Repairing a ?blown? JBL PB12 is it possible/worth it

Discussion in 'Subwoofers' started by riverwalk, Jan 11, 2009.

  1. riverwalk Audiophyte

    riverwalk
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    I know almost nothing about home stereo systems, but do enjoy watching movies and listening to music. I have a pair of Yamaha NS-777 tower speakers hooked up to a simple Yamaha RX-596 stereo receiver. I like the sound, but I would like just a little bit more bass. I have an old 5.1 surround sound system in my closet (I'm in college so moving between apartments and such I just am to lazy to set it back up right now). The sub I bought for the 5.1 system is maybe 3 years (I don't remember) old. It's a JBL Powerbass PB12 subwoofer. Nothing fancy just a circuit city subwoofer. I hooked it up to the receiver, and then hooked the speakers off the sub. When I turned it on it made a loud pop (which brought back my memory that I think part of the reason its in the closet it that its broken). If I turn the music all the way up and you put your ear up to it you can just barley hear it playing faintly. I unhooked the 777's and then turned it up and again its just barley playing faintly. I took the sub(cone/what-ever-you-call-it) out and looked and the fuse is not blown and the circuit board inside has nothing broken on it. All the wires are attached inside and none are melted (as far as I can see) or broken. My guess is that it's the (do you call it cone?) big round speaker part that you unscrew from the bottom of the sub and unhook a red and black wire from - has a big magnet on it (sorry about the lack of education). Can I just replace that part? It it worth it, or should I buy a new sub. I don't really feel like spending 3-400$ again. I figure though that it might cost half that now because it's 3 or 4 years old, so I'm sure they make better ones now. Again I'm just looking for the cheapest way to get a 12in sub to complement my ns-777's for listening to music off my computer or radio. Thanks for your help!

    P.S. If / When I get a new sub, how do I know how loud I can listen to it without blowing it again? Or is it not the volume but some incorrect settings that blow subs. I like to turn it up (the point of subwoofer/speakers), but I don't know when to stop, or what is safe. Thanks again!
  2. TLS Guy Audioholic Overlord

    TLS Guy
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    It could be either the speaker (driver) or the plate amp. The loud pop suggests that it is the amp most likely.

    Take it to a service center and see if it can be repaired at reasonable cost. Those sub amps seem to give more than their fair share of problems, especially the cheaper ones, I've noticed watching these forums.

    If it is the amp, I doubt the volume level you were using had anything to do with the failure.
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  3. STRONGBADF1 Audioholic Spartan

    STRONGBADF1
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    Hi riverwalk, and welcome.

    The JBL PB-12 was notorious for having bad amplifiers. I couldn't tell you if that's whats wrong with it or not but I wouldn't put any money in mine if it dies tomorrow. (knocks wood) I'm also not a student...:)

    JBL in the past has helped out PB-12 owners with discounts toward a new JBL subwoofer. I don't know if they still do or if the deal is even worth it but it's worth checking into.

    Hopefully one of the other members here can be of more help.

    Good luck,
    SBF1
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  4. riverwalk Audiophyte

    riverwalk
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    Thanks for your help. Should I go to best buy (where I got it) or to a small stereo place?
  5. TLS Guy Audioholic Overlord

    TLS Guy
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    Worst Buy will jack up the cost of repair to higher than buying a new one. Try and find a good independent service tech in your area.

    The other thing is that you could modify your sub to take a different plate amp. If you are handy that would be your best solution.

    If those plate amps are junk, it will still be junk after you fix it.

    If you don't think you could change to a different plate amp, you are probably better off getting a new sub, as long as you don't buy the cheapest on offer.
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  6. jamie2112 Banned

    jamie2112
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    I would not try to fix the sub. Most likely it won't be worth your time. I would buy a cheaper SVS or HSU sub. There are many on ebay and audiogon...just my opinion of course....
  7. riverwalk Audiophyte

    riverwalk
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    Is the plate amp the circuit board thing that you unscrew from from the back of the sub (with the red/black attachment points on the back side)? I think I am handy enough to replace that thing, and I have a bunch of tools. Where would I get one? Thanks again!

    Also, if the JBL is bad can I put a different companies plate amp on. The sub otherwise sounded good to me and I'd hate to just add it to the landfill if a small part can be replaced. Thanks
  8. TLS Guy Audioholic Overlord

    TLS Guy
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    The plate amp is the unit that has the mains plug, and you connect your receiver to with the controls on it.

    Here are a collection of plate amps.

    You will have to remove the current amp and find away to fit the new one. There must be a complete seal and no air leakage. If this is not possible, leave the current amp in place, and make an external enclosure for the new amp. Route the leads to the driver out and connect it to the amp.
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  9. riverwalk Audiophyte

    riverwalk
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    Thanks! My sub is 250w, so would this 240w http://www.parts-express.com/pe/showdetl.cfm?Partnumber=300-804 probably be the best? Also I think I could leave the other amp in there, and just cut a hole with my jigsaw to fit this one in. Just before I buy something like this, is there any way I can be sure its the plate amp and not the sub itself that's broken? Thanks
  10. TLS Guy Audioholic Overlord

    TLS Guy
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    Yes, that amp will do the trick. I think your plan is a good one.

    Yes, you can check your woofer. Move the cone gently to and fro, by pressing on the center of the dust cap. Make sure it is free and there is no grittiness.

    Measure the DC resistance of the woofer. It should be a bit less than the rated impedance of the woofer.

    Take a 1.5 volt battery and connect it to the woofer terminals, take one of the connections on and off the battery. The woofer cone should fly to and fro.
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  11. riverwalk Audiophyte

    riverwalk
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    Thanks, I will try that, and let you know. Also, I looked at that website of yours. Really neat setup! Are all of those older looking electronics so you can play old formats if you need to or are they for some special use.
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  12. TLS Guy Audioholic Overlord

    TLS Guy
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    You are just starting out, but I'm an old geyser, and alleged on these forums to be curmudgeonly also.

    I have been collecting music seriously since I was quite young. In those days there was a mix of 78s and LPs. The LP introduced in 1948, a year after I was born gradually took over from the 78 during my early childhood. I have been a serious collector of LPs since age 7 and have a fairly decent collection. The turntables you see have been with me most of my life, those three since my early college days, although some of the arms have changed.

    The turntable on the left is mainly for playing 78 rpm discs. The dedicated preamp for this turntable is a Quad 22 tube model from the early sixties. It contains all the EQ curves for most 78s ever pressed. Different companies required different playback curves for their discs. The two outer turntables are Garrard 301s, which were first produced in the late fifties and set an entirely new standard in disc reproduction, and is one of the great landmark products in the history of audio. The arm on the left is the Decca ffss, with 78 and LP heads. Designed by Stan Kelly, and introduced at the dawn of the stereo LP era was another landmark product. The other arms are SME, one a Mk III the other a MK II improved.

    Now to the tape decks. Above the turntables in the middle is a mastering professional cassette deck from 1984.

    On the left of that is a Revox A77 MK II two track high speed. This machine I restored from a pretty sorry state. It plays and records tape in one direction, there two audio tracks for stereo. It plays at 15 and 7.5 inches per second. Just above it is a dbx II encode/decode noise reduction unit. There were a few prerecorded tapes made in this format by Barclay Crocker and I have a few.

    The machine on the right, is a machine that I acquired in pretty decent shape, but it did not work due to a couple of serious faults. The most difficult to solve was the fact that the capstan motor clocking chip was down and NLA. This is a Revox A 77 MK IV four track standard speed. Tape speeds are 7.5 ips and 3.75 ips. The tape has two sides. On side one stereo tracks are 1 and 3, on side two when you turn over the tape, stereo tracks are tracks 2 and 4. There were a lot of tapes prerecorded in this format by the major companies. The stacked four track head was introduced by RCA in 1959. The stereo tape proceeded the stereo LP by quite a few years. In the early to mid fifties, tape machines were introduced with staggered heads for left and right stereo channels. The problem was that there was no standard head spacing, so different recording companies tapes were made for different brands of machines. I have a few staggered head tapes in my possession and can record them to wave lab and synchronize the tracks.

    The staggered head machines quickly gave way to the stacked two track machines. Prerecorded 7.5 ips two track tapes from this era have astonishing fidelity. I have quote a few. The introduction of the prerecorded four track tape in 1959 was a retrograde step as far as fidelity was concerned, but doubled playing time. Later tapes were made with Dolby B noise reduction, and this improved things greatly. I have some of those that have very good fidelity. The unit with the polished fascia above the four track A77, is an old Advent Dolby B noise reduction unit.

    The machines in the rack are a bespoke Brenell MK 6 with parabolic tape path.

    This is a very rare machine. There were quite a few made for the BBC. This machine was made for me in 1974. Speeds are 15, 7.5 and 3.75 ips. It can record and play back two track stereo. In addition to two track erase, playback and record heads, it has a fourth head which is a quarter track playback head, so it can play back quarter track tapes. This machine is one of two machines I obtained in 1974 to make professional recordings.

    The machine below that is a Revox MK I two track high speed. It has professional Dolby A noise reduction units above it, one for each track. This machine I restored from an absolutely worn out state. This was a landmark machine, produced by Herr Willi Studer, of Studer Revox Switzerland. It was the worlds first solenoid operated, feedback tachometer controlled tape deck.

    It was superior to the huge studio machines of the era. The early Beetles Abbey road recordings were made on machines just like that.

    The other tape machine, the big one the other side of the audio work station, is a Revox A 700 two track three speed machine. This is the other tape machine I acquired in 1974 to record professionally.

    Above the A 700 is a Panasonic Digital Audio Tape Deck, (DAT). This was a digital cassette based rotating head format, now obsolete. Sampling rate is 48 K rather than the CD 44K. Above that is a professional dbx 1 encode/decode noise reduction unit for the A 700.

    Above the audio work station is a now very rare PC X-1 digital recording system.

    The first digital recording system, was the Digital Audio Stationary Head System (DASH). This was developed jointly by the BBC and 3M. 3M did not pursue it long, but makers of studio tape machines such as Studer produced them for many years. CDs were mastered on such machines for many years. The advantage was that it allowed for razor blade editing, like the analog machines, they developed from. PC based editing was still way in the future.

    The PC-X1 system was a VHS tape based rotating head digital recording system. It appeared in 1984, when I adopted it. After a few years it was superseded by DAT.

    I used to do almost all the outside broadcast classical music recording for the UND public radio station KFJM FM in Grand Forks ND for many years as a public service. The big analog machines were very heavy, but the big issue was that the machines used a reel of 10.5" diameter tape every 30 minutes. Running costs were about $40 per hour. So the digital system reduced my out of pocket expenses considerably.

    Now of course recoding is done on the digital work station, which allows PC based digital editing and processing. CDs can be compiled to the Red Book standard. The DACs for encode/decode are in the blue rack mounted unit. This is an RME Fireface 800 connected to the mother board by Firewire 800 connection.

    So this is the reason you see that equipment. It makes this set up part museum which fascinates visitors, but it also makes this studio one of the most comprehensive archiving studios on the planet.

    I suspect this equipment is unfamiliar to you, and there is a good chance you have never seen a reel to reel machine in operation or heard one.
  13. Pyrrho Audioholic Ninja

    Pyrrho
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    You can hook up a subwoofer directly to ordinary speaker wires from your receiver (I mean the subwoofer driver itself, not the external terminals on the plate or outside the box). If it plays bass when you put on music, it works. If it does not play, or sounds distorted, then you have a problem. It is best if you test it mounted in the box with the plate amp covering the hole (but not wired to the subwoofer driver), but it is not necessary if you keep the volume fairly low. It would not be a bad idea to to check the subwoofer first with a meter, on the off chance that there is a short in it, but that is not likely.

    The above, by the way, would require greater care if we were talking about a tweeter, because tweeters can be damaged by bass. But woofers cannot be damaged by treble at a normal power level for the woofer.
  14. riverwalk Audiophyte

    riverwalk
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    Very impressive TLS guy. I am mainly to young to be farmiliar with those older formats, but I do have a turntable of my dads, tape decks, vhs&beta, cd, dvd, 5.25in floppy 3.5in floppy etc all stored in my closet because I figure it would be neat when I am older to look back at how things have changed with technology. I like the JD on your other website and just curious but what is that other tractor in your avatar? It looks like a really old front end loader almost.

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