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Help diagnose Sonic Gear 2.1 woofer no bass
Help diagnose Sonic Gear 2.1 woofer no bass
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Old 23rd November 2020, 03:50 AM   #11
Mikey5791 is offline Mikey5791  Malaysia
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Join Date: Nov 2020
Location: Kota Kinabalu, Sabah
Hi,

You mentioned "Follow the wiper wire to the pcb disconnect pot & connect the walkman there." Sorry i am not sure how to connect the walkman input on the pcb . That is why i just disconnect the bass pot, replace with a 100k pot(B104 type as per img 084810) as i don't have stock of 50k pot.
Connected the walkman jack to audio input as per my set up on img 104953.
There is normal audio from the two satellite speakers but nothing from bass speaker. Volume knob and treble knob is fully adjustable but there is no respond from adjusting the 100k bass pot. I am getting the same result as the original 50k pot.
Is this meaning the original 50k bass pot is still good? Off circuit i tested the resistance reads 48.8k ohm on 200k ohm scale.

Should i now proceed to put back the original 50k bass pot and install 2 pieces 8 pin socket for the 4558 op amp? So i can swap test the op amp anytime. Btw, i have not replaced any of the suspect open resistor. Also i remembered 4 of the resistors was desoldered on one leg and i haven't soldered it back to pcb as they are open and resistance are almost none. Will this affect or interrupt the bass output?
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File Type: jpg IMG_20201123_104953.jpg (251.5 KB, 7 views)
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Old 23rd November 2020, 11:36 PM   #12
MAAC0 is offline MAAC0  Portugal
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Join Date: May 2010
Quote:
Sorry i am not sure how to connect the walkman input on the pcb
It would be connecting the walkman output to the PCB pin where the blue wire (wiper) connects. If this works, it means Your problem is before the pot or the pot itself is bad.

You have to find out if the fault is before or after the pot. In the pre-amp / filter or in the Sub amp.

You could here also connect the wiper to another amp to find out if signal is reaching the pot.

Quote:
Is this meaning the original 50k bass pot is still good?
Probably Yes, since the new one also doesn't work.

Quote:
and i haven't soldered it back to pcb as they are open and resistance are almost none.
Open / broken resistors measure infinite. If they measure low resistance, they are shorted / burned, but they appear Ok on the pictures. Follow the color codes on them and replace if they don't correspond. Lower than a few Ohms doesn't make sense. Of course if they aren't soldered the circuit isn't complete & things don't work as they should. Perhaps You've found the fault already, but if they are low resistance that also means higher current flowing thru them & the 4558's could be been stressed. That's why I suggested changing the rightmost (Sub pot connected) op-amp. If it is also a 4558, Can't see by the pictures, any dual op-amp will do like NE5532, TL072 etc for testing. Replace resistors first.

There are meters which can measure resistors in place since the voltage is low enough to not activate semiconductor junction Voltage (>0.6V) My Fluke does that, but most don't. That's why You should lift one leg in order not to measure components in parallel with them which gives You incorrect readings.

It is hard to follow by pictures, I see solder pads on the underside , but don't see any wires or components at those supposed positions at the top PCB for example. Since there are few components, You could try to make a schematic and then follow it.

Last edited by MAAC0; 23rd November 2020 at 11:48 PM.
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Old 24th November 2020, 02:20 AM   #13
Mikey5791 is offline Mikey5791  Malaysia
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Join Date: Nov 2020
Location: Kota Kinabalu, Sabah
Connected the walkman output to pcb input red and black wire on vr2 as per my setup img 093117. I removed the vr2 shielded brown wire which connects to the 100k bass pot. I made sure i didn't connect or shorted to white wiper wire. Switched on mains but hear nothing, no sound at all. Is my testing setup done correctly?

But if i insert the 3.5mm jack into audio input, then there is audio from the 2 satellite speakers. Seems like the problem is somewhere after the bass pot.
Fyi, i have not replaced any of the resistors yet as the only electrical shop selling parts is not open due to the current covid pandemic. Also, i need to make sure which parts need replacement and then once the shop reopens, i will do the complete purchase.

Sorry to inform that i have no knowledge of drawing a schematic by tracing all the tracks on pcb.
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Old 24th November 2020, 03:01 AM   #14
Mikey5791 is offline Mikey5791  Malaysia
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Join Date: Nov 2020
Location: Kota Kinabalu, Sabah
Fyi the 4 resistors that i had desoldered one leg off board are as follows:

R16 green,blue,brown,gold=560 ohm. Dmm reads 544 ohm
R8 orange,orange,gold,gold=3.3 ohm Dmm reads 104.4 ohm
R6 green,blue,brown,gold=560 ohm. Dmm reads 542 ohm
R25 blue,grey,brown,gold=680 ohm. Dmm reads 686 ohm

Funny to note that even though these 4 resistors were one leg off the pcb, the 2 satellite speakers are still working normal with adjustable volume and treble pot. Except the bass pot still no sound.
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Old 24th November 2020, 11:27 PM   #15
MAAC0 is offline MAAC0  Portugal
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Join Date: May 2010
Quote:
R8 orange,orange,gold,gold=3.3 ohm Dmm reads 104.4 ohm
Are sure about the colors ? 3.3 Ohm is a strange value. 100 Ohms is 100* higher. That could explain many things. Try looking with a magnifying glass or another light source. Sometimes Colors appear different. If it is really 3.3, short it to test.

Quote:
i have no knowledge of drawing a schematic by tracing all the tracks on pcb.
I would ask a friend for help. As I told You it is a bit difficult this way.

You start with a pencil at the input for example and then draw the components following the tracks. At the end You have the schematic.

I can't determine by the pictures if the rightmost op-amp is indeed a 4558. Could be a single op-amp like a TL071. The pot seems to connect to pin2 but the supply pins are different between the two. Sorry I can't help more.
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Old 25th November 2020, 07:58 AM   #16
Mikey5791 is offline Mikey5791  Malaysia
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Join Date: Nov 2020
Location: Kota Kinabalu, Sabah
MAACO,

Thank you for helping me up to this stage, although is is sad to hear you can' t help more.
It has been an enriching insight and learning experience into the area of subwoofer. Btw, the rightmost op amp and the middle one are 4558 type. I will get the 8 pin socket too.
Hope that the shop selling electronic parts will open by next month and i most likely will go buy those needed parts.
Best of wishes to you.
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