Yamaha yst-sw320 idling pot replacement - Page 1
Forum section: electronics / repair

author: alexAB
author_rank: Newbie
timestamp: April 27, 2022, 05:12:24 am
content: 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 regardsAlex

author: FIXITNOW2003
author_rank: Regular Contributor
timestamp: April 27, 2022, 06:15:07 am
content: i would just lock the original pre-set with some Loctite SF7400. if it was mine

author: Chriss
author_rank: Frequent Contributor
timestamp: April 27, 2022, 06:45:04 am
content: Locktite for sure and problem solved.

author: Jeff eelcr
author_rank: Frequent Contributor
timestamp: April 28, 2022, 01:49:15 pm
content: 25 turn pot does not sound like a good idea as a bias pot.As others said locktite.Jeff

author: alexAB
author_rank: Newbie
timestamp: April 28, 2022, 03:40:23 pm
content: 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 !

author: David_AVD
author_rank: Super Contributor
timestamp: April 28, 2022, 08:56:26 pm
content: Old pot is 3 pin, new pot is 3 pins. What is causing confusion?

author: Jeff eelcr
author_rank: Frequent Contributor
timestamp: April 28, 2022, 09:40:56 pm
content: 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 todo 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 resistorpart goes across the two points and the wiper goes to the center hole, the PCB is mostlikely already connected to one of the other points but check anyway.Jeff

author: alexAB
author_rank: Newbie
timestamp: April 28, 2022, 11:18:31 pm
content: 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.pdfSo 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 ?ThanksAlex.

author: CaptDon
author_rank: Super Contributor
timestamp: April 29, 2022, 12:48:05 am
content: 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.

author: Jeff eelcr
author_rank: Frequent Contributor
timestamp: April 29, 2022, 01:39:06 am
content: Either way will work but to be correct short wiper and 3measure 1-3 turn clockwise does reading go up?If so correct if not reverse.Jeff

author: Jeff eelcr
author_rank: Frequent Contributor
timestamp: April 29, 2022, 01:54:48 am
content: CaptDon, Nice to know!One carver amp 30+ years ago PWM direct ac voltage input realy threw meat the time and yes an adjustment fixed it only PWM amp I had seen then wasthe 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 engineerwho designed it.Jeff

author: alexAB
author_rank: Newbie
timestamp: April 29, 2022, 03:56:21 am
content: 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 $ eachbought 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 productsAnd 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 .

author: Jeff eelcr
author_rank: Frequent Contributor
timestamp: April 30, 2022, 02:44:25 am
content: Alex note the O on all the outputs, your replacements should also.Jeff

author: alexAB
author_rank: Newbie
timestamp: May 01, 2022, 07:51:57 pm
content: 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 .

author: SpecialK
author_rank: Regular Contributor
timestamp: May 02, 2022, 01:14:21 am
content: Quote from: alexAB on April 29, 2022, 03:56:21 amThat 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.

author: alexAB
author_rank: Newbie
timestamp: May 03, 2022, 08:13:24 pm
content: 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.