Matrix-12 Power Supply

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garranimal
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Matrix-12 Power Supply

Post by garranimal » Mon May 30, 2011 6:47 pm

My most favorite, go-to synth. I had it turned on for 5 minutes, just sitting there. Then suddenly the system locked up:
- the LCDs went dark
- the LEDs came on
- one LED was blinking.
- and an ever so faint wisp of smoke. :(
Needless to say I powered-down in a New York minute.

I've heard the power supplies can develop issues after time. Here's what I've done so far:
- pulled the PS connectors to all the boards so the chips don't get fried
- with a DVM I checked the PS board voltages that feed the voice boards, and they were all over the place.
(BTW the xpander service manual test procedure says to test voltage at the voice boards. Huh? I don't want to fry them if the PS is acting up.)
- checked the memory battery and it's good at 3.01 V.

I'm very good w/ soldering, re-work, building electronics from plans - but I'm not an electrical engineer by any means.
This power supply is quite strange to me as it uses pulse-regulation which I do not understand at all.
The closest repair tech I trust is in another state, but that's a last resort as I do not have a steady income right now.
And I am very competent and thorough at replacing components, researching data sheets, and have an oscilloscope as well.

I am thinking about replacing the large PS caps, but they look fine - really.
Any advice on tests I can perform, issues common to this model, etc would greatly be appreciated.

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Re: Matrix-12 Power Supply

Post by rhino » Mon May 30, 2011 7:26 pm

Check any +5-volt regulators. There may be one on a board OTHER than the PS.
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Re: Matrix-12 Power Supply

Post by garranimal » Mon May 30, 2011 10:53 pm

The 7905C at location U5 input tests wonky too, +/-1V on the input, much more deviation on the output.

The Xpander manual shows/describes the +5V feeding into a main power regulation IC at U6 (U4 on the Xpander). U6 being a NE5561 - pulled the datasheet, yes it's being used for voltage regulation. It looks fine, not fried.

So, can anybody elaborate from experience what tends to go wonky on the M-12 power supplies?

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Re: Matrix-12 Power Supply

Post by garranimal » Sun Jun 05, 2011 8:05 pm

Quick update - I've put the larger PS caps on order and should have them popped-in this week. Hope this does the trick :beg:

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Re: Matrix-12 Power Supply

Post by garranimal » Wed Jun 08, 2011 10:59 pm

Replaced all the big caps but this did not resolve the issue. I noticed both of the 2W 0.1-ohm resistors are cracked.

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Re: Matrix-12 Power Supply

Post by snod_donkey » Thu Jun 09, 2011 9:50 am

I'm not that sure but this looks like a switching type power supply. Cant't find the circuits anywhere for it though

Most switch modes require a load to work correctly so if this one works the same you might find your readings are way off if its not plugged into anything

If your not sure about testing this i would replace most of the parts in the switching secttion to start with. Start by removing the power transistors on the upright heatsinks and check them with a meter. If you have burnt out resistors the the problem may just be around that part of the circuit


Looking at the expander circuits the power supply looks the same, there is a good description of how this works and yes its a switching unit. I believe it wants a load on the 5 volts so the switching circuit will work. Think you may have to plug one of the boards in unless you have a "dummy load" you could hook up

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Re: Matrix-12 Power Supply

Post by garranimal » Thu Jun 09, 2011 3:43 pm

snod_donkey wrote:I'm not that sure but this looks like a switching type power supply. Cant't find the circuits anywhere for it though
Exactly! Been looking at partial information from the Xpander.
snod_donkey wrote:Most switch modes require a load to work correctly so if this one works the same you might find your readings are way off if its not plugged into anything
Well ok that's what I've been suspecting, and it still doesn't make sense but I sure am glad to hear somebody say that!
snod_donkey wrote:If your not sure about testing this i would replace most of the parts in the switching secttion to start with. Start by removing the power transistors on the upright heatsinks and check them with a meter. If you have burnt out resistors the the problem may just be around that part of the circuit
I think this would be a great way to proceed. I can't find any data or pinout for the "Q1" Thompson SK9165 (as marked) which I think the parts-list/schematic for the Xpander is calling IRF531/UFN531. I can pull the v-regs and test them, and go ahead and replace the 2W resistors which couldn't hurt - if those are shorted open it would put a greater load on the switching circuit?
snod_donkey wrote:Looking at the expander circuits the power supply looks the same, there is a good description of how this works and yes its a switching unit. I believe it wants a load on the 5 volts so the switching circuit will work. Think you may have to plug one of the boards in unless you have a "dummy load" you could hook up
This is scary business now when I hook up the boards: something on either the display or processor board immediately starts screaming/hissing which sounds kinda like a car when a bearing goes out on the belt drive.

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Re: Matrix-12 Power Supply

Post by schmidtc » Thu Jun 09, 2011 4:29 pm

Those cracked resistors may have fried open, probably the result of a butt load of current going through them. Using a dummy load, and not the synth while you're testing sounds like a good idea. Good luck!

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Re: Matrix-12 Power Supply

Post by garranimal » Thu Jun 09, 2011 10:33 pm

Feeling much more comfortable about using a dummy load. So, next question being: what power (amperage) should the 5v dummy load be?

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Re: Matrix-12 Power Supply

Post by garranimal » Wed Jun 15, 2011 12:27 am

Re-soldered the leads to T2, I've heard these can come lose over time.

Pulled and checked diodes, zeners, etc and they checked out except 'D10': the schottky rectifier diode, a DVM diode test shows 0.11V breakdown voltage one way, then no reading the other way as expected. But a resistance test on the DVM shows .6k ohms one way, then the other way it flashes between no reading and .6k-ohms. Sooo, seems ok, or is it? I'm thinking about just going ahead and replacing it. Parts are cheap and I want to get this thing done.

BTW my schottky diode in my M-12 is VSK63. Don't know if that's what the factory calls for on the M-12, so I'm looking for a suitable cross-reference.
- Xpander service manual calls for Motorola 1N5823 aka VSK520.
- The Motorola datasheet is unavailable online.
- digchip.com suggests to use sk502. That's not a to-220 package and can't be put on the heat-sink. :(
- Ugh, another dead end. Please help me find a cross-reference.

EDIT: Also, I pulled the -5V voltage regulator and it seems fine.

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Re: Matrix-12 Power Supply

Post by schmidtc » Wed Jun 15, 2011 1:04 pm

Switch mode power supplies are F*CKING DANGEROUS, so you might give it to some with more experience at this point. That diode drop sounds small, but it looks reasonable from the 1N5823 datasheet. For the dummy load, look at the resistance at the power terminals to ground with nothing hooked up, use it only if it seems reasonable with ohms law, otherwise start something like 1 watt or greater at 1K ohm. Make very sure the load can handle the current before you hook it up.

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Re: Matrix-12 Power Supply

Post by garranimal » Wed Jun 15, 2011 4:31 pm

schmidtc wrote:Switch mode power supplies are F*CKING DANGEROUS, so you might give it to some with more experience at this point.
I'm almost ready to turn it over to someone. Just want to make sure it's not something obvious and in my power to find/fix. I'm sticking to pulling/testing individual components for now and staying away from dummy loads.
schmidtc wrote:That diode drop sounds small, but it looks reasonable from the 1N5823 datasheet.
Yeah those Schottky diodes can be way less than .6v so that might be okay.

Here's another big clue I've discovered. During power-on, the high-pitched whine sound is actually coming from T2 so now I'm thinking there is an excessive load coming from the T2 circuit, perhaps shorted components. I'm going to go ahead and replace all the small electrolytic capacitors as one or more of those might possibly have dried up.

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Re: Matrix-12 Power Supply

Post by sam » Wed Jun 15, 2011 5:54 pm

I would test the caps for shorts first, Electro caps don't short as a rule but tantalums do and I suggest replacing caps may not be the answer.

I hope that helps..
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Re: Matrix-12 Power Supply

Post by garranimal » Wed Jun 15, 2011 8:06 pm

Thanks sam, the small electro caps test fine so I put them back in (they are a weird size, 15uF).

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Re: Matrix-12 Power Supply

Post by sam » Wed Jun 15, 2011 8:43 pm

garranimal wrote:Thanks sam, the small electro caps test fine so I put them back in (they are a weird size, 15uF).
Electro caps don't really fail but I always change them in the power supplies however tantalums do short.

Your problem could be a faulty op amp....But without getting in there i don't know.
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