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Replacement PSU for Cheetah MS6

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Replacement PSU for Cheetah MS6

Postby BillB » Fri Dec 28, 2018 1:47 pm

My MS6, referred to here
https://www.soundonsound.com/forum/view ... it=Cheetah
has finally given up the ghost.

After several instances of popping its fuse (and tripping the mains circuit breaker) I pulled it out of the rack and have been waiting for a chance to look at it. Opened it up today, popped in a higher value fuse than previously (500mA rather than 200mA) and it didn’t pop anything, but it didn’t power up either.

I’m not inclined to troubleshoot the power supply because a) the PSU (not least the transformers) is well known as the weak spot of these synths and b) someone has modified it in the past and c) it would be pretty awkward to get at all the parts/contacts whilst keeping the unit in a state suitable for sound testing.

Based on this https://www.2btech.co.uk/refimages/ms6_power_184.pdf (obtained from somewhere on the inter-webs in the past) the MS6 requires +/- 5v and +/- 12v. Ignore the 12v AC overlay, that is not my intention. There are no official schematics, so everything for a Cheetah found on the web has to be treated with some caution. But in principle this corresponds with the 7805/7905 7812/7912 regulators in the MS6.

Based on this, and fact that I saw an example of someone doing a similar thing on the web (somewhere), I am wondering if there is a single supply available, ideally in a laptop PSU type format, with a 5 pin connector delivering 0v +/- 5v and +/- 12v. If so, it would probably be best to desolder all of the PSU components and run the external PSU (via connector) to the power lines on the main PCB.

So, any thoughts on the approach and any suggestions for a PSU? Google hasn’t helped me yet..., or at least, I haven’t turned up anything which delivers all four voltages (-5v being the problem).

Many thanks

Bill
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Re: Replacement PSU for Cheetah MS6

Postby Sam Spoons » Fri Dec 28, 2018 2:06 pm

I haven't seen a laptop with multiple voltages from the PSU for donkey's years, they all have had a single DC supply presumably with the 3.3V/5V/12V DC stepped down internally from the 15-20VDC typically supplied by the line rat.

Would a desktop PSU in a suitable box be an option?
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Re: Replacement PSU for Cheetah MS6

Postby BillB » Fri Dec 28, 2018 2:18 pm

Hi Sam

I meant laptop format (neat box on a mains leads) rather than an actual laptop supply, which I agree normally just supply a single voltage. So something like the example here
https://www.matrixsynth.com/2013/03/che ... -with.html
- although this looks home-made and might only be bypassing the transformers, not the regulators.

Desktop supply in a box might be OK, but again, don’t think I have seen one with -5v?

Thanks

Bill
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Re: Replacement PSU for Cheetah MS6

Postby Sam Spoons » Fri Dec 28, 2018 2:30 pm

Of course, didn't read your OP properly. Looks like a custom build might be your only option then. :(
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Re: Replacement PSU for Cheetah MS6

Postby BillB » Fri Dec 28, 2018 2:43 pm

In which case, the question may be - does anyone know of a kit or a design which would deliver said voltages?

All suggestions welcome.
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Re: Replacement PSU for Cheetah MS6

Postby Sam Spoons » Fri Dec 28, 2018 3:17 pm

Maybe drop an email to Jon at Orchid Electronics http://orchid-electronics.co.uk? He could certainly build you a custom job.
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Re: Replacement PSU for Cheetah MS6

Postby garrettendi » Fri Dec 28, 2018 3:51 pm

Sam Spoons wrote:Maybe drop an email to Jon at Orchid Electronics http://orchid-electronics.co.uk? He could certainly build you a custom job.

I think we need a separate subforum for everything Orchid can do!

Hope you get it sorted OP!
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Re: Replacement PSU for Cheetah MS6

Postby Sam Spoons » Fri Dec 28, 2018 4:23 pm

I'm sure you know of this website http://www.maad.net/ms6/ The 'official' schematics are here http://www.maad.net/ms6/ and it looks as if the PSU schematic you posted came from those.

I has assumed the PSU was external BTW but, obviously, it isn't.
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Re: Replacement PSU for Cheetah MS6

Postby Folderol » Fri Dec 28, 2018 4:30 pm

Buggrit :protest:
I did write out what was needed for a DIY jobbie, but Sam had sent in a post seconds earlier, and thinking I'd just got an acknowledgement I hit return.
Poof! all gone.
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Re: Replacement PSU for Cheetah MS6

Postby Sam Spoons » Fri Dec 28, 2018 4:32 pm

Sorry Foldy, that happens to me all the time... :blush:
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Re: Replacement PSU for Cheetah MS6

Postby BillB » Fri Dec 28, 2018 4:32 pm

Thanks, Garrettendi.

Sam, yes, well aware and I have the MAAD Rom upgrade fitted, so it is worth making an effort for the old MS6. I can solder, so a kit would be fine. The biggest issue as a DIY-er has always been the PCBs. I have made them in the past but they took a long while to draw, etch, clean and drill. As I now tend to value my time more highly than in the past, going to Orchid or similar may be a good option.

Buying something off the shelf would be even better, so any other suggestions welcome. It would appear to be a non-standard set of voltages, though.
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Re: Replacement PSU for Cheetah MS6

Postby BillB » Fri Dec 28, 2018 4:36 pm

Awww, Folderol!

It’s not all gone though... it’s still somewhere inside your head.

Cue sci-fi image of Folderol strapped into a chair with lots of electrodes strapped to his head.... Mmmwwwahh-ha-ha!

:ugeek:
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Re: Replacement PSU for Cheetah MS6

Postby BillB » Fri Dec 28, 2018 4:42 pm

Sam Spoons wrote:I has assumed the PSU was external BTW but, obviously, it isn't.

Sorry, should have said, for anyone not familiar with MS6 internals: everything, including PSU, and excepting front panel switches and display, is on a single circuit board. As the board nearly fills the inside of the rack, left to right and front to back, that’s what makes it a bit tricky to work on.
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Re: Replacement PSU for Cheetah MS6

Postby Sam Spoons » Fri Dec 28, 2018 4:48 pm

Yeah, found a low res pic of the inside on that site. It does look as if it should be fixable though to my fairly inexperienced eyes.
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Re: Replacement PSU for Cheetah MS6

Postby BillB » Fri Dec 28, 2018 6:12 pm

Here’s an old photo of the interior around the PSU, before I replaced the internal battery and Rom chip.

https://www.2btech.co.uk/refimages/MS6- ... -59-53.JPG

Two transformers, one for each +/-5/12v supply. I understand, from comments on the web, that the transformers are hard to come by. Given the mods around the regulators, including a flying resistor that doesn’t connect to anything else, I would prefer to bypass the lot.
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Re: Replacement PSU for Cheetah MS6

Postby Folderol » Fri Dec 28, 2018 6:59 pm

OK, I'd thought this was an external PSU as well.
Moving two of the regulators onto a strip of ally was probably done to keep them cooler (they can get hot enough to be uncomfortable).

The floating resistor is bad. I'd be inclined to make a note of where it was, then remove it.

I'm guessing that lump in the top corner with the pretty coloured wires is a voltage selector. If you've been blowing fuses, it's possible the contacts on that have been damaged. Some of these can be pulled right out, then repositioned for the alternative voltage range, but many are captive and you can't see the pins - I take it you have checked it's on the right setting.

I'm also assuming you've got a simple test meter.

The big 4700u cap is on the +5V supply, so (as you can easily get to both ends) you can quickly check to see if anything is happening at all. Depending on exactly what the transformer windings are, I would expect about 8-9V across it (it is wired between the transformer and the regulator so is a convenient test point.

If that's OK, I would want to get to the other three biggish caps, but that might not be so easy.
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Re: Replacement PSU for Cheetah MS6

Postby BillB » Fri Dec 28, 2018 7:55 pm

Correct on the voltage selector at top left and yes, it is set to 240, not 120 volts. Yes, I have a multimeter.

Voltage across 4700u cap is 7V.

I can take out the main PCB to get access to all components but it will be a little tricky to make all the connections safely and hold the board securely whilst applying power. Do you think a repair is a better option than a PSU replacement?
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Re: Replacement PSU for Cheetah MS6

Postby ef37a » Fri Dec 28, 2018 8:27 pm

Don't know synths but, IMHO I would strip out the internal supply and go one of two ways.

5 years ago I would have bought a nice chunky toroid (20VA ish?) and the bits to make a 12+5V supply and put it in a tin and connect with a 5 pin XLR.

Now? Just buy two decent SMPSU rats. Snot them together and connect as before with XLRs.
If going the SMPSU route, maybe put some extra LC RF filtering in the kbd case?

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Re: Replacement PSU for Cheetah MS6

Postby Folderol » Fri Dec 28, 2018 9:16 pm

BillB wrote:Correct on the voltage selector at top left and yes, it is set to 240, not 120 volts. Yes, I have a multimeter.

Voltage across 4700u cap is 7V.

I can take out the main PCB to get access to all components but it will be a little tricky to make all the connections safely and hold the board securely whilst applying power. Do you think a repair is a better option than a PSU replacement?

Everyone has their preferences, but I generally prefer in-situ repairs where possible.

That voltage is lower than I'd like, it doesn't leave much headroom for the regulator. Still... One of the smaller caps should have a similar voltage across it - maybe slightly higher, and the other two should have about 15V.

If running with the board partly out use copious pieces of cardboard to make sure everything is insulated from the case. Cardboard is best as if it gets in contact with something very hot it just smolders. Paper might catch fire, and plastic will melt, with bits quickly poking through.
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Re: Replacement PSU for Cheetah MS6

Postby James Perrett » Sat Dec 29, 2018 12:05 am

I know it is too late but fuses are generally specified at a certain value for a reason - NEVER replace one a higher rated fuse. I'd go along with Folderol's suggestion to fix what you have rather than rebuild. It sounds like a basic regulated supply which won't be hard to fix if you take a logical approach.
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