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Dave A

MTM question

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OK today I take this pair of pristine MTM's to the shop to check for ohms and frequency sweep before listing them and I can't get an ohm reading and the frequency generator barely makes a noise when hooked up. So I pull the back cover off and try the frequency sweep on just one disconnected tweeter and it will run you out of the room. Both behave this way so I am figuring there is a crossover routing problem and I would like to get input on how to trouble shoot this or even better yet a schematic.

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Let me suggest that you measure the resistance at the input terminals of the woofer(s).  You should disconnect at least one wire feeding the woofer(s).  Of course a good woofer will measure about 3.5 to 4 ohms.  A bad one will be "infinite" or whatever your meter shows when the probes are not touched together (on the ohms scale).  It is unlikely you will get any reading in between.

 

Yeah, pictures! Let us know.

 

WMcD

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@Dave A You should be able to do continuity tests on all the tweeter voice coils and on the 2 inductors. And a value test on the resistors. Oh yea and you did check the fuse right?:)

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My assumption since a random test of a tweeter checked out fine that voice coils are not the issue. I did get a very faint output from the whole unit so something is getting through and since that is the case I did not check the fuse but perhaps I should anyway. I will dig deeper into it as time permits.

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It has a crossover and auto transformer so it is also level controlled.  Your frequency generator probably can't drive all that to any decent level.  I would try connecting an amp to it.  It probably expects a few watts or more.

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Looks like you have the piezo version which will have a high impedance by design and will not measure like a K77 for example.

 

Make sure the two series wire wound resistors measure good.

 

miketn

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1 hour ago, mark1101 said:

It has a crossover and auto transformer so it is also level controlled.  Your frequency generator probably can't drive all that to any decent level.  I would try connecting an amp to it.  It probably expects a few watts or more.

I wondered about that since the max input is 30w and I doubt the sweep generator gets that high. I will try a hookup tomorrow and see.

 

4 minutes ago, mikebse2a3 said:

Looks like you have the piezo version which will have a high impedance by design and will not measure like a K77 for example.

 

Make sure the two series wire wound resistors measure good.

 

miketn

I don't think this has been used and yes ohms was way high which threw me. These are not K-77's for sure and I bet they are much louder.

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MCM-1900.pdf

 

I should clarify it’s my understanding the piezo tweeter impedance can measure high at low frequency and low at higher frequency like a capacitor for example.

 

If you look at the spec sheet it shows 4ohm with 3ohm minimum but I believe that to be in their specified frequency range.

 

Not having a schematic its hard for me to be certain but looks like the + signal terminal goes to the (2) series resistors to the fuse and then a series capacitor to the auto transformer (?) which means if you try reading resistance with a meter at the input terminal it will not read 4 ohm but more like an open circuit due to the capacitor.

 

 

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Piezo tweets don't age well, if they are original just replace them.

 

Motorola KSN1016A ?

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47 minutes ago, mikebse2a3 said:

which means if you try reading resistance with a meter at the input terminal it will not read 4 ohm but more like an open circuit due to the capacitor.

Which is precisely what happened.

 

10 minutes ago, jason str said:

Piezo tweets don't age well, if they are original just replace them.

I would figure they would if not used. These are basically old new stock and I can't imagine they have a spoil by date.

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Re just the woofer crossover.  Every woofer crossover I've seen has a d.c. path to ground through the woofer.  I.e. the path from the hot (red) input lead goes though a resistor or two and then to the hot lead (red) input of the woofer, and then the other side of the woofer goes back to the input terminal ground (black) as ground.  There may be one or more capacitors across the woofer and these will cause a momentary lower ohms reading on the meter when the probes are first hooked up.

 

This means that the checking ohms across the input to the box will give you the series (adding) resistance of the crossover inductors (usually less than 1 ohm) and the woofer voice coil.  If this shows an open (usually infinite resistance) then you very likely have an open voice coil(s) in the woofer(s).  Both woofers would have to be burned out for the unit to make no bass noise.

 

OTOH, the midrange and tweeter crossovers do not have a dc path to ground.  Their inputs are arranged in parallel with the input terminals and bass crossover input.  But because we are measuring DC resistance they do not affect the reading.

 

The ohm meter puts a small amount of current through the circuit which comes from a battery in the meter.  I think of this as a zero Hertz signal.  The woofer crossover will pass a zero Hertz signal.

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Well today Mr Brain hooks these up and runs a tone from Audacity. So I start off with 19khz and think what the heck hardly anything. Then I go for 15khz and much more and by 14khz gobs of sound. It's funny how we can forget common sense while testing. These are certainly odd birds compared to regular tweeters and my first experience with Piezos.

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Methinks maybe no output at the higher frequencies cuz you can't hear that high anymore? Any dogs around to confirm?:)

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3 hours ago, babadono said:

Methinks maybe no output at the higher frequencies cuz you can't hear that high anymore? Any dogs around to confirm?:)

Pretty much what I said.

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1 hour ago, Dave A said:

Pretty much what I said.

I meant no offense. I am an old fart, I know I'm lucky if I can hear 15kHz.

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On 9/3/2019 at 1:12 PM, Dave A said:

the frequency generator barely makes a noise when hooked up

Been there:

Most of the time I have had something for tunes in my office. Picture the setting of me and a fellow Engineer / Addict in my office, a Luxman R1050 and Klipsch Heresy doing a HF test. The first time through, the wattage lights were lighting, but i didn't hear a thing, so I turned it WAY up and played that Hz again. I thought that he was messing with me  by holding his ears.....until several others scrambled to my office door asking what that noise was. 😑

 

I'm enjoying what I can, while I can. 😎

 

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