Replacing transformer on Jx-8P

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fingerbib2000
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Replacing transformer on Jx-8P

Post by fingerbib2000 » Sun Jun 15, 2008 4:54 pm

Hi, I previously psoted a topic about my JX-8P not powering up and then miraculously resurrecting itself: http://www.vintagesynth.org/phpBB2/view ... highlight=

anyway, it's dead again. I think maybe the transformer is broken since i cannot read any current coming from the transformer, but it seems fine going into it.
here's a picture of it:
http://www.minds.may.ie/~lineone/JX-8p/100_0745.JPG


So, to my question: where can i get a replacement for this transformer? are they standard components and not specific to the synth? maplin have a selection:
http://www.maplin.co.uk/module.aspx?Mod ... 7&doy=15m6

these seem a lot easier to hook up than the one currently on the board (i.e. the connecitons aren't hidden under the plastic)

so, can any one tell me from looking the pic of the jx-8p, what transformer i need? sorry for beiong such an electrical n00b but i'd like to replace it myself if that's not impossible for someone with a little soldering experience but pretty much zero knowledge of circuits...

thanks!
Sequential Circuits Pro-8, Rolands TB-303, MKS-80, JX-8P, SH-09. Waldorfs Microwave XT, Pulse. Yamahas RS7000, A4000, DX-200. 2 x Cheetah MS-6. Novations Supernova 1 & 2. Korg ER-1. Creamware Pulsar 2.

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Post by Bross » Sun Jun 15, 2008 10:36 pm

I'm curious how you measured current going to and from the transformer. Current must be measured in series so you would have to cut wires and put your multimeter between them. I think your "no current" measurement (and conclusion that it's the transformer) is not correct.

Have you measured voltages coming out of the power supply board to the left of the transformer? Check the voltages at CN1 and CN2 as shown in the service manual.

I think you should find a local tech person to work on it. It doesn't need to be a vintage synth expert, just someone with some musical instrument electronics repair experience who won't kill your synth. Machinegeist gave some very likely probable causes in your original post that you could share with the tech.

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fingerbib2000
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Post by fingerbib2000 » Sun Jun 15, 2008 10:56 pm

Bross wrote:I'm curious how you measured current going to and from the transformer. Current must be measured in series so you would have to cut wires and put your multimeter between them. I think your "no current" measurement (and conclusion that it's the transformer) is not correct.
.
yes, i would think you're right about this...but here's how i came to that conclusion:
I just bought this cheap voltmeter and stuck one of the tips on the contact for the red wire marked AC out (on the filter board side), it lights up (the voltmeter has 3 LEDs: 110v,220v,380v. if i place one tip of voltmeter on the ac out and then on another bit of metal, the 220v lights up. if i just place one tip on the ac out contact, a fainter LED illuminates ) same on the black wire marked AC in and the contacts 6 & 7. neither of the white wires light up the voltmeter (this means the circuit is broken somewhere?)

if i place the voltmeter tip anywhere after the transformer, it doesn't light up. so i assumed that is the point where the circuit is faulty? i can't afford to have a tech charge me £50 just to look at the synth when it may just be an easily replacable part...

again, i realise what i'm describing may sound silly, but i'm learning as i go here...
Sequential Circuits Pro-8, Rolands TB-303, MKS-80, JX-8P, SH-09. Waldorfs Microwave XT, Pulse. Yamahas RS7000, A4000, DX-200. 2 x Cheetah MS-6. Novations Supernova 1 & 2. Korg ER-1. Creamware Pulsar 2.

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Post by nathanscribe » Sun Jun 15, 2008 11:47 pm

That transformer looks like a multitap to me, so you're not going to get one from Maplin. Try googling the part number (the number on top). The service manual might have more details. Might have the PDF of it somewhere....

Yup, have it (from the manual):

Orange/Orange = 9.5V DC @ 0.8A
Red/Black = 19V DC @ 0.3A

DC? From a transformer? Hmm. Methinks they might mean AC.

It'll be one of Roland's custom parts probably.

I wouldn't mess about with any of the mains supply stuff unless you really know what you're doing. As for fault-finding - again, if you don't know what you're doing, you can be at risk, your gear could be at risk, and you might not find anything out anyway. Best to hand it to someone who knows about it.

Having said that, I'm self-taught, with a little help from some knowledgable friends & colleagues, and a dozen years of trying not to fry things. If you want to learn, take it step by step, get a couple of books on electronics (Babani publish pretty cheap ones, and they do one on power supplies which covers some basic info on mains transformers etc) and experiment. I'd still recommend leaving off the mains stuff until you have a bit of background though. Learn about what's going on at each stage of the circuit before you even attempt to poke at it. Forewarned is forearmed, and all that.

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Post by fingerbib2000 » Mon Jun 16, 2008 12:26 am

ok , i'll think i'll the mains stuff alone for now then! cheers for the info on the part number etc...i'll probably go and read up on it some more and take it to a tech if i'm not making any headway. cheers!
Sequential Circuits Pro-8, Rolands TB-303, MKS-80, JX-8P, SH-09. Waldorfs Microwave XT, Pulse. Yamahas RS7000, A4000, DX-200. 2 x Cheetah MS-6. Novations Supernova 1 & 2. Korg ER-1. Creamware Pulsar 2.

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Post by hfinn » Mon Jun 16, 2008 1:47 am

Usually the transformers are fine. At least in my experience. I have yet to see a bad one.

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Post by Mooger5 » Mon Jun 16, 2008 8:52 am

I´ve seen a few faulty transformers. When the circuit they feed demands more current than what they can supply, they saturate and eventually burn out. Happens mostly in amplifiers and wall-warts (even "manufacturer-approved" ones). With instrument built-in transformers it´s rare as they usually are more than enough rated, but it can happen.

fingerbib2000, does that transformer smells like it´s burned?
If you can read AC voltages at the primary and nothing at the secondaries it must not be conducting, but see if a reading at the bridge rectifiers confirms that. Select AC on your multimeter and probe the pins marked ~.

Transformers are usually custom-made so the best you can do is to take yours to your local coil winder. They´re used to repair that kind of stuff and don´t even need to know the VA ratings of the transformers. Their machines unwind the coils and count the number of wire turns. Next, they put new magnet wire in and wind the same number of turns. And that´s it.

I used to order custom made transformers for DIY valve amps from those shops. For that repairing job I´d expect to pay around 25/30 euros.

This, of course, assuming the fault is in the transformer. Please be certain before rushing out.
From the pictures you posted it looks like that synth was serviced in the past, as the regulators are fixed to the heatsink in a not so usual way, and they don´t look in good shape. Also by their looks the reservoir capacitors need to be replaced.

Good luck.
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Post by fingerbib2000 » Mon Jun 16, 2008 9:18 am

wow, great tips there Mooger5! The the transformer doesn't smell burnt out. i'll get a multimeter and check the components more thoroughly...
Sequential Circuits Pro-8, Rolands TB-303, MKS-80, JX-8P, SH-09. Waldorfs Microwave XT, Pulse. Yamahas RS7000, A4000, DX-200. 2 x Cheetah MS-6. Novations Supernova 1 & 2. Korg ER-1. Creamware Pulsar 2.

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Post by Mooger5 » Mon Jun 16, 2008 9:22 am

nathanscribe wrote:
Orange/Orange = 9.5V DC @ 0.8A
Red/Black = 19V DC @ 0.3A

DC? From a transformer? Hmm. Methinks they might mean AC.
You´re right, it´s a typo. The 9.5 and 19 AC voltages are nominal, so they go above and below the values mentioned. After rectification they can´t go below the regulator voltages which must be 5 Volts DC for the digital section and 15 Volts DC for the analog section respectively. They can´t go much above because of the safety range with which the regulators will work without getting fried.

As a last resource, two separated transformers, one 230/10 and one 230/20 will work. The ratings should be no less than the afore-mentioned 0.8 and 0.3 Amperes, but the higher, the better.
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Post by Mooger5 » Mon Jun 16, 2008 9:37 am

fingerbib2000 wrote:wow, great tips there Mooger5! The the transformer doesn't smell burnt out. i'll get a multimeter and check the components more thoroughly...
You´re welcome, but please follow the advices given by the other forum members, as voltages can kill.
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