Altec Lansing quad system with woofer was a great system but now only produces very low volume, even at high volume settings. What might I need to fix?

A constant low hum is a sign of a power supply issue. You need to check the capacitors that filter the ripple current and you need to check the diodes that convert ac to dc

If other speakers are working, then 745 is defective or the volume switch is set to mute. Please check that, repair it if required.

Normally these low end system components depend on each other and cannot be interchanged with components from other system. So if one of the components (subwoofer or right control satellite speaker) breaks or gets lost the whole system is unusable...
I was in the same situation. Got a subwoofer from VS4121 without original satellite speakers. I acquired satellite speakers from Altec Lansing 251 which used same connectors but the internal wiring was different and incompatible. Speakers are also glued together so it is impossible to open without damaging it. Obviously there is no service documentation anywhere..
But if you like electronics you can try to make something based on the information below:

Altec Lansing VS4121 connector pinout:
Take the male connector (from right satellite speaker) and number pins in first row from left to right as 1-3, pins in the second row as 4-7, pins in the bottom row as 8-9 and the circle enclosure as 0. Then:
0 - Ground
1 - Speaker Right
2 - Treble (seems just like voltage control but may need to be confirmed: Min=0V; Max=voltage from Volume control Pin 9)
3 - Head Set Right
4 - Bass signal (not sure what the exact original design is as I have not opened the speaker) But seems like they use 50K potentiometer and some sort of RC low pass filter configuration. When potentiometer is set to Min it is shorted with ground. When set to Max it is taking signal from Right and Left Head Set (pins 3 and 5) combined through some larger resistors and low pass filter.
5 - Headset Left
6 - Ground for Headset (grounds 0 and 6 seems to be connected in the subwofer but within the right satellite speaker they seem to be isolated with separate leads to sub)
7 - Power ON/OFF. When OFF or headset jack plugged then 0V. When ON and headset jack unplugged then 5V
8 - 5V Power in
9 - Volume (min=0V; max=5V). It seems they use 50K potentiometer but probably anything >10k should work.

At minimum for standalone subwoofer you will need to implement:
- Power On/Off (switch between pins 8,7)
- Volume control 50k potentiometer (probably anything between 10k and 50k will do) between pin 7 and 0 with variable output to pin 9
- Bass control - join two resistors R (I used 7k) and call this join point A. Attach other ends of the resistors to pins 3 and 5 respectively. Attach 50k potentiometer (I used 10k) between point A and ground 0. Attach Variable pin of the potentiometer to pin 4 (Bass input signal). Attach capacitor C between Pin 4 and Ground pin 0 (I used Electrolytic 1uF with negative end to ground) . Resistor R and capacitor C should make simple passive low pass filter (Google for formula) that should short any frequency higher than 80-120Hz to ground and feed frequencies lower than 80-120Hz to pin 4.

PS: I managed to hack 251 control satellite speaker to somewhat work with VS4121 Subwoofer but that is another subject. All electronic parts were salvaged from old broken TV so total cost was $0 but took several days of tinkering for speakers that I could buy used for $10-20 :)

Hi there,
I think that some wirings inside the main woofer or the tray holding it might have loosed. It could be the speakers were accidentally dropped or anything.
You can bring it to a local store Best Buy, Staples, etc. have it checked by one of them technicians or to see if it can be put back to it's place. I'm no speakers expert but that's the only reason I could think of.
If you are comfortable opening the speaker covers and checking inside the speakers then you can also try to check for some loose parts.
Hope this helps!

I have the same problem I tested the power volume controller with in my friend sub-woofer and same issue, is not the sub-woofer neither the speaker, is the power volume controller definitively.

Your woofer is bad, probraly blew the megnatic coils, and if mention that it's giving you a tearing sound, then the woofer cone is tear somewhere and is not get the reflex to produce the sound.

This is a loose connection between PC and the Sub-woofer. The cables may damaged or it may be a hard ware problem.
Think this is helpful !

The problem I have found is the contact glue between the back panel and circuit board inside the sub woofer. The glue becomes conductive after some time (2 years..!!) and needs to be scraped away from across tracks, you can tell the problem areas as the glue is pale yellow but goes brown and conductive over time. Good luck.
Regards
aj

if the volume controls of the speaker is set to 100% together with the volume control on your computer then still get a low volume,try some other device where you can use your speaker like mp4,if still got same problem,bring it to a service center there might have a problem with the amplifier.

hi there are fast checkings for you: Quick Connect Card http://www.alteclansing.com/downloads/641QCC.pdf manual: http://www.alteclansing.com/downloads/641_MAN.pdf is the system working fine exept the sub? my guess: the headphone jack works fine then the speakears are too if soo then the sub amp is bad or some connection is torn try opening the sub and see if everything is hooked right and if soo then the amp is the problem every electronics shop can fix this good luck