JLB PSW-D112 - Worth repairing?

I'm not familiar with that sub so you will have to provide some details.  Ported or sealed?  Ported - what is the tuning frequency of the enclosure?  Stamped steel frame driver or a more beefy driver?  I'm just trying to guess the excursion potential.

It is never a great idea to work with a factory sub but at least replacing the amp you have more control over the situation.  Worst case scenario, you replace both the amp and the driver.  You then have a DIY sub without having to build a box.

Tuning frequency can be found if you know the enclosure volume, port diameter, port length and number of ports.

Based on the the manual, it is tuned to 28hz.  That's the low end of the frequency response.  Also, the sub-sonic filter is 25hz.  There is also mention of some type of EQ boost but no mention of the fequency, amount or the Q of the boost.  Adding a new amp may sound a little flat.  That could be good or bad depending on how it sounded before.  Also, the driver is 6.1 ohms DC so it is an 8 ohm nominal driver.  The Parts Express amps are all rated with 4 ohm drivers so you will get less power than the specs say.

It's going to cost more than $100 to get a proper amp.  In fact, I'd recommend an amp/driver swap for the best performance increase.

The Dayton BASH 300 or the Dayton 240w amp ($149 and $139 respectively) are both good choices.  But with either, you would have to modify the subsonic filter by swapping out some resistors.  A basic knowledge of soldering is required for that.

One option would be to seal up the sub, use one of the amps above and replace the driver with the 12" DVC model - http://www.parts-express.com/pe/showdetl.cfm?Partnumber=295-185.  That is a GREAT driver that I have used in a 1.2 cubic foot, sealed enclosure with excellent results.  Not boomy at all and plays much lower than expected in such a small enclosure.  If you go with the amp or the amp and sub, wait a few weeks if you can.  PE always runs a 12 days of Christmas sale with great deals.  They always have an amp for sale during that time.  You need to watch every day because the sale is good for 24 hours.

I saw those numbers and those are the outside dimensions.  Is the grill included in that measurement?  I would then need to know the thickness of the walls to determine the net enclosure volume.

I hate to break the news to you but this is not a huge sub compared to others that are considered quality subs.  Whether you want to pitch it or repair it, you need to have specific goals as well as constraints in mind for your new or repaired sub.  My guess is you want something smaller - they make smaller subs.  Do you want it to play loud as well as deep?  Small and loud is easy.  Small, loud and deep is difficult unless you throw a lot of money into the sub.  That means small, loud, deep and cheap is impossible.  Most of us make a compromise based on our goals.  I don't have a problem with large so the subs I build tend to be quite big.  I'll be cutting wood for a pair of 10" subs this weekend and each will be larger than your JBL.  I need them to be that big to play as low as I want.

Each amp comes with a set of instructions that include the resistors to replace as well as the values for different filter and boost combinations.

If you keep the driver, then stay ported.  They chose that driver/enclosure combination for a reason.

Hopefully you have a few woodworking tools because you will need to adjust the size of the amp cut-out to fit the new amp.  If you need to mount an amp vertically to cover the entire hole, then you probably want to go with the BASH model.  The 240w model has fins and they need to be oriented vertically for optimum cooling.  The BASH amp runs cooler so it can be mounted any way you want to.

Just swap the amp now.  Use the BASH 300.  With an 8ohm driver, you are looking at between 150 and 200w of power going to it.

Based on this sheet - http://www.parts-express.com/pdf/300-750.pdf, I'd use the 2db of base boost chart and the filter FC=25hz.  Later, you can swap the driver and make a few mods to the port to lower the tuning.

Well, a lot of questions were answered by you and the owern's manual.  Sealed is the easiest way to go when you have a lot of unknowns.  If I had not known the tuning frequency was around 28hz, recommending the BASH amp in its standard configuration would have the potential to destroy the sub driver.  You want to limit playing signals under the sub's tuning fequency.  Just swapping the amp is also the cheapest way to go for now.

That is just an arbitrary JBL part number.  Since it is not sold by itself on the DIY market I know nothing about it.  I have the tools to measure the Theile/Small parameters but that would only be helpful if you wanted to build a new enclosure.

That does look like a decent enclosure so there is potential if you decide to swap drivers later.

The D112 and PB12 (both similar subs sold around the same time from JBL) are notorius for the amp failing.  If you read the reviews at PE on the Dayton amps, several folks there mention the PB12.  I have a PB12 that the amp failed in after 3-4 years and replaced it with the 240W Dayton amp.  The swap was simple - just remove the factory amp and slip the new one in - you will have to make the cut out just a little larger for the Dayton amp.  While I had it apart, I sealed the enclosure a little by adding some silicon to the seams, as they were just glued.  Hookup is simple, just two wires (the PB12 has a couple of green/red LEDs on the front that you can hook up as well if you want, but I didn't fool with that because I didn't know where to hook them in).  Sounds better than it did from the factory by far!  I did have some noise from the amp due to ground issues (read the reviews on the Dayton amps) but resolved that by removing the grounding plug, which isn't a good idea but worked :)

Yeah I let it sit for a day or so before buttoning it back up, as I was waiting for the amp to arrive.  I got lucky there, I had forgotten about the fumes from silicon - back in the day building car boxes we would let them sit for a day or so before closing them up for that very reason.

The 240w amp with no bass boost.

Agreed, that's what I'm running on mine.  Sounds better than new, for sure.

Correct.  That is the only 240w sub amp without bass boost.

You may have to enlarge the cut out where the amp goes - I know I did on my PB12.  I don't know if the two shared the same amp or not.  As far as filters, I didn't do anything.

I don't know the stock filter on that amp is.  Based on your information above, the filter needs to be set to 25hz with as little bass boost as possible.

You will have to make changes.  Flat to 30hz is just wasting the potential of this driver.  I'd go with flat to 15hz and tune the 5cf enclosure to 17hz.

I'm pretty sure my last post was meant for a different thread.

After looking at the stock values, the filter is very, very shallow.  I don't think you will have a problem using that setting.