Need guidance to replace power transformer please

[Markoneswift]

I'm not seeing this on the SM, should be no centre on primary.
What are the wire colours of left, centre, right

Unplug the unit and measure the resistance at the power cord, between the prongs.

Also SM shows only one primary for +/-12Vac

View attachment 1169002

Just looking at the underside in the image, I guess the centre pin on the primary side doesnt actually connect to anything but if thats the case, I wonder why I read continuity between the left pin and centre.

I will do the tests you suggest on the chord pins etc and report back.

Im definitely learning more about transformers and how they are represented on a schematic so this is already a great learning excercise for me.

 

[Bassblaster]

I guess the centre pin on the primary side doesnt actually connect to anything but if thats the case, I wonder why I read continuity between the left pin and centre.

Its an unused center tap. for switching between 120/240V most likely.

 

[mbz]

Its an unused center tap. for switching between 120/240V most likely.

Agree. So the lack of continuity is a bad sign. Will wait confirmation on the measurements at the power plug.

 

[Bassblaster]

Agree. So the lack of continuity is a bad sign. Will wait confirmation on the measurements at the power plug.

i have had a bad power cord before, only once, but it is possible.

 

[Markoneswift]

i have had a bad power cord before, only once, but it is possible.

Believe me, I checked that before I even posted here. I may have sounded like a dumbass with my initial confusion of AC / DC voltages but to not try another chord would have put me beyond any help

 

[mbz]

Believe me, I checked that before I even posted here. I may have sounded like a dumbass with my initial confusion of AC / DC voltages but to not try another chord would have put me beyond any help

I just wanted to be sure that the transformer was at fault before spending $$$$;s on a replacement.
If the resistance measurement between the outer pins of the primary (post #21) gives open, AND the center pin to one other pin gives open
then I'll have to agree the primary is shot. A replacement transformer should be too hard to find. Will wait on confirmation before crunching the numbers.

 

[Markoneswift]

Ok well I can confirm that with the transformer back in the circuit and everything hooked up, I read open circuit between the power chord pins so its down to looking for a suitable replacement now I guess.

As I said before, in our small market the chances of finding a like unit are slim to none. A different sub altogether is a distinct possibility, especially if the price of replacing the transformer gets silly. My son only paid $140 NZ for the whole setup which was a great deal, so I'd like to get the original matching sub going again if possible.

 

[Bassblaster]

If the main trans is truely shot and needs replaced, check this post i made last page.

if its only an AC-CT-AC transformer, consider looking at AnTek toroidal's. i don't know what voltage yet i havent went through the manual. but for being rated 200W at 6 Ohm, im going to guess its around 30V or higher
EDIT: Page 23 of the service manual.
power amp is an STK404-140SC. rated 180Wx1 at +/- 73V. so a 52V secondary would be needed
but the unit actually runs on +/- 62V. so a 44V secondary would be needed
the 12V is from the regulator circuit for the op-amps in the crossover

Would be way easier getting a new trans that trying to source a used one from a donor unit. i would suggest either the AS-3440 or AS-3445. AS-3440 will get you a little less power due to lower voltage, and the AS-3440 will get you a little more de to the extra volt.
but check to make sure they will work cause that subwoofer has low voltage input stuff. the both i listed have 2 15V windings so it shouldn't be an issue. pics and more pics

 

[mbz]

Looking at the specs, power figures are a bit questionable.
Output 200W/6ohms, Consumption 184W. Fuse is 1.6Ax230V gives
370VA maybe add a margin so 400-500VA?

Ideally 240V primary for NZ, but will probably have to settle on 230V.
Means the secondary will be slightly higher than rated voltage.

For the secondary, the target is 62Vdc, allow 5% for 230/240V supply and
maybe another 5% for no/low voltage (transformer will deliver secondary
voltage at rated VA/consumption, voltage will be slightly higher at no/low
load) and allow a volt or 2 diode drop (62-3-3-1) gives about 55Vdc/1.414
= 39Vac, say 40Vac.


Will check RS for possible candidates.
230/240V primary
40V secondary
1.6->2A
400-500VA


Just a small point, the continuity test at the power cord needed to be done
with the unit switched on (the switch breaks the circuit).

 

[Markoneswift]

Thanks for that detailed help @mbz

I made sure the mains switch was closed before checking the continuity so it is indeed dead, Jim.

Cheers

 

[Bassblaster]

that 200W rating is rubbish as the chip is 180 max at like +/-72V. and i wouldn't think what is about a 150W amp would need 4-500VA.

 

[mbz]

I vaguely recall that RS delivers to NZ?

I did a few searches, a 500VA torroidal (smallest they had) is about AUD130- (#$%@!)
They did not have a suitable chassis mounted item.

Maybe someone else/@Bassblaster can shime in with a better source/price...


https://au.rs-online.com/web/c/power-supplies-transformers/transformers/toroidal-transformers/
https://au.rs-online.com/web/c/powe...s/transformers/chassis-mounting-transformers/

Agree, 400-500VA seems high, this was determined by the main fuse and the mains voltage.

If we can assume 200W then 250-300VA would be plenty (200W/0.8)

 

[Bassblaster]

You wont be able to board mount a new trans, you'll have to mount a new trans on the inside, and run the wires over to the holes on the board. will took kinka tacky but the electrons dont care.
kinda why i posted about the AnTec torroidal's, really the go to for audio projects. 300VA for about $40. other than that i dont know of many options. iron cores are stupid expensive for some reason. $30 for a 50VA 24VCT on Mouser. Not many places to get large power trans as most audio manufacturer will get theirs custom wound. unless you could track a switching supply with +/- rails. the fuse will always be higher then what the unit will draw even at full power. i have a Sansui amp fused for 7A. 120Vx7A=840W fuse. amp rated for 100Wx2. its a 320VA transformer.

 

[Markoneswift]

The biggest toroid I can get domestically is 160VA (and costs $65.90 !!)

https://www.jaycar.co.nz/12v-0-12v-160va-toroidal-transformer/p/MT2112

I guess one big problem with importing one of a suitable size will be shipping costs (especially if it's iron).

I think this is looking like a 'throw out and buy something else' project. Damn shame as it's a decent sub with nice internals and cabinet. I might keep the amp board for parts (and maybe the woofer too).

I reckon we sign off now guys, @Bassblaster and @mbz thanks heaps for your assistance on this.

 

[Bassblaster]

that 12-0-12 wont even be enough for the amp IC to turn on. but if you want to call it a loss keep that heatsink, cooling fan, maybe large filter caps and most of all, the STK power amp. those things are hard to come by. i dont know how brands like sony and LG use them in all their new shelf stereo systems.

 

[Markoneswift]

that 12-0-12 wont even be enough for the amp IC to turn on. but if you want to call it a loss keep that heatsink, cooling fan, maybe large filter caps and most of all, the STK power amp. those things are hard to come by. i dont know how brands like sony and LG use them in all their new shelf stereo systems.

Economies of scale, I suppose. When they're buying in the 10s or 100s of thousands they get cheaper