How to ID L-Pads and crossover....? [Archive] - Audio & Video Forums

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piece-it pete
03-24-2004, 09:47 AM
Hello everyone!

I've got a neat old speaker that needs attention, a University TMS-2 "stereo-in-one-cabinet" 1950s' number, rearfiring 12" dual VC woofer handling 150 hz & down, two 8" mids and horn tweets firing out of each side. I love this, it really rocks, which as I age includes the ability to do justice to jazz & orchestral pieces :) .

The problem I'm having. At lower levels on side is quieter than the other. I've tested all the drivers and they appear to be fine.

Each channel has L-pads for the mids & tweets, and are my first suspect. I have tried cleaning them but the "second layer" of the pad has no holes in it!!

How do I find out what values they have, I think I need resistance in ohms? so I can replace them.

Also, the crossover components are waxy paper cylinders that I have heard go bad, and are possibly a upgrade target anyway.

The components:

To tweeter: 1" dia. x 2" long cylinder, marked 85_LPT, MFD_V.D.C., 2._50, USA _[pyramid symbol]472

To mid: 1" dia. x 4-1/16" long cylinder, marked MallorY, 150_MFD_V.N.P., 44A155B, 235913X.

To woof: 1-5/8" h x 2" w thing, looks to me like an inexpensive transformer, marked 72B114B, MEC.

I'm thinking these are great candidates for a complete electrical renovation. I've already drawn out the circuit (if that's what you'all call it!).

Any advice/suggestions are appreciated, I'm a newbie to DIY crossover work!!

Pete

pelly3s
03-24-2004, 04:32 PM
well L-pads are rated by the wattage they handle. now the rest of the parts you can measure or get part numbers and try to crossreference them

jeskibuff
03-26-2004, 03:48 AM
How do I find out what values they have, I think I need resistance in ohms? so I can replace them.Really the best way is with an ohmmeter. You can get one fairly cheap nowadays. If the L-pads have spotty contacts, you still should find enough positions where you get a solid reading to determine its range, especially since you have two of them (theoretically identical) to measure.


To tweeter: 1" dia. x 2" long cylinder, marked 85_LPT, MFD_V.D.C., 2._50, USA _[pyramid symbol]472Most likely an 85 microfarad capacitor. Capacitors are used to pass higher frequency signals and block out low frequency signals when wired in series with a tweeter.


To mid: 1" dia. x 4-1/16" long cylinder, marked MallorY, 150_MFD_V.N.P., 44A155B, 235913X.Most likely a 150 microfarad capacitor. This one should allow lower frequency signals to pass to the midrange, but filter out the bass signals.


To woof: 1-5/8" h x 2" w thing, looks to me like an inexpensive transformer, marked 72B114B, MEC.Most likely an inductor (also called a coil). Works the opposite of a capacitor...allowing low frequencies to pass, but not the high frequencies.

I think your best bet is to take a speaker (or one crossover) to an electronics repair technician or supply house (Radio Shack being a last resort, if that's all you've got). They should be the best resource to help you determine what replacement parts to use.

Another thing you should be able to try is to just "jump" the L-pads to see if this makes the speaker volumes equal. The range of each L-pad should vary from a short circuit (zero ohms) to a higher value (which you are trying to determine with an ohmmeter). Jumping each L-pad by connecting a wire across them would isolate your balance problem. If it "balances" the volume levels, the L-pads were the problem. If it doesn't, the problem is with some other component.

skeptic
03-26-2004, 08:02 AM
"The problem I'm having. At lower levels on side is quieter than the other. I've tested all the drivers and they appear to be fine."

Unless I have misread this, it seems to me that if you haven't touched the L pads on the speakers, then this suggests a problem with the volume control in your preamp or receiver, not the speaker.

Try turning the balance control to the side that seems quieter and see if adjusting the volume control doesn't result in some cutting out. If the two channels of the volume control on the preamp don't track the same, try cleaing them or if necessary replacing it.

Try this equipment with another speaker and if it still happens, you have your answer.

L pads are invariably rated for impedence and wattage. 8 ohms and 25 or 50 watts are typical although not necessaryily universal. Replacing them is usually a lot easier than trying to clean them. Sometimes after many years they have to be taken apart and cleaned with de-oxit. They usually aren't that expensive. If you disconnect one, try an ohm-meter from let one to leg three.

Good luck.

piece-it pete
03-26-2004, 09:08 AM
I'll jump the L-pads hopefully this weekend. I've tested the receiver & it's fine.

I know the pads are dirty 'cause they crackle. I've tried de-oxitizing them but can't get at the "second layer" of the thing, and do not yet have the confidence to take them apart. If I replace them maybe I'll try then :) .

So, my read so far is: Check the L-pad resistance in ohms with an ohmmeter. Then buy similar L-pads with what ever wattage rating I choose.

Then, if that doesn't do it perhaps it's a crossover component. Jeskibuff, thanks for the info - I assume (uh-oh :) ) that if I simply replace them with like values I should be OK. The problem with taking this thing to a supply house is that it (there's only one) measures approx. 30 x 25 x 13 + legs, and the components are simply attached to the inside around the woofer on the back panel, held on by I think 1,000,000 screws :) . I could probably disconnect the whole back panel (involves cutting most wires) & take it in, but then again if I do this I'll want "at least" VG quality parts that usually are only affordable to me online.

I'm going to be renovating the cabinet later this summer anyway, since it'll all be ripped apart I thought I'd go for the gusto! Including the entire pathway from binding posts up to the drivers, learning on the way, & hopefully not screwing anything up too bad :D .

I've still got to decide: keep the original exterior look (early American with the original gold & brown weave grill cloth, which I might be able to clean but will more likely have to replace)(the speaker really looks like a small console stereo), or going for a more modern look which involves new fabric, new hardware, new legs, and perhaps a new top. I should have some pics in the next week or two & will post one in the speaker forum.

Gosh, I love this stuff!! Thanks again!

Pete