See Second Post (I messed up this one)

 

Hello!

(Sorry I submitted my post by accident before completing it and ran past the time limit before I could edit it so had to send the updated post here. Thank you for understanding)

Need some help with repairing a Altec Lansing 2100 2.1 Speakers System:
Problem:
- Right channel output is not working (actually its very quiet)
- Swapping left and right speaker indicates that problem is not with speakers or cables but with the Amplifier unit which integral with the Sub Woofer Unit

Pictures of Speaker System:
Altec Lansing 2100 2.1 Speakers: (https://www.amazon.com/Altec-Lansing-2100-Computer-3-Speaker/dp/B00005S8KY)

PCB pictures inside the Sub Woofer unit:


U1, U5, U10: 4558C --- "ST4558C - en.CD00000478.pdf"
U2: PT2256 (compatible with TC9235P) --- "pt2256.pdf", "TC9235P - datasheet.pdf"
U4: TC4013BP --- "TC4013BF_datasheet_en_20140301.pdf"
U7: LM324N --- "LM324N - en.CD00000457.pdf"
U11: TDA7377 --- "TDA7377.pdf"

J1: Stereo Input
J4: Sub Woofer Speaker internal PCB connector
J7: Right channel output
J8: Left channel output


PCB layout (I think):
Audio In Jack (Stereo in) --> OpAmp U1 --> Volume Control IC (PT2256) --> OpAmp U5 --> OpAmp U10 --> Amp (TDA7377)

Amp (TDA7377) --> L/R Output
Amp (TDA7377) --> ? Sub-woofer Circuitry ?


Troubleshooting and things tried so far:
1. If only the right channel of audio is allowed to enter the Stereo input jack (J1) by changing the Left-Right balance on input source (mp3 player / computer) and the output pins (pins 1 and 7) are shorted on the 4558C OpAmps (U1 / U5 / U10) or the Volume control IC (pins 2 and 15 on PT2256) it can be seen that right channel audio reaches the left speaker correctly in all cases except when output pins are shorted on OpAmp U10. It would seem that the problem seems to be around the OpAmp U10.



2. If pin 3 is momentarily grounded the right speaker works momentarily at correct volume (for about 10 seconds)

3. Removed OpAmps U1, U5 & U10 and replaced with a DIP8 socket. After replacing all 3 OpAmps with NE5532 (compatible replacement) right speaker still doesn't sound.

4. On removing U10 then U5 and then U1 the audio becomes quieter and quieter and finally goes off but all the time the right channel doesn't work.

5. Tested most of electrolytic capacitors in-circuit using an ESR meter and they seem to be OK.


Please can someone help with things I can test or replace to fix this Speaker system. Thank you so much!!!

 

Are you sure that the problem is not in your "Audio source" or settings. If you supply the audio source from a computer, try changing it with a different cable and your phone instead.

If the problem really is in the sub-buffer itself, try to see if there are any setting you can change.

I seriously do not believe that this problem is from the buffer(you can check with a multimer what comes in right after the jacks, the result should be the same for all jacks).

 

Thanks for your reply! I am pretty sure the problem is not from the input side because I have used at least 4 devices with different input cables with the same result. So unless the Jack is the problem I don't believe its an input issue. But let me check with a multimeter like you said close to the input side - you want me to check DC voltages between signal and ground on each channel with a constant audio source?

 

Yes, if possible with the same source(switch the input from the first jack to the second one, then try to use 2 inputs at the same time even if they are not the same and switch them again).

 

The TDA7377 is the output amplifier with L R and subwoofer,. The lm324 is probably the tone control preamp. You could swap or short the inputs to the tda7377 to prove it.

 

Yes, if possible with the same source(switch the input from the first jack to the second one, then try to use 2 inputs at the same time even if they are not the same and switch them again).

Sorry I couldn't understand. How do I use "2 inputs at the same time"? There is only one 3.5mm stereo receptacle on the Speaker unit. I have multiple stereo cables though and multiple devices that can produce audio (mp3 player, mobile phone, laptop, etc.). I have tried using different cable and devices in a number of combinations with the same result.

I checked the DC voltage at the jack stereo jack (male) of the cable coming from my audio source and then at the terminals on the pcb after plugging the jack into the 3.5mm receptacle on the Speaker unit. Its really small (in millivolts) but about the same for both channels at the jack and at the terminals. Is this the correct way to check?

 

The TDA7377 is the output amplifier with L R and subwoofer,. The lm324 is probably the tone control preamp. You could swap or short the inputs to the tda7377 to prove it.

The TDA7377 has 4 inputs If I short the 2 inputs for the L R speakers I get audio in both speakers (L & R) i.e. I get Left channel audio in both speakers. So the problem seems to be before this amplification stage? Correct?

Audio from the Sub woofer comes out just fine so I didn't look at that input. Yeah I figured the LM324 was only for the Sub Woofer.

 

Are you sure that the problem is not in your "Audio source" or settings. If you supply the audio source from a computer, try changing it with a different cable and your phone instead.

In my post under "Troubleshooting and things tried so far:" please see point 1:

1. If only the right channel of audio is allowed to enter the Stereo input jack (J1) by changing the Left-Right balance on input source (mp3 player / computer) and the output pins (pins 1 and 7) are shorted on the 4558C OpAmps (U1 / U5 / U10) or the Volume control IC (pins 2 and 15 on PT2256) it can be seen that right channel audio reaches the left speaker correctly in all cases except when output pins are shorted on OpAmp U10. It would seem that the problem seems to be around the OpAmp U10.

Basically each OpAmp handles 2 channels of sound. At each stage I am shorting the channels to see if the Right channel (the channel which I am having problems with) reaches the left speaker. This happens in all cases except at OpAmp U10.

Would this be ample evidence that the cause for the problem is not an input issue? Or am I not troubleshooting this correctly?

 

Looking like the preamp stage then U10, no right chan.

Can you swap chips?

 

Looking like the preamp stage then U10, no right chan.

Can you swap chips?

Thats what I thought and tried to swap the chips but alas. Infact I put DIP8 sockets in the place of the opamps so I can try different chips. Didn't fix the problem. In my post under "Troubleshooting and things tried so far:" please see point 3:

3. Removed OpAmps U1, U5 & U10 and replaced with a DIP8 socket. After replacing all 3 OpAmps with NE5532 (compatible replacement) right speaker still doesn't sound.

 

U10 might be the channel combiner for the woofer.

"2. If pin 3 is momentarily grounded the right speaker works momentarily at correct volume (for about 10 seconds)"
Pin 3 of U10?

This makes me suspicious that an electrolytic capacitor is involved. It may be in a bias network or a coupling capacitor. The capacitor itself may be OK and something else is causing the problem, but there just isn't much chance anything not somehow related to charge on a fairly large capacitor is likely to cause ten seconds of of restored operation. Note that the small electrolytic cap closest to the big filter cap is (I think) in the output path from of the amp in U10 that has pin 3 as an input.

[EDIT]: I think pin 1 of U10 is the output to the power amp for the woofer. It looks like it probably goes to pins 11 & 12 of the power amp. If this is true, it would suggest shorting pin 3 is changing something between pin 3 and the circuitry closer to the inputs.

 

See if you can measure the DC voltages at the chips, that might give a clue..

Ideally need the service manual.. .

 

Ok I think I have narrowed it down to this capacitor between U5 and U10:


There are 4 of them but only this one checks out as open with my esr meter. Also when I give only left channel input signal and I short the equivalent points on left and right channel on the pcb before and after this point I get a output in the right channel only after this capacitor.

It has the markings .068J 100HT. Is this a Film Capacitor? I didn't even know these could go bad. Can someone help me figure out a good replacement? Thank you so much for all the help so far!

 

68nF capacitor.

 

It is a 0.068 miicrofarad (68 nanofarad) 5% tolerance (denoted by the J) 100 VDC. I don't know what the HT might mean. It is almost certainly a metalized polyester film type, often referred to as "MKT". That particular physical style is not very popular in industrial electronics but was/is very common in consumer equipment. Any similar capacitor you can fit physically should be OK. A voltage rating of 25 is probably entirely adequate. 63 VDC is a common rating in industrial components.

Caps like that are not prone to failure in signal applications.

If you have any sort of capacitor somewhere in the range of 30 to 100 or even 220 nanofards (0.03 - 0.22 µF) on hand, you could use it as a test replacement. The sound might not be "right" but it would verify your hypothesis.

 

Thank you!

I removed the 4 capacitors and tested them. They measure around 69.5nF so I guess 68nF it is! The bad one measured around 64.5nF hot (just after de-soldering)...after cooling down it didn't measure anything. Also my ESR meter gave me a resistance of about 22 ohms on all and wouldn't measure the bad one.

@ebp I am going to try testing my hypothesis like you said. I think I have some 630V 0.1uF PET capacitors I could use. Will get back with results. Thanks for all the information!

 

Ok that seemed to have FIXED IT! Used 0.1uf PET capacitors in place of the 0.068uf and to my ears atleast it sounds perfect!

But I seem to have found another problem : This seems unrelated to the first problem in my mind. The volume is controlled using a push button on the remote as can be seen in the pictures in my starting post. When raising the volume from the lowest volume by click the button continuously the left channel increases gradually but the right channel stays silent until roughly the "midway" point. At the "midway" point the right channel suddenly matches the left speaker and then they both gradually increase together correctly i.e. matching each other in volume as they increase. This happens at exactly the same number of button clicks from the lowest volume. If I reduce the volume one click after that "midway" point the right channel volume suddenly drops out. I couldn't notice this before because the right channel was always too quiet even when setting it at max volume with the remote.

I believe the two chips involved with volume control are the PT2256 and the TC4013BP (both data sheets are attached with my starting post). I already replaced the PT2256 chip with the TC9235P in an effort to solve the original problem so I don't think there should be any problem with that.

Thank you so much for helping me solve the original problem! But do you guys have any ideas about this new problem? I don't know where to even begin troubleshooting this one.

 

The only thing I can see that might have something to do with it are the "loudness tap" pins, 4 and 11. Pin 4 is tied to AGND through surface mount resistor R11. Any chance that resistor might have been damaged or desoldered at one end in changing the IC? It looks like R10 does the same for pin 11, but it's through-hole. Very odd mix of SM the TH parts!

 

Very odd mix of SM the TH parts!

Indeed! Strange mix and this is not the only place it has been done on the PCB.

Any chance that resistor might have been damaged or desoldered at one end in changing the IC? It looks like R10 does the same for pin 11, but it's through-hole.

Nice catch!!! I tested (in-circuit) between loudness tap (LT1 & LT2) and AGND. On the TH side it was 3.18 K ohms on the SM side it was open circuit!!! The SM resister doesn't seem to be damaged visually and seems to be soldered correctly but clearly it seems to have a problem. Unless the two sides are not symmetrical. Marking on SMD resister is 392 (3.9 K ohms I think) and TH side is 4 band orange,white,red,gold (3.9 K ohms). Your recommendation? Replace SMD? I might have a close TH resistor. How important is the value?

The only thing I can see that might have something to do with it are the "loudness tap" pins, 4 and 11.

You mean pins 4 and 13? I really don't understand this loudness tap. What exactly is it for?

Thank you so much @ebp !!!