    Any idea what could cause a driver to stop working, but start working again after you turn it over a bunch and run voice coils individually free air for a bit? Is your suggest likely to damage the motor? Should I be seriously looking at getting this driver replaced?

    PS- I thought it was the amp that overheated and caused the sub to stop but I've now ran the pod race, lobby scene, chaingun, helicopter explosion (10dB under reference) and now several songs from the Blue Man Group DVD (not quite as loud) while typing this and it's still going strong.

    Will

    What you hilighted was what I was taking the picture of, that's what concerned me most. Also shows you how much I know about electronics. I didn't know those were diodes, thought they were resistors :b
     
  10. jeff lam

    jeff lam Screenwriter

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    An easy way to tell most diodes from resistors are many diodes are made of a glass type material. Some aren't and are usually plastic instead but diodes usually only have one ring next to one of the leads specifying the cathode connection (at least I'm pretty sure it's cathode). Resistors usualy have color code rings 4-5 total.

    But soon everything will be using surface mount components and all the schooling we had on that stuff will be useless.
     
  11. Ryan Schnacke

    Ryan Schnacke Supporting Actor

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  12. Bryan Michael

    Bryan Michael Supporting Actor

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    the rings are numbers that tell the restance of the ristor it goes black brown and the last 2 bands tell the tolerance of the ristor it is usaly gold and silver
     
  13. Will Pomeroy

    Will Pomeroy Stunt Coordinator

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    Just so ya know...

    its the first two colours tell corespond to a number each, like 4, and 5, lets say, the 3rd is the mulitplier, like 100, and the last band is silver for 10% tolerance, and gold is 5%. So that would mean that its a 4500 ohm resistor, within 5% (lets say gold band) of that.

    there is a 5 band code with just an extra middle number, like 4590 ohm or something...
     
  14. Brian Bunge

    Brian Bunge Producer

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  15. Patrick Sun

    Patrick Sun Studio Mogul

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  16. Jeff Rosz

    Jeff Rosz Second Unit

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    heh,
    i would tell ya'll my way of remembering the resistor color codes, but i'd get booted right off the forum. [​IMG]
     
  17. jeff lam

    jeff lam Screenwriter

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    Jeff, I learned a much cleaner version of remembering the color codes in high school from my electronics teacher: "bad boys (race) our young girls but violet (generally) (wins)"

    Much better than the other one that replaces the words in ().

    Brian,

    I remembered on LED's that the cathode was the shorter lead but couldn't remember on the diode with the ring. Guess I need to do some brushing up.

    Bryan + Will,

    Usually the 5 band resistors are the lower tollerance ones (1% or less) and they need that extra character for accuracy. 3 sig figs and a multiplier + tollerance band.
     
  18. Kevin Deacon

    Kevin Deacon Second Unit

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    Come on Jeff, tell us know-nots what original words were in the parens. Please!!!
     
  19. Jeff Rosz

    Jeff Rosz Second Unit

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    jeff,
    heh, i like the dirty one better.
    kevin kevin kevin,
    lets just say its VERY BAD [​IMG] , unless of course your name is violet [​IMG]
     
  20. Mary M S

    Mary M S Screenwriter

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    Are you guys gentle with neophytes? Or change that to dangerous novices. (can DIY area include self-repair on purchases gone bad, too?)
    Sub went bad opened it up and found fuse blown. Replaced fuse..no go. further investigation showed one resistor? (I think) fried. Went to 4 locals and canNOT find the correct resistor which appears to be (from deductive process of seeing every other W size either side of this one) prob a 1/2w 20ohm 2%.
    Husband got inpatient, got the soldering iron out last night and replaced using a 1/4w 20ohm 2% into the slot (I believe the amp board). I did not know he was up to this and was suddenly surprised by a blaze of white-light-sparking glory, which appeared to be about to take my husband into the other world; right there in my living room. I did not eye the board after that riveting moment last night, - I know prior to the solder job, no burn marks on the circuit board (looked perfectly pristine) except for the blown fuse and resistor.
    Now more resistors are fried. [​IMG] (have not looked to see if NOW the board looks smoked).
    If we go through the bother of determining and buying all correct (and newly fried) resisters. What would any of you possibly estimate odds that this amp, (per scanning all above posts) has a major issue somewhere else in the power chain, rather than just possibly having been over pushed (thermal related)? The cost (I believe ..was about 200 or so for this SW) I have wanted to replace it for a long while, so begin to wonder if it is worth all the part-testing mail-order track down time.
    The bad thing about not 'trying' to fix it now is I truly wanted to spend months researching LFE and sub’s before I purchased the next one. I don't know if we can live without one that long.
    Really curious about what broke initially but I guess we will never know unless we do start checking everything till its functioning again.
    My husband stated the day before ...we would mail order for the correct resistor...he mentioned he had no idea what a different W rating would do on the board, but felt the ohm rating and percent would be more critical as long as the W rating was not larger, but then stated NOT knowing, he would order the proper part. (So I don't' know what got into him...he missed his LFE??? he had a new Blue Man DVD).
    ...my assumption would be (hindsight) W matters a lot?!!!
    PS. as an aside as I just started researching subw build, and prob will cont. to do so even if we fix the current unit. Is there an issue (since I tend to overkill) with purchasing a very powerful sub if I can shoehorn it (I’ve already noticed the at or < 20Hz zone and was intrigued by the Everest .) Concerned by the extremely small room due to the lengths of those long soundwaves being deflected off close walls? Is it possible to end up with some sort of negative compression effect due to the tiny tolerances [enclosure] of the room itself? What keywords should I use to google/research that type information. I have so much to learn!
    no matter how much I pick up along the road, I'm sure I will remain.
    Yours truly; resident village idiot.
     

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