Audio Altec Lansing MX 5021 Subwoofer Problem

mndar

mndar

Disciple
I have a Altec Lansing MX 5021 and there seems to be a problem with it. This is what happens.

When playing music; the controller, sub-woofer and satellites switch off after 5-10 secs. The LEDs on the controller switch off as well. After this, sometimes the whole thing powers back on itself (in under a sec, so I here a bhoop-bhoop!). Sometimes, I have to power it on myself using the controller.

I experimented quite a bit and the problem turned out to be the woofer. I opened the subwoofer and disconnected the woofer (de-soldered it, didn't cut the wires) and now the speakers don't switch off. Of course, I have no bass but the speaker system works without a hitch.

This site (Jimmy’s Junkyard » Blog Archive » Inside Altec Lansing MX5021: Amplifier Mod) says that a TDA7295 drives the woofer.

From what I described, where do you guys think the problem lies? Should I look for a replacement woofer or could there be a problem with the amplifier?
 
Sorry, need a small clarification
By 'sub', do you mean the 6.5" Woofer or the output coming from the PCB/amplifier?

The 6.5" woofer is disconnected from the amplifier (de-soldered). I checked with multimeter probes connected to the two connectors of the woofer and it shows short (i.e it's very low. I checked with an analog multimeter).

The output from the PCB/amplifier is not short.
 
the woofer coil. Continuity mode wont work. subs are typically 4 ohms, and they will beep. Set it to Ohms range ( you are expecting something 2/4/8 Ohms - it should be written on the back of the sub). if its much less than that, you need a new sub. Else you might have a problem with the output stage
 
greenhorn said:
the woofer coil. Continuity mode wont work. subs are typically 4 ohms, and they will beep. Set it to Ohms range ( you are expecting something 2/4/8 Ohms - it should be written on the back of the sub).
Click to expand...
I need to get a better multimeter to do that.
There are no ratings written on the back of the sub. But this site Altec Lansing MX5021 Review - Introduction & Contents says it's a 4 ohms sub.

The back of the sub has this written on it
"ALTEC LANSING", "H65-002A-04-J", "040412"

if its much less than that, you need a new sub. Else you might have a problem with the output stage
Click to expand...
The sub produces correct output for the 5-10 secs that it functions after being switched on. I'm not sure what to make of that.

I connected a speaker from an old stereo system where the sub was connected and tested the speaker system for some time. It worked alright, no unexpected resets or shutdowns.

The site I mentioned above says that the subwoofer is a
6.5" long throw woofer
50 Watts @ 4 Ohms
Frequency Response: 40 - 150 Hz
Will it be possible to get a similar woofer? Any idea how much that might cost?
If it's not expensive, I'll buy one and see if it solves my problem.
 
You've pretty much licked the problem. It IS the woofer if you're getting sound in a substitute speaker. I would suggest, if that speaker can be used as cannon fodder, that you turn the volume up real high (not max it out, about 40-50% of max volume) , and let it play for an hour or so. That way you can eliminate any thermal issues.

Note that the MX5021 greatly (though not totally) depends on the movement of the subwoofer cone to keep the amplifier cool. The heatsink is a large slab of aluminum with no fins, so if you have low air velocity and high ambient temperatures, the amp could be shutting down. Also, a 4 ohm sub s a pretty heavy load so you may not be maxing out the stress with a small speaker played at low volume.

Try looking for 6" car subwoofers from China in your local market. The woofer isn't anything exotic, but even then most solutions available in India will be suboptimal - at least it will work. Note that you need a sub and not a midwoofer. This could be a little challenging.
 
cranky said:
You've pretty much licked the problem. It IS the woofer if you're getting sound in a substitute speaker. I would suggest, if that speaker can be used as cannon fodder, that you turn the volume up real high (not max it out, about 40-50% of max volume) , and let it play for an hour or so. That way you can eliminate any thermal issues.
Click to expand...
I'll try that and post the result.

Note that the MX5021 greatly (though not totally) depends on the movement of the subwoofer cone to keep the amplifier cool.
Click to expand...
Right now I'm using the system without the woofer, so there is a big hole on the front. Is this enough to keep the amplifier cool?

Try looking for 6" car subwoofers from China in your local market. The woofer isn't anything exotic, but even then most solutions available in India will be suboptimal - at least it will work. Note that you need a sub and not a midwoofer. This could be a little challenging.
Click to expand...
Any idea about the pricing of a 6.5" woofer in Mumbai/India?
 
Standing air can't cool much, hole or no hole. In any case you are testing for thermal trip so a bit of heat is not an issue. The chips are protected against thermal failure internally. You *want* it to get hot.

You should be able to get something from Pandam Audio on Lamington Road. It won't be the same, maybe better, maybe worse. It won't be matched to the cabinet. Expect to pay between 800 and 1200 for one sub.
 
The problem turned out to be something completely different.

The Voltage Regulator Module, L7809CV wasn't making proper contact. It was a little loose, you could move it with your finger. I guess the vibrations form the woofer were causing the VRM to make and break contact with the PCB. Soldering it properly fixed the problem.

Have been using the speaker system for 2 days now, everything seems fine.

greenhorn and cranky; thanks for your inputs.
 
The system has now developed a severe problem.

After powering on, there is a hum and kind of click-clak sound from the sub-woofer for a few seconds. During this period there is noise from the left and right speakers. Sometimes you can hear a faint audio from them.
After a few seconds, the controller switches off by itself and so does all audio.

I opened it up, the L7809CV seems to be getting very hot. Its output voltage if 9V at first and drops to 5V after few seconds. Replacing it has not helped. The new one gets very hot too and the output voltage drops to 5V after few seconds.

What can I try next? Any suggestions?
 
cranky, Thanks for the quick response and more so for the eBay link. Being under $10; it shouldn't attract any customs duty, should it?

I am in Mumbai. Do you think it might be available locally?

Also, once I get it; any idea where I can get it soldered in Mumbai?
My soldering skills aren't good enough replace that chip.
 
No you shouldn't have to pay customs, but small shipments have a nasty habit of disappearing, unless you pay for registered post. Seller should be able to advise you - normally adds a $3 shipping fee.

Any mobile repairing shop should be able to do the job for you in under 5 minutes. Soldering is usually not the issue, but removing the chip is a very painful process.

Note that a blown 7433 is a possible issue, but not certainly the only one. You should disconnect the 7809 and visually trace the output line to see what else is feeding off it - maybe that little dock?

It's been 8 years since I sold mine so have very little memory of it, unfortunately. You'll be on your own from hereon in. Sorry about that.

@Spacescreamer: Yes it was a very good unit in its day, but some components aged and have been discontinued, plus demand was probably too sluggish. In many ways it was ahead of its time, then computer speakers went downhill and became more about looks than sound. I have no interest in that category anymore as nothing worthwhile ever comes up.
 
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cranky said:
Note that a blown 7433 is a possible issue, but not certainly the only one. You should disconnect the 7809 and visually trace the output line to see what else is feeding off it - maybe that little dock?
Click to expand...

Yeah, will look into it before buying a 7433.
Is a diagram of the dock's connector available? Is there a way to power on the speakers without connecting the dock?

I forgot to mention one thing in my earlier post. The first time I faced this problem was while audio was being played from the speakers. There was a hum, a click-clak sound and then the system switched off.
Anticipating that something has blown, I opened up the sub, took out the PCB, the sub-woofer and the transformer, connected everything outside and powered on the system. For some reason it worked.

So, assuming that this was some freak incident, I put everything back in but the problem came back.

Right now I am wondering if I should first go for replacing the 7433 or to give it to some repair shop (possibly on Lamington Road). Of course, first I'll try tracing the 7809's output line as you suggested.
 
All attempts from my side to repair have failed. Gave the system to a local repair shop. He hasn't been able to fix it so far.
So, I was thinking of getting an amplifier since the woofer and speakers are working. Went to a local shop. They have locally made amplifiers but they don't have any datasheets for any of them.

According to http://www.neoseeker.com/Articles/Hardware/Reviews/mx5021/ , these are the specs for MX5021

Satellites:
1x 1" Loaded Horn Tweeter
2x 3" Full Midrange Drivers
20 Watts/ Channel @ 6 ohms
Frequency Response: 150 - 18000 Hz

Subwoofer:
1x 6.5" long throw woofer
50 Watts @ 4 Ohms
Frequency Response: 40 - 150 Hz

I am going to have to get a 4 channel amplifier and bridge 2 channels like this guy has done https://forum.lowyat.net/topic/2182924/all

A quick Google search revealed that not all amplifiers support bridging. Without any datasheets available, it will be a big risk buying a locally made amplifier.
Does anyone know shops in Mumbai, may be on Lamington road where I can get amplifiers. Hopefully with datasheets.
 
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@mndar and anyone else has any idea that from where I can get, speaker cables for mx5021. I have a mx5021 lying unused since last 2 years with me. Assembled it a few days ago, it powered on but no audio output came through. I think the speaker cables have been damaged and needs a replacement.
 
cranky said:
The only thing I am aware of powered by that 7809 is the TDA7433 main control chip, and if it has failed (which it sure sounds like it has) you need to get one:

You're very lucky, this is usually not very easy to find and was impossible to get a few years ago, even on the Bay.
Click to expand...
You were right about the 7433 having failed. The repair guy came to the same conclusion.
Bought the 7433 from the ebay seller you mentioned.
The 7295 and the two voltage regulators (7809 and 7909) had gone bad too, so had to replace those.
Thanks a lot.

Would you recommend putting a heatsink on the 7809 and 7909 ?
 
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