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Sound Dynamics subwoofer schematics

Serge125

Nov 29, 2017
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Hi all

I'm trying to repair a Sound Dynamics THS 1000 HT subwoofer and I have a non working volume POT and I have no idea of the value of the POT so that I can replace it. The problem with it was that the volume was stuck on high so the speaker would distort badly on action flics and no way to turn it down. I have resolder around the pot and did pray some lube/cleaner and no go. I would like to know what value of that pot because nothing is written on it (normally it should be) and if I had a schematics it would say what I needed but I don't have any repair manual. I was tempted to try a 5k from another subwoofer but afraid that might damage it further. The value I got with my tester set at 1k was 724. I wonder if it's normal value or not. I need your help with this please any info on how to fix this will be welcomed!!!

THANKS!!!
 

kellys_eye

Jun 25, 2010
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Got any pics of the parts?

Does the pot have three terminals? If so, measure between the two outer, then measure between one outer and the middle pin and, whilst adjusting the control, see if the resistance value changes.

Do this from the other outer pin (to centre pin) too.

Let us know your findings.
 

Serge125

Nov 29, 2017
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Got any pics of the parts?

Does the pot have three terminals? If so, measure between the two outer, then measure between one outer and the middle pin and, whilst adjusting the control, see if the resistance value changes.

Do this from the other outer pin (to centre pin) too.

Let us know your findings.

Ok here are some pics of the pot and board. The reading that I get is 781 on the two outer terminals, my meter is set at 1K. To check the middle and outer terminal I use an analog meter so I see more clearly at what it does. So on the meter it starts at 0 then the moment I move the adjuster a bit the needle goes completely at the other end. When the subwoofer was on and I played a movie it's like to volume was stuck on high and no mater if I turn the volume low or high it stays on high. so if you need more info I'll try my best to get it for you.

THANKS!!!!!
 

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73's de Edd

Aug 21, 2015
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Sir Serge125 . . . . .


So on the meter it starts at 0 then the moment I move the adjuster a bit the needle goes completely at the other end.


To check the middle and outer terminal I use an analog meter so I see more clearly at what it does.

What you do now is test between the middle terminal and the OTHER end terminal, this tim I'll bet that it will read from zero and then start increasing in resistance, until up until encountering that open spot at the other end of the pot

I certainly believe that you have yourself a 1 K audio taper pot there.
The way that you will know that is if you put the pot wiper at its mid point the resistance reading will NOT be
1/2 of 1K or half of your given reading of 724 ohms, since the seems to be a loss of ~ 276 ohms due to that bad spot on the pot, near the other terminals end.

If your original unit was an audio taper log pot . . . . . . that unit should have read ~100 ohms from one end terminal to the center rotor or ~ 900 ohms from the other end terminal to the center rotor.

With that original unit being a PCB mount and those two extra tabs on the sides being for mechanical mount and support, you just might have to settle on a more common unit with just your basic 3 terminals and rely upon a
square block of wood on the backside that is silicon rubber mounted to the back of the pot and to the pcb proper for an insulated means of establishing mechanical support.

NOW . . . .here is where you go to the likely sites . . . . . depicting that controls mechanical profile, with its relevant electrical specs and fully research them out . . . .

https://www.google.co.in/search?tbm=isch&slpassthru=1&q=1k Log Taper Potentiometers

ADDENDA:
Any chance that this is THE unit and the involved pot is the second one down ?

THS-1000-V__-F-007-906-SPZ.jpg






73's de Edd

 
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kellys_eye

Jun 25, 2010
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Pot.png

The marking on the pot.....

Conventionally this would mean 1MΩ log law.
 

Serge125

Nov 29, 2017
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73's de Edd

Aug 21, 2015
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Sir Serge125 . . . . .

U say . . . .
So iI (sic) must get a 1MΩ pot right?


Just for further confirmation . . . .

I think that you are possibly short on your comprehension versus your observations.
You did not tell which pot it was , as was requested . . .if being one of the two on that back panel.
I can not see a 1 meg value of pot being used, that is being an EXTREMELY H IGH value, when being used in a solid state design of unit .
I can only relate to it as being associated with FET circuitry or using integrated circuitry, with it containing that FET family of internal input circuitry.
Your INITIAL info of HIGH uncontrollable distorted audio . . . . EXACTLY RELATES with the very end of a pot that gets that terminal connected to ground, having opening up.
Now you say that the unit has continuity from end to end .
But not having or knowing your instrumentation . . . . that reading of 724 on the K scale could relate to 724 ohms or actually be 724K.
If you ohmically read between the units center tap and to either end and rotated the shaft, the resistance should be changing when testing towards either end.
That would suggest the ground end of the pot is not being open circuit near its very ends range . . . therefore the capability to decrease the volume would be valid.

Observation . . . . . .
I perceive of this unit as being a good bass supplement to a low powered audio amp system as you might find within a desk computer or a laptop.
Look at the back of that unit and you see two RCA input jacks which I relate to a low level inputs of the .5V RMS
thresholds .
Then you see the two sets of RED-BLACK wire inputs of what you would equate to being interconnected to that minor type of stereo amplifier, of which I earlier attested to.
Look at that designated " High Level Input " label, being just above the terminals.
That would relate to a low impedance speaker type of input, with an internal blending of the R and L channels to a further monaural BASS processing.
ALSO . . . look at the LEVEL control and its specific YELLOW marking of the 2 . . . that should be suggestive / implied that a " 2 " control setting TYPICALLY should give proper volume tracking between the bass output versus your basic stereo amp audio and its speakers outputs that is feeding this unit.
( Any chance that your main stereo amplifier unit feeding into this unit is being a kajillion watt unit . . . . but you just want to grab yourself some 'mo BASS ? . . . . . trouble resides there . . .if that is the case. )

Lastly . . . . .
Just in case you are unsure of your ohmming out of resistance with your meter, look at the board . . . . (which components we could not see) . . . . suspecting the resistors mounted on the board to be using three colored bands for identifying their value. Or luckily, they might be numerically identified.
Picking out several resistors with BROWN as their third bands (closest to the center of the resistor) should result in reading out as hundreds of ohms for their value readings . . . . . like your 724 ohms woud properly relate to a 1 K pot.
OR . . . if you pick out YELLOW third bands on resistors, their readings would relate to 724K readings which be in order for that pot being a 1 meg unit.
But if you are using the speaker inputs for your incoming audio, I would want to believe that for that low incoming impedance, related to speaker matching, that a 1 K, audio taper level control pot would be in order.

PEE ESS . . . . .
That pot, of which internals you have shown, certainly has a better quality of fabrication / construction than the typical cheap audio pot that is using a spray or stamped on resistance coating. It is a stable fired and glazed resistance element . . . . .. akin to copying the GOLD STANDARD " PWEMIUM BWEWED " units that laid out a thick furrow of resistance element within a porcelain /ceramic base and then fired it, as was done with select Allen Bradley units in times past.

What say ye ?


73's de Edd
 
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Serge125

Nov 29, 2017
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Nov 29, 2017
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Sir Serge125 . . . . .

U say . . . .
So iI (sic) must get a 1MΩ pot right?


Just for further confirmation . . . .

I think that you are possibly short on your comprehension versus your observations.
You did not tell which pot it was , as was requested . . .if being one of the two on that back panel.
I can not see a 1 meg value of pot being used, that is being an EXTREMELY H IGH value, when being used in a solid state design of unit .


In fact yes it is the bottom pot on the back, sorry for not mentioned it but I thought that you had seen it that it was LEVEL = VOL and that I have not mentioned CROSSOVER pot. The reason that I said it might be a 1mΩ is because kellys_eye in one of his answers said that the markings on the top of the pot labelled IMA maybe meaning that it might be a 1MΩ. As I said there isn't any info on the pot to know what it is that I need BUT the reading as I said is about 781 (on a K settings).

My knowledge of electronics is very limited, yes I do know components and what they do, BUT I can't find a short or any type of problems in a circuit. This is something that I SO WISH that I could do but don't know how to do this. So I fix what I see, like broken wires, redo wiring, add a switch, modify etc…. Let's say that I take chances and learn from the taking. I so DEARLY WISH to trouble shoot so much it's not funny and I envy all of you who can do this and bow before thee!!!!

Right now I know that the pot IS the problem and I need a new one but so much to chose from where I get my parts that I'm unsure witch one that I really need. What do you suggest?

You said if I move the shaft either way that I should see an increase or decrease but I don't see any of it. Yes one of my probe is on the centre and the other on either side and nothing so that is why I know that this pot is gone.

Here is the 2 photos of the circuit where the pot was;

THANKS!!IMG_0027.JPGIMG_0028.JPG
 

Serge125

Nov 29, 2017
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And.... Anything? If you need more info let me know I'm trying to give as much info that I can. If I try a 1K what's the worst thing that could happen maybe burn something else.

Thanks all!!!
 

sashijoseph

Dec 3, 2017
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Dec 3, 2017
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That most certainly is a 1Meg pot.
The 1M marking clearly indicates that,as pointed out earlier.

Also it looks to be wired as a feedback resistor in an inverting summer amplifer(one part of the LM324).
The summing resistors look to be 2.2M,so 1M feedback resistor is in order.

A 1K pot won't do any harm,only the output of the sub may be too low.

As a temporary measure you could wire in a 1k/2k/10k or thereabouts resistor in place of the pot (see the pic) and check....
IMG_0028.JPG
 
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