Just before Christmas 2020, I got sent a SoundLab SST-19 guitar pre-amp and a Cheetah MS6 synth module. Both needed power supplies. The SST-19 was immaculate and the customer was confident that it was working (when it had a power supply, of course). You can read all about that here soon. The MS6 however, was a different story all together. The customer told me that one day it just went bang. In fact, it tripped the electricity supply to his house! After a brief conversation via e-mails, he sent me his machine requesting a complete Cheetah MS6 power supply build.
I was told that the machine was working prior to the bang but of course I had no idea if the unit had been damaged by the fault. It was a gamble and there weren't too many resources available for this thing, either. Here’s a list of useful stuff that I did find:
Readable schematics were also thin on the ground and when I did find a copy that was reasonably okay, that’s all it was, a schematic with very little other technical reference.
To speed things up in the long run, I spent a couple of hours cleaning up and organising the best schematics I could find. Hoping that my efforts will be helpful to someone else and my small contribution to the the Cheetah MS6 community, my reorganised (and readable) schematics can be downloaded here. 🙂
So, where were we? Oh yes... The unit turned up and I took off the lid. Ah! So, someone’s been in here before. Three of the four regulators had been replaced. They’d been attached to a thin aluminium strip which had then been glued to the inside of the case and I'm sorry but the soldering was atrocious! After taking the board out, it was obvious that this box had sustained fluid damage at some point in its life. Hmm… looked like I had my work cut out. Oh and the original firmware had been swapped out for the Maad KM 3.0 custom version. Sceptical at first, after some research, I have to confess that I've only heard good things about this new code. 🙂
Anyone who’s familiar with the Cheetah MS6 will be aware that the power supply is well, bloody awful. It’s not uncommon for example, for the case to go live. Yes, you read correctly. On top of that, the underrated transformers (yes, there two) are underrated making them quite unreliable and the original heatsinking for the voltage regulators is totally inadequate. In fact in the image below, you'll notice that only two of the four regulators are actually attached to the heatsink. It's just such an incredibly bad design and such a shame for a cool little analogue synth.
My initial approach to the project was to build an external box which would house two uprated transformers and then provide a suitable connection from the box to the MS6. The latter part would require a hole or two to be drilled into the rear case of the MS6 but I've done this kind of thing before on other machines like the Line 6 Pro racks (Echo Pro, Mod Pro and Filter Pro).
For the external enclosure, I chose one of my favourite, 'go-to' boxes; the Camdenboss RTM5006/16. Earthing this very substantial diecast steel enclosure would create a Faraday cage and hence, provide excellent screening ; perfect for housing the uprated transformers.
There are four regulators in the MS6, supplying +/-5V and +/-12V. Removing the original transformers was a bit tricky but once they were out, my idea was to drop in connections from the power socket which would be fitted, to the main PCB where the secondaries of those transformers connect to the board. Complimentary (balanced) voltage rails, meant there was a centre-tap for the AC feeding each pair of supplies so a 6-core cable was needed.
While that was going on, I cleaned up the PCB, getting rid of as much sign of the fluid damage as I could.
Next was the post transformer part of the power supply which as I’ve already mentioned, was a real mess. The PCB seemed quite fragile around the rectifier diodes, capacitors, etc so I had to be careful. While I was in design mode, I knocked up a drawing for a proper heatsink.
Well, I took the stripped-out case, the enclosure for the external power supply and my drawings to Lenton Engineering in Watford and a couple of weeks later, I got the call informing me that everything was ready to collect.
The transformers, switches and connectors all fitted perfectly into my cool steel external enclosure, so if nothing else, I would end up with a groovy dual-voltage AC power supply. 🙂
The heatsink also fitted perfectly which was a relief as when I was designing it, the regulators on the MS6 I had, were a bit all over the place. It took me a minute to realise that the distance between the TO-220 mounting holes was an imperial half-inch and NOT metric.
By now, I'd made the cable that would connect the external box to the MS6 so all that was left to do was to screw the connector socket to the rear of the MS6 chassis and then wire it to the main PCB.
So, all was done pretty quickly and it was test time. As usual, I cleared a big space, just in case it all went bang... again. Well, it didn’t. In fact, nothing happened at all for a few seconds. While the unit was still on, I quickly measured voltages and all looked good; +/-5V and +/-12V. Then characters appeared in the display but not as expected. All that was shown was ‘FE’ which was flashing and every now-and-then, some random stuff.
I'm aware that when the memory is empty, the Cheetah MS6 loads factory pre-sets from ROM and doesn't require re-initialisation as such, or loading of factory patches via some fancy button hold-down routine. Since I'd already removed the original battery (because it had leaked), automatic re-initialisation is exactly what the machine was trying to do. There was another issue however; the buttons on the front-panel were totally unresponsive.
After a few minutes of power, I noticed that one of the multiplexers was seriously hot, so I immediately switched off. I replaced the cooking 74138 and tried again. Wow! This time it all fired up perfectly! It still displayed 'FE' but this time successfully went into its initialisation sequence and of course, the front panel buttons were now working.
I hooked up a MIDI keyboard and attached the output to my mixer. Hey, I’ve got sound. More than that, all the voices were working. This was all quite a relief.
So what about the memory back-up battery? Well, I decided to implement my Live Forever battery mod.
This required a slight modification to the battery back-up circuitry as the original battery was a rechargeable Ni-MH. I was dropping in a lithium battery which I really didn’t want the MS6 to charge, when it was powered on!
Once that was done, I tried again. You know what? This thing doesn’t sound too bad. Of course there's better but you have to bear in mind the price bracket of this box when it first came out.
The power supply in these machines is notoriously bad. In fact, I’ll go so far as to say “it’s not a matter of if but when it'll blow”. On top of that and as has been proved by the customer of this unit, they don't need to fail 'quietly'. This is a real shame as the synth is actually really quite good with six analogue voices and even multitimbrality. It’s also a shame that the MS6 has an old-style Ni-MH battery. If not charged regularly, there’s a significant chance of this leaking… all over your main PCB!
So, if you have a Cheetah MS6 that you love dearly, please don’t hesitate to contact me regarding
- an external power supply,
- my Live Forever memory back-up battery mod
In my humble opinion, I think it's worth it but please do something before it blows up... and trips your house electrics!
UPDATE - 10th May 2022
I've now built quite a few of these power supplies for customers who just love their Cheetah MS6. As such, I've also managed to work out a price. The following is everything I did to get to a reliable and safe solution and for which I do NOT charge:
- Reverse engineering the original MS6 internal power supply.
- Reverse engineering the MS6 battery back-up facility and adapting it to take a lithium cell.
- Designing the external enclosure for the new power supply.
- Designing the cut-out required in the MS6 chassis.
- Designing a proper heatsink for the regulators.
- Selecting reliable components for the new power supply and the battery back-up circuitry.
- A considerable amount of testing! 🙂
When working on your MS6, here's what I do and what I charge for:
- Preliminary check of your MS6.
- Taking apart and dismantling the entire unit.
- Removing redundant power supply components.
- Removing back-up battery and associated components.
- Preparing and delivering metalwork of external enclosure and MS6 chassis for drilling.
- Building external power supply into steel enclosure and custom power cable.
- Installing custom heatsink.
- Modifying the battery back-up circuit and installing the new lithium battery.
- And of course, reassembling and then testing the unit.
To do the whole job (excluding metal work, of course), easily takes a couple of days and I simply can’t charge for all of that. If I did, this MS6 power supply would cost more than the MS6 itself!
So, prices for my Cheetah MS6 power supply, now start from 590 GBP. Yes, I know it's expensive but there's a lot of metalwork involved and this is done by another company which I've been using for forty years and do excellent work for me.
UPDATE - AUGUST 2023
Since posting this article however, I'm truly humbled by the number of enquiries I've received and how many of those enquiries have turned into customers trusting me with their units. In fact, after having done two or three, I decided to design a label for the power supply.
I used the term 'start' in the previous update because the price of the red diecast enclosure has gone up by 66% since I started building these. To save you a little money, I'm now offering exactly the same power supply in a black version of the steel diecast box that I used to use and still keep the price at 590 GBP. I can still do the red case but it's an additional 30.00 GBP now! 🙁
The price of the Neutrik connectors that are used, has also gone up but I'm absorbing that cost. 🙁
Please remember that Plasma Music Limited is VAT registered so VAT at 20% needs to be added to builds purchased within the UK.
Today, I've received another MS6 with the customer wanting the works. This prompted me to update this post.
I'm delighted that MS6 owners are finding this page and responding to my advice to do something before it's too late. These are really lovely machines with a classic eighties sound. Don't let them die!
UPDATE - 13th October 2022
Working by myself on things like valve amps and dodgy old synth modules like err... the Cheetah MS6, is sometimes concerning as I often run the risk of electric shock and of course, there's no one with me to assist if things go badly wrong.
I've currently got three MS6s with a forth due in shortly. The owner of tonight's machine didn't want my external power supply and after the repair, while doing the final tests, I got hit... the big ZING. Ouch! Straight off the case, no less. Yes, you read correctly; off the case. As many Cheetah MS6 owners may be aware, this is an annoying MS6 issue.
It took a while to isolate things and make the machine safe but guys, I can't emphasise just how bad the power supply is, in these machines. PLEASE GET IT SORTED!!!!!
UPDATE - 24th October 2022
Below is a picture of another of the three MS6s I have in, prior to any work being carried out. The PCB is quite different to others I've seen and I believe this one to be either a prototype or a very early version.
In common with all MS6s, it has the badly mounted regulators, only two of which are mounted to a thin aluminium strip (the heatsink). I've highlighted the reset line and the diode connection between +12V and the processor's STBY, NMI and IAQ1 ports.
This machine was a challenge and after building the external power supply, modifying the rear of the case, making one of my custom heatsinks, implementing my Live-Forever battery mod', the bloody thing wouldn't boot. 🙁
I was really careful to make sure that the flying wires were in the right place. I double checked my work and although the unit would power up, it wouldn't boot. That led me to believe it was a processor reset kind of issue.
After a bit of hunting around, I sussed out that the +12V regulator was duff. Although the digital circuitry is running at +5V, as mentioned above, processor STBY, NMI and IAQ1 are derived from the +12V line.
I swapped out the 7812 and voila, another rescued Cheetah MS6 that's going to be around for many years to come.
UPDATE - 1st November 2023
Weirdly and three years later, the first MS6 I received and which is featured at the beginning of this post, found its way back to me... without the power supply that I built for it. Read the whole story here. 🙂
UPDATE - 13th December 2023
My Cheetah MS6 power supply has proved to be a lot more popular than I imagined. I've therefore decided to include it in my on-line store: