Yamaha yst-sw320 idling pot replacement

[alexAB]

Hey guys,

I have a Yamaha YST-SW320 which is a 250 watts powered subwoofer , I want to change the pot that serves as an idling current adjustment , the thing is the 1 turn pot that is already there ( which is a flat horizontal type of pot ) always change its value due to vibrations inside the speaker cabinet , I bought some Vishay 25 turn vertical pot to avoid this problem , i need some advice to mount this in a correct manner , here is a screencap of the schematic :



The pot is named VR1 on the schematic , so my question is what is the correct way of mounting the new pot ? Do I need to solder the pin 1 of the new pot after R79 ? Or the pin 1 should go before R69 ? It seems like the wiper is shorted either with pin 1 or pin 3 like in this picture :



Thank you for your help .
Best regards
Alex

[FIXITNOW2003]

i would just lock the original pre-set with some Loctite SF7400. if it was mine

[Chriss]

Locktite for sure and problem solved.

[Jeff eelcr]

25 turn pot does not sound like a good idea as a bias pot.
As others said locktite.
Jeff

[alexAB]

Ok guys , but as Jeff eelcr says why this is not a good idea with the 25 turns pot if it has the same resistance value (here 470 ohm trim pot ) and superior wattage rating (flat pot is 0.25 watts Vishay is 0.5 watts ) , do they have a different behavior as a bias pot except for the number of turns? I just try to understand why ? Thanks !

[David_AVD]

Old pot is 3 pin, new pot is 3 pins. What is causing confusion?

[Jeff eelcr]

If bias is off you might have to take to long adjusting a 25 turn pot (overkill for sure)
also wire wound?  There are some adjustment disadvantages to hi turn pots having to
do with the contact point of the wiper at some positions.
 What it will do in your unit I do not know as you have them you can try it, the resistor
part goes across the two points and the wiper goes to the center hole, the PCB is most
likely already connected to one of the other points but check anyway.
Jeff

[alexAB]

Yes the new ones are 3 pin pot , if the bias is off i can just measure what is the actual resistance value of the flat pot between the wiper and the pin that is connected with it and adjust to the same value with the 25 turn one before soldering it and fire it up; this can be a good start , and i know it's overkill but the shop where i got them , i got a discount coupon around so i got them for nothing in terms of price , they have only 1 turn pot and 25 turn , and my bad it is some Cermet trim pot not Vishay.

Here is the pdf for @Jeff eelcr just in case :

https://cdn-reichelt.de/documents/datenblatt/B300/3296-SERIES_ENG_TDS.pdf

So the good way to solder the new one is like this right ? :



Or the opposite , pin 1 and wiper connected together , and pin 3 solo ?

Thanks

Alex.

[CaptDon]

The 270 degree pot for PWM adjust on the Carver PM-2.0T / PT-1250 / CBA-1000 amplifiers is EXTREMELY touchy and when they get dirty you can blow up the SMPS instantly when trying to adjust the 60us pulse width!!!! I replace them with 10 turn PCB mount pots adjusted to mid-point and carefully go through the PWM alignment with an external 25vdc supply before cranking it up on line voltage. 25 turns may be overkill but I see no reason why it shouldn't work.

[Jeff eelcr]

Either way will work but to be correct short wiper and 3
measure 1-3 turn clockwise does reading go up?
If so correct if not reverse.
Jeff

[Jeff eelcr]

 CaptDon, Nice to know!
 One carver amp 30+ years ago PWM direct ac voltage input realy threw me
at the time and yes an adjustment fixed it only PWM amp I had seen then was
the Bose 1800 and a few of the early GM/Bose ones.
I do not remember the model number but I do remember talking to the engineer
who designed it.
Jeff

[alexAB]

Ok , for more precision the sub is a second hand i got it from a guy that toasted the woofer , also the bridge diode was toasted too don't know the reason why but i replaced all the elecrolytics capacitors at least a good part of it especially from the main power board and some around the bridge diode :

 

The big caps were the most pricey , 8200 uf 80 V ,  38 $ each  bought them directly from a yamaha service repair , put some rtv-167 under them  , for the bridge diode i used a sony equivalent , at first it was an unbranded thing .

Then i replaced all the silicon insulation thermal pads behind the transistors here is before after for you to see :





This was before , and after :





I got lucky because non of them was toasted , but just in case i ordered a bunch of them cause now they are discontinued products 

And then the trickiest part was to find a woofer replacement , the original one had a Protection grill on top of it , so you could barely see what was behind (cone or suspension ) and with their weird impedance , 6 ohm , it's not so common , i finally bought a woofer from an yst-sw 315 (brand new boxed ) , which has the same power rating and same impedance but a higher stiffness , i destroyed the old woofer by cutting the grill , the suspension and the spider and yes the voice coil was toasted , i mean like really toasted . Anyway, this thing sounds great for the price i managed to get it .

[Jeff eelcr]

Alex note the O on all the outputs, your replacements should also.
Jeff

[alexAB]

Yes , O version has higher DC Collector/Base Gain hFE and DC current gain hFE value , in my replacement parts there are some O and some are R so i'll keep that in mind not to solder the R version if something go wrong in the future .

[SpecialK]

And then the trickiest part was to find a woofer replacement , the original one had a Protection grill on top of it , so you could barely see what was behind (cone or suspension ) and with their weird impedance , 6 ohm , it's not so common ,

That didn't used to be the case.   4 ohm seems to have always been the common choice for car audio.  But for home audio North America was 8 ohm , the UK seemed to have made quite a few 6 ohm drivers.  I've never owned a Japanese loudspeaker, but a lot of AV receivers quoted power is in to 6 ohms.  I suppose that the output impedance from a single AB transistor pair is the optimally matched there.

[alexAB]

To be precise they are all 5 ohm impedance woofer ( yst-sw 320 , 315, 515 , ns-sw 700 and other Yam 250W 10 inch active sub) only the 320 was different from the others (woofer part ref XY889A00 ) ,so it's either X4457A00 (discontinued) , X4457D00 or X4457B00 for the replacement part , i don't know the difference between A, D, B woofer ? The only visible difference is : no protection grill , a vented pole and higher spider stiffness.
XY889A00 has none of them.