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Battery leak in Simmons SDS7

This Simmons SDS7 battery leak occurred because the battery wasn’t charged regularly.

Recently in, was a Simmons SDS7.This is my second most favourite electronic drum systems of all time, after the Simmons SDX. In fact, you can read how much I love this instrument here.

Anyway, this SDS7 was apparently just fine a couple of years ago but when the owner recently got it out of storage, there was no sound. Powering up but otherwise completely dead, only the voice-card name illumination seemed to be coming on with nothing showing in the programmer display. Well, I had a pretty good idea what was wrong with it.

Like a lot of music technology at the time, the pre-MIDI Simmons SDS7 used NiCd (nickel-cadmium) batteries to back up its memory. Some manufacturers like Cheetah, continued to use NiCads all the way up to the end of the eighties, perhaps even a little later. Unlike lithium batteries which soon took over, NiCads need to be regularly charged. If left uncharged for an extended period, NiCd batteries die and WILL LEAK.

Fortunately, the Simmons SDS7 was built with vertically mounted PCBs. Despite the fact however, that there was also a version (like this one) which, instead of having a single battery soldered directly to the memory-board, used a triple AA battery holder (3 x 1.2V in series = 3.6V), battery acid managed to leak out over those already old solder joints, tracks and components. I soon noticed for example, that the C̅E̅ line was down and in fact there were several other issues preventing code from running and therefore not allowing the machine to boot.

There was also, a fair bit of damage to tracks on the rear buss-board (marked up as 'SDS7 BACK PLANE'), which couldn't be resolved by cleaning alone and which required some hot-wiring. Annoying but to be expected.

Battery acid damage on Simmons SDS7 rear buss PCB
Although the memory-board was okay, some tracks were badly damaged on the rear buss PCB. The one highlighted in red had completely perished and only the white trace was left.

Above you can see that I'd already jumped one track but there were another eight which although buzzed out okay, I really didn't like the look of. One in particular looked as though it had completely succumbed to battery acid.

Technical literature on really old machines like the SDS7 is thin on the ground. Techs often have to make do with copies that were scanned many years ago on what would now be classed as, low-resolution scanners. I therefore, decided to make myself a little reference diagram.

Simmons SDS7 connection reference

Unlike the image above which is a standard 1920 x 1280, the original pdf document is pretty big but without going to that kind of trouble, I'd be shooting in the dark. In the end, I actually decided to install my wires around the back of the back plane PCB.

You'll notice in the two images above, that the polarising key in the CPU card edge connector (P13) is somewhere where it shouldn't be and that the key in the edge connector to the right (that takes the memory-board) is missing altogether. In fact, there was a polarising key on one of the voice-cards that was also out of place and so the card didn't work properly.

Anyway, while I was buzzing things out on the back plane, I carried on cleaned up the memory-board.

Battery acid damaged Simmons SDS7 battery holder
The battery holder can't be salvaged but I have other plans anyway. I've made a start on cleaning up the memory-board. You may notice that I changed the two grey battery wires. They are now colour-coded!

When I say "cleaning up the memory-board", it actually took several days to buzz out connections and hot-wire tracks that had been damaged by battery acid. After all that, this SDS7 finally managed to consistently boot but there was another problem; the data encoder was unresponsive. Grr... The problem turned out to be a simple but very annoying, dry joint under a buffer that reads the signal off one of the optical receivers. Phew...

My own SDS7 is serial number 71722. The serial number on the back of this one was #70320. If sequential, that's 1,402 units difference. More than just a batch or two!

Indeed, as I went over the insides of this SDS7, it became apparent that it was quite an early version. It had the optical encoder as opposed to the electrical contact type. The polarising keys in the edge connectors on the back plane were the small bits of black plastic type, instead of the yellow clip types used later. There were also various wire jumpers on the CPU-board that aren't on later revisions.

On a side note... There was also some work to do around the back of the unit as the socket pins although not corroded (because they can't), were very black and so needed a good clean.

Shiny socket pins on Simmons SDS7
The XLR pins on the audio outputs were black so needed a good clean. I think I'm also going to have to do the XLR receptacles on the pad inputs.

The lid was smacked in at the front and was also badly scratched so after several days of patient straightening with a combination of heat, G-clamps and weights, I sent it off to be refinished at a local company .

Straightening the top-case on Simmons SDS7 SN70320
Removing the creases on a piece of metal like this is a time consuming task but it'll be worth it. This SDS7 is otherwise in pretty good cosmetic condition.

So, back to batteries... Amongst instruments like the afore mentioned Cheetah MS6 and true vintage gems like the Sequential Circuits Prophet V, a Simmons SDS7 battery leak is something I've seen quite a few times over the years, or should I say decades. Don't think that everything can be fixed. It took me two years to concede that the last Prophet V I had in with a leaked battery wasn't salvageable. It's incredibly sad and something that would potentially 'brick' an iconic piece of vintage music tech' so please take note: If you have a piece of old gear like this, do check what kind of battery it has. If it's got a NiCd battery, then make sure it's charged regularly.

! ! !  WARNING  ! ! !

Floating around on the Internet, is a post of someone who's substituted a NiCd battery with a lithium battery in (coincidentally), a SDS7. PLEASE DO NOT DO THIS!

Memory retention is designed for specific batteries. The chemistry of a lithium battery is very different to that of a NiCd battery. For a start, the former can't be charged and if you try to do so, you may end up with getting more than your fingers burnt... literally.

For over thirty-five years now, equipment has been designed to take batteries like the CR2032 which we're all familiar with and which outputs 3.0V. Older NiCd batteries are rated at 3.6V. That 16.7% difference can sometimes be significant.

You should also double check when buying batteries as many unscrupulous or perhaps just ignorant retailers advertise NiMH (nickel metal hydride) as NiCd and lithium-ion as lithium. In all fairness, sometimes it's just not clear. As we all know, a Google search for example, will often throw up results that are close to your search term so please check before you buy. Yes, I know it's frustrating, especially when NiCd batteries are becoming increasingly more difficult to procure but please don't be tempted to take the easy option. Batteries are all different. Even those that can be charged, have different charge rates, depending on their respective chemistry. Charging circuits for NiMH batteries for example, are usually designed to measure the status of the battery, thereby dynamically reducing the current that's being fed into them. This is NOT the case with NiCd charging circuits. If you substitute NiMH into a system that normally takes NiCd, then you stand the risk of overcharging the battery. This might not be disastrous but will reduce the lifespan of the battery.


It's quite difficult to find information on the Simmons SDS7 and so I thought it prudent to offer some useful links for those interested.

For a start, if you're in the US, you need The Simmons Guy on your side. Ed is not just knowledgeable and experienced, he's methodical, tidy and VERY helpful. Just one thing; because of all that, he's amazingly busy!

Ed (The Simmons Guy) is also very active on social media and it's worth checking out his Facebook page and his YouTube channel. In fact, in amongst his videos, you'll see my own SDS7 #71722 before I bought it from him, LOL. Ed, where on earth do you get the time?

This site has a lot of really useful SDS7 information and may be of help to those who just need a few facts.

And of course, there's the Simmons Virtual Museum which also has a few useful resources.

UPDATE - 30th May 2024

Since writing this article, I'm delighted to advise that the SDS7 featured above, is now fully up and running! 😀

It took a while and as I focused my attention on the memory-board, actually there were a couple of tracks on the back plane that slipped my eye. Grr...

Since posting however, I've had a few more older items in with completely leaked out batteries.

Three very old NiCAD batteries

If you think they look bad, then you wouldn't want to see the insides of the machines that these all came out of! One of them had to be scrapped as it was totally unsalvageable.


Simmons Electronics Plc

I often get asked, which part of my carrier was my favourite. Well, to be honest, if you’re into music, it’s a bit like someone asking which is your favourite band or song. The seventies and the eighties were very exciting times. As an electronics engineer wanting to leave defence and branch out into music technology however, the eighties were truly magic. Indeed, my time at Simmons Electronics Plc in particular, well "kid in a candy store" comes to mind! Fast, hi-tec and an amazing bunch of people, Simmons was one of the many cool electronics companies around, uniquely being the only British electronics manufacturer at the time, to have an office in Tokyo.

What used to be the old Simmons factory in St. Albans
What used to be the old Simmons factory in St. Albans (picture taken September 2021). Above the main entrance / reception area and at the top of the stairs (to the left of where it says 'Marshall Volkswagen') there used to be a massive Simmons drum kit.

Within a few months of starting with the company, I was very familiar with machines past, present and future and working on them daily wasn’t always enough. I ended up buying a SDS9, a SDS7, an EPB, a couple of SPM 8:2s and I even bought a Silicon Mallet. And then, in 1987, 'we' released SDX and I just had to have one! Fully expanded (and costing a small fortune), SDX was capable of holding a whopping 8MB of RAM. Well stupid me, I decided that 8MB wasn't enough for drums and cymbals so I ended up buying two fully loaded SDXs, both complete with pads.

My two Simmons SDXs
Here are my two Simmons SDXs from many years ago, in a previous (life) studio. I still have them and hope I can get them back into my studio soon.
Simmons ZI pads for the SDX used FSR technology
And here's an old snap of my 'small' kit. This expanded to double-kick, a few SDX ZI cymbals and a lot more toms. Geesh... What was I thinking? I'm not even a drummer!

Young and stupid (I think I've confessed to such already), I sold my SDS7. I can’t remember why but it might have had something to do with buying the two SDXs which if memory serves me correctly, started at 13,000 GBP each for like the basic version which only had 2MB of memory and fewer pads. Little did I know that years later, selling my lovely SDS7 would be a decision I'd regret and so my long quest to find a really nice replacement began.

In recent years, I'd regularly check out the usual auction sites and scour the Internet looking for a nice SDS7. Well, over thirty years after selling the one I bought while at the company, I finally found one that looked too good to be true.

I'd heard about a Simmons tech in the US called The Simmons Guy and the unit I found was one that he was selling. That in itself, immediately made me feel comfortable as from what I've read, this dude seriously knows his stuff. The thing is that apart from being totally functional, this particular example, looked in really good condition.

So, on 11th December I bit the bullet and today (20th December) my new Simmons SDS7 serial number 71722 arrived.

I was recovering from my first bought of COVID but I simply couldn’t contain my excitement. It took me a while to get through the superb packaging. Of course #71722 is a US unit and therefore the transformer is wired for 120V operation. In such incredible condition, I reluctantly opened the unit and converted it to UK 240V, which included stepping down the fuse.

Simmons SDS7 arrived just as I came out of COVID
My new Simmons SDS7 on the bench and powered up for the first time and after its conversion to UK 240V.

The Simmons Guy is 'Ed Rose'. We got to know each other pretty well, while I waited for my new Simmons SDS7 to arrive. Such a lovely dude, he threw in so much with this unit including a beautifully discrete Tubbetec uniPulse MIDI conversion on a 3.5mm socket, loads of EPROMs, a patch-changer and many internal tweaks such as EPROM size selectors on each voice card and the hi-hat fast decay mod'.

Three Simmons SDS9 pads
These lonely SDS9 pads have been on the landing of the new studio for months. It's like they've been waiting for something...

Launched in 1983, the SDS7 didn't have MIDI. In fact, the first MIDI keyboards, the Roland Jupiter 6 and the Sequential Circuits Prophet 600 only appeared that year and even the MIDI Manufacturers Association wasn't established 'till 1985. Indeed this was a very exciting time.

Fitted with a Tubbutec uniPulse MIDI to CV converter which I have to say, has been done quite discreetly, #71722 can now be triggered via MIDI IN. Even the hi-hat open / hi-hat closed can be controlled via MIDI controller 18.  Tubbutec supplies a configurator for Windows and OS X. With no MIDI Out though, things are a bit 'one-sided'. Having said that, the uniPulse hardware is fitted with a momentary switch which allows the system to 'learn' (and memorise) the incoming MIDI channel. A very convenient touch.

1983 Simmons SDS7 with Tubbutec UniPulse MIDI to CV
1983 Simmons SDS7 with Tubbutec UniPulse MIDI to CV.

All trigger inputs and audio outputs are on 3-pole XLRs. In the case of the audio outputs, one might be forgiven for thinking that they're all balanced. Unfortunately, that's not the case. In fact, being high-impedance as well, I've wired things to accommodate a couple of active unbalanced to balanced converters between the SDS7 and my Yamaha DM-2000 mixing desk.

My new Simmons SDS7 in the control room
I would normally have got this racked up but #1722 is in such amazing condition that I decided it should have pride of place.

I've also hooked up the SDS7 to a TP-Link Kasa smart power outlet and a schedule has been created which powers on the SDS7 every Monday at 12:00 mid-day for a couple of hours. That way, the NiCd battery receives regular charging.

My new Simmons SDS7 in its new home
Simmons SDS7 #1722 in its new home.

At some point, I might consider modifying the memory back-up battery circuitry so as to take lithium batteries, similar to the modification I offer as part of my external power supply package for the Cheetah MS6.

What the google hook-up also means however, is that I can say "Hey Google, switch on my SDS7" and guess what happens! 😮


How practical will my new Simmons SDS7 be for contemporary music production? Who know! Who cares!! What I do know is that I will be using it and I'll absolutely love using it. Yes, it's a huge chunk of mega nostalgia for me but I'm a firm believer in keeping the creative process fun and that's totally subjective.

UPDATE - 28th December 2023

I'm totally over COVID and #71722 is now MIDI'd up to the rest of my studio. For the time-being, stereo outputs are going directly to my Yamaha DM2000 and I AM SO HAPPY!!!!!

My new Simmons SDS7 responded immediately to MIDI and sounded just as I remember back in 1986 (ish). Oh boy! I'm gonna have some fun with this. 😀

NOTE: While I wait for my active unbalanced to balanced converters to arrive, the stereo outputs from the SDS7 are running unbalanced to my DM2000 and I have to admit that I can't believe how quite this box is. While I don't recall having any noise issues with my original SDS7 back in the day, users do refer to a noise which sits at 7.3 kHz. This particular SDS7 however, is dead quiet. The DM2000 meters will show up failing chorus chips in something like a Roland JX-8P which might not be immediately apparent on the monitors and yet the unbalanced audio outputs from my new Simmons SDS7, aren't showing anything when not played. To be brutely honest, I didn't expect that, especially from a machine that's forty years old.