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Old 06-25-2012, 07:52 AM
 
8,402 posts, read 20,663,341 times
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I have an M&K VX-7 powered subwoofer from the 90's. The fuse connector broke off 10+ years ago, so it's been sitting unused, until my recent purge of all redundant or unnecessary possessions. I bypassed the fuseholder and connected the sub. It plays, but:

1. It doesn't thump, it mostly hums in time to the music. No real percussion.

2. Not much output even with the volume at full.

3. Tried low and high level inputs, same issues.

4. Cleaned the VC and x-over pots with tuner spray.

5. The driver isn't locked up and I don't hear or feel any voice coil scrubbing as if it were blown.

6. Everything inside looks new. No blown or leaky caps, no corrosion, etc.

7. It's not ground loop hum as was suggested in a thread I saw.

I was in the A/V business for over 25 years, have multiple other subs, and know all about the connection and use of any A/V gear. This is acting as if the pots are dirty or the sub is shorted, but neither seems to be the case, except that tapping on the VC sometimes results in the volume jumping.

I'd like to sell this, but don't want to put a bunch of money into something I can get maybe $100 out of. Thoughts?

Last edited by vmaxnc; 06-25-2012 at 08:06 AM..
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Old 06-25-2012, 11:48 AM
 
Location: Somewhere in northern Alabama
18,540 posts, read 55,453,855 times
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"except that tapping on the VC sometimes results in the volume jumping."

Have you resoldered the connections to the VC? Sometimes the lead wires can be broken without showing the break. If the VC is on a PC board, look at the traces on it. If it is a multilevel board, you may be outta luck, but it is worth a try.
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Old 06-25-2012, 12:05 PM
 
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Quote:
Originally Posted by harry chickpea View Post
"except that tapping on the VC sometimes results in the volume jumping."

Have you resoldered the connections to the VC? Sometimes the lead wires can be broken without showing the break. If the VC is on a PC board, look at the traces on it. If it is a multilevel board, you may be outta luck, but it is worth a try.
The VC is on a single level PC board, but I didn't look closely at it. It was 95 degrees and humid the day I poked around in my garage work area with the sub, and I didn't have much patience for it after being out there all day. I may bring it in and try again. Is there anything else you suggest? So far you haven't exceeded my knowledge, although I've never soldered on a PC board.
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Old 06-25-2012, 03:50 PM
 
Location: Somewhere in northern Alabama
18,540 posts, read 55,453,855 times
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LOL! If you worked in A/V for 25 years, I'd be very surprised if I "exceed" your knowledge.

I took you post to mean you were having a bad day, and maybe a brain fart or two like I get from time to time.

If the VC tapping creates major changes, it seems logical to check that out thoroughly. Could there be a bit of dust or bug part on the wiper? Is the wiper spring "sprung?" Is there a convenient way of replacing or at least bridging out the component? This is where the old-school wire-wound pots were impressively reliable.

Soldering - Damp wadded tissues around the area, small tip, low wattage, magnifying loupe if you have one, light touch.
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Old 06-25-2012, 05:08 PM
 
8,402 posts, read 20,663,341 times
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Quote:
Originally Posted by harry chickpea View Post
LOL! If you worked in A/V for 25 years, I'd be very surprised if I "exceed" your knowledge.

I took you post to mean you were having a bad day, and maybe a brain fart or two like I get from time to time.

If the VC tapping creates major changes, it seems logical to check that out thoroughly. Could there be a bit of dust or bug part on the wiper? Is the wiper spring "sprung?" Is there a convenient way of replacing or at least bridging out the component? This is where the old-school wire-wound pots were impressively reliable.

Soldering - Damp wadded tissues around the area, small tip, low wattage, magnifying loupe if you have one, light touch.
I spent very little of that time looking inside the boxes! I did car install, then design/sales/project management of home audio/video systems.

I'd been in the garage for several very hot humid hours, sorting a bunch of cables and other a/v stuff for sale. The sub came into play because I wanted to add it to the garage sound system. I gave it a few minutes of time but gave up on it.

I shot a good bit of tuner cleaner into a hole on the side of the VC and X-over pots. It's possible there's junk in there but not likely. I'm not positive what you mean by sprung (pulled away from the wire-winding?), but the pot feels like I remember, as far as I can tell. I could bypass it, but where I'd use the sub I have no sub level control, so that wouldn't be the best solution. Maybe I can replace the pot. Nothing to lose at this point.

Interestingly enough, my other M&K sub, a V-100, may have the same issue. If I can't fix this, I'll be dragging that one out, if not to use in the garage, then for sale.
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Old 06-25-2012, 10:39 PM
 
Location: Somewhere in northern Alabama
18,540 posts, read 55,453,855 times
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"I'm not positive what you mean by sprung (pulled away from the wire-winding?), but the pot feels like I remember, as far as I can tell. I could bypass it, but where I'd use the sub I have no sub level control, so that wouldn't be the best solution. Maybe I can replace the pot. Nothing to lose at this point."

Pots have to hold contact between a wiper arm and the carbon or whatever is being used as the variable resistance. Your cleaner will usually do a good job of cleaning grease and muck off both contact areas (especially if you twist or slide the control a few times as you are cleaning.) The pressure of the wiper arm on the other media usually comes from a spring or possibly a strip of metal that is placed into position in a "spring" state. Metal fatigues over time, and the pressure diminishes. You may or may not "feel" the difference.

Replacing components is a fun exercise in itself. Highly recommended. Do yourself a favor though, and take a junk circuit board unsolder and re-solder a few components before doing this. This may sound odd, but TRY to mess up on those practice runs so that you can see what happens. It is a fast way of learning (even if it is kinda stinky).

Makers have a good motto - if you can't get into what you own and fool around with it, you don't "own" it.
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Old 06-26-2012, 08:59 AM
 
Location: Central Texas
13,720 posts, read 25,904,706 times
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Nothing in the description of the problem fits a shorted driver. What exactly would that mean anyway? When the voice coil of a driver fails, it is one of two things - the insulation of the wire in the coil winding fails, which causes a short - which almost instantly will cause the amplifier to cut out (if it has short circuit protection), blow a fuse, or fail; second - the coil fails "open" which produces no sound at all. If the flexible leads from the cone/coil fail you could hear intermittent sound or "scratching."

If the sub just produces audible hum then you know the driver is working. The amp has failed in some way. It could be the power supply.
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Old 06-26-2012, 09:08 AM
 
8,402 posts, read 20,663,341 times
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Quote:
Originally Posted by hoffdano View Post
Nothing in the description of the problem fits a shorted driver. What exactly would that mean anyway? When the voice coil of a driver fails, it is one of two things - the insulation of the wire in the coil winding fails, which causes a short - which almost instantly will cause the amplifier to cut out (if it has short circuit protection), blow a fuse, or fail; second - the coil fails "open" which produces no sound at all. If the flexible leads from the cone/coil fail you could hear intermittent sound or "scratching."

If the sub just produces audible hum then you know the driver is working. The amp has failed in some way. It could be the power supply.
It certainly does. When a driver is shorted (or "blown") it can play, and often will, albeit erratically. Turning the volume up will frequently cause a non playing driver to suddenly come on, because it will have forced the driver past the point where the physical short can keep the driver form moving. I've seen it hundreds of times, more often in car audio. That is one of the characteristics of my problem. I didn't say it was shorted. I said it is acting as if it is shorted. As I said I don't feel or hear any scratching. On "blown" drivers it can also be felt between the voice coil and the magnet.
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Old 06-26-2012, 11:02 AM
 
Location: Somewhere in northern Alabama
18,540 posts, read 55,453,855 times
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I think what you just described is not a "short" but an intermittent open circuit. If you have a multimeter, just check for resistance across the speaker coil. (Disconnect one lead from the speaker to the circuitry, then measure the resistance across the coil.) If there is no change in the meter, or a continuity test shows no continuity, then your speaker coil has an open circuit.
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Old 06-26-2012, 03:11 PM
 
Location: Central Texas
13,720 posts, read 25,904,706 times
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Electrically a short is a bridge between two contacts that should not have one. Usually sparks are generated.

A loudspeaker is a pathetically simple device. A voice coil is just a coil of wire that generates an electromagnetic force when electricity is passed through it.

A "blown" speaker is a speaker with an open voice coil. That means no electricity will flow through the speaker, producing no sound. It could occur because the actual coil itself failed (a strand of wire is broken) or the tinsel leads are broken.

A "shorted" speaker would be one that behaved as if you put a jumper across the + and - terminals. That causes the amplifier to see zero ohms instead of 4 (or something higher) and will usually trip protective circuitry in the amp. No sound will be produced.
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