M14s: I may have a pair

[Alien_Shore]

Why not mount the 908 on the Manta..This will give you the opportunity to examine the 902 to see what is wrong with it.

Good thinking, AB...did just that very thing. In fact, I toyed around with several drivers on some spare MR horns. Also, I rebuilt the prototype XO, using the inductor and L-pads from the 9842 XO, and all new caps - playing around a bit with the values.





This is getting much better now. The prototype left channel is now sounding better than the stock right channel. Progress!

I need to get a measuring program for my laptop. Any suggestions out there for something that is accurate but doesn't cost an arm/leg?

[aditya]

This is getting much better now. The prototype left channel is now sounding better than the stock right channel. Progress!
I need to get a measuring program for my laptop. Any suggestions out there for something that is accurate but doesn't cost an arm/leg?

Congratulations on your progress

I use a db-meter, and the free version of ARTA for my measurement work. The free version can do everything except saving a graph, and is not fussy about the sound card also.

ARTA Download

[Alien_Shore]

After a couple attempts at getting an L/C meter that would work accurately with these multi-layer coils, I have some fairly reliable measurements of the coil in the Model 14 crossover. So here's the updated schematic. I'll measure some of the other crossovers I have later this week.

Model 14 XO schematic

For what it's worth, the 4 crossovers I have all measure a bit differently, but that's predictable given the tolerances of these devices. I'm feeling pretty good about the 0.21 mH being the average across the ones I have.

[Alien_Shore]

So in the process of updating these crossovers, I decided to test the woofers. Here's the comparison of the two:

This first one is looking and sounding pretty good.



Something is off with this one. The Re is low, and a big difference in the impedance plots.



Anyone got any insights? I think the second one may need a trip to OKC...

[Alien_Shore]

Talked to Bill @ GPA today. He said there's something "definitely not right" with that woofer. I'll be packing it up and sending Monday.
Until then, I'll be playing with my newest tool in the arsenal:



I picked up a Behringer Ultradrive Pro. Wow. This thing changes everything. It's amazing to me how much different the sounds are with the different filter types, and slope of the crossovers. I have a lot to learn...

[Alien_Shore]

OK - I've taken the plunge and got a test rig up and running.
Here are some of the results - you guys go easy on me - I'm a noob on this measurement stuff.

All measurements done in-cabinet, on-axis, and near-field (mic right up next to the driver):

Cabinet 1 Woofer:



Cabinet 2 Woofer:



Cabinet 1 HF Driver:



Cabinet 2 HF Driver:



Definitely looks like I have a problem with one of the 902's, or maybe I screwed up the measurement somehow.

[Alien_Shore]

Evolving the technique:

Cabinet #1:



Cabinet #2:

[Alien_Shore]

Since I was taking apart the cabinets to pull the speakers out to send off to GPA, I did some physical measurements of the dimensions of the Model 14 cabinet. I couldn't find a drawing of this cabinet online anywhere so I sketched up a quick drawing:





There are more details needed to actually construct on of these cabinets. Perhaps I'll add to this if the need arises.

[Alien_Shore]

OK, while I wait for the woofers to be "GPA'd", I needed to work on these cabinets.
Several things are in obvious need of repair. I started with the base cabinets:



They all have some refinishing issues, but there are some structural problems too. First off, there was a woofer mounting bolt missing. Turns out that there was a big chuck of particle board broken away and the tee-nut was gone as well. In case you're wondering, the bolts are 1/4-20 x 1 in. (finished in a lovely dark color). The tee-nut obviously is a 1/4-20 thread (more on the parts later). First thing was to take some industrial strength wood filler and patch the hole. You can see right there at six-oclock, this is after 3 applications and sanding in-between. It actually filled in pretty nicely:



There was a similar issue with one of the cabinet tops where the horns mount. One of the horn mounting bolts was just turning the tee-nut. Turns out it was cross-threaded like never-before-seen. I asked Bill @ GPA for some advice and finally got both the bolt and the tee-nut out without damaging the horn. The repair job was similar to the base cabinet - wood filler, sand, repeat.

The snazzy knurled bolts for the horn (and the crossover) are socket head bolts (you can find these at home depot online - not available in retail stores near me anyway). I couldn't find the woofer mounting bolts anywhere, so I figured what I would do is convert them all to socket head bolts so I can just use the same tool to remove the horns and the woofers, plus they look very cool. So here is the hardware I'm using for the repairs:

Per speaker -
(6) #10-32 x 1" steel socket cap screws (MR931 horn bolts)
(8) 1/4-20 x 1" steel socket cap screws (Woofer bolts)
(1) 1/4-20 Brad hole tee-nut
(1) #10-32 Brad hole tee-nut
(8) Shepherd 1-1/8 in. Plastic Base Nail-On Cushion Glides (for the foot)



With the holes patched, and the new tee-nuts installed, it was time to do the re-finishing. I scanned this site for the recommended stuff. Howards Restor-a-Finish gets the nod. I also picked up the Howards Feed-N-Wax. Let me tell you, this stuff is unreal! Talk about easy, no-mess, and fantastic results. I'll let the pictures tell the story. Here is the top-cabinet before I started - lots of water stains, sun damage, etc.:



Then comes the Restor-A-Finish (I'll start calling "Jesus in a can") you just slather this stuff on and rub it in for a few minutes with a soft cloth:



Twenty minutes later you just wipe it down:



As if it didn't look dramatically improved enough already, just rub in some of the Feed-N-Wax ("Apostles in a bottle"):



Holy divine furniture re-finisher, batman! Unbelievable. This stuff is highly recommended... A+!

OK, on to the pedicure. The feet of these 30+ year olds were looking rough:



I figured what I'd do is tear off the crappy particle board feet, and replace them with hardwood. The nearby Lowes had some nice oak 2x2's...that'll do just great. I cut them to size (18" x 14" in case you're wondering), glued and screwed. Here they are in the jig waiting for the glue to dry:



Stay tuned, as the epic M14 refurb continues...and thanks for watching.

[voice of the theater]

Wow--great job Allen! Those 14's are coming along nicely. They're sure to look as great as they sound now......