Mike Walden's replacement CPU board for the Simmons SDS6 is an amazing rescue resource for one of the rarest eighties music machines ever! On my recommendation, one of my customers bought one of Mike's boards as soon as they were available. Here's the story...

The Simmons SDS6 was an analogue sequencer with eight trigger outputs and a hi-hat open / closed output, all intended to drive systems like the Simmons SDSV and SDS7. Of course the trigger outputs could be fed into many electronic drum systems of the time.

Simmons only made a hundred and ninety SDS6 sequencers. It was a beast at the time and by today's standards, is positively over the top.

Famous for its massive LED display, the first revisions (yes, there were several, even for a hundred and ninety units) only had tape sync and didn't include MIDI. Memory back-up was via the expansion port at the rear and users looking to copy / back up data had to do so via Simmons' proprietary Memory Expansion Pack. Incidentally, my 'Advanced Memory Expansion Pack' (AMEP) works just great with the SDS6.

Like many machines of that era however, the SDS6 used Ni-Cd batteries for memory back-up and if not charged regularly, these damn things leak and badly. If not caught quickly, the result is more often than not, a bricked Simmons SDS6.

For me, things started about this time last year when a customer turned up with a Simmons SDS7 and a SDS6, neither of which was powering up. As it turned out, the memory back-up batteries in both machines had long expired and had badly leaked.

It took several months to repair the SDS7 but the SDS6 was a lost cause. Having only made a hundred and ninety units, seeing one in this state is quite upsetting, to say the least.

After a few weeks of trying to repair the damage, I was forced to draw a line as it became obvious that my efforts were quite futile. I proposed to my customer, that I design a replacement CPU board.

Well, round about the same time, a certain Mike Walden had recently purchased a SDS6 which by the sounds of it, had suffered the same fate. Mike had the same idea. After having spent most of 2024 designing a whole load of bits and pieces for the Simmons SDS7, I really didn't have the time to take on something like a whole CPU reverse engineering project. Mike sounded keen so I left him to it.

Reverse engineering something like this is a mammoth task but you also need to bear in mind that there are very little resources on this stuff, out there. On top of that, there are technically two types of SDS6; the standard trigger output version and the MIDI version. Both were substantially different. Mike's failed SDS6 was a MIDI version which was great for me or rather my customer, as his was a MIDI version, too. Hey but more about that later.

On 27th April 2025, Mike announced on Simmons Drums Users - Past and Present (private) Facebook group, that his project was finished and that his replacement CPU board for the Simmons SDS6 would be available on ebay.

As soon as Mike's replacement CPU board for the Simmons SDS6 was on ebay, I telephoned my customer giving him the ebay item number. Yes, it was the middle of the night but this was a big deal.

Usually, the minimum production quantity for PCB projects is five. Mike had four boards available on ebay so I'm guessing that one of them ended up in his own SDS6. As you can see in the image below, I'm also guessing that the one Peter bought is, well... No. 2!

Peter immediately bought one of Mike's boards and then contacted me again to arrange drop-off.

The next day, Peter drove down to Hemel Hempstead from Cambridge and I simply couldn't wait to see Mike's board. Packaged extremely well, there were a few anxious moments before all was revealed.

On first sight, the first thing that went through my head was complete and utter respect. I'm an electronics design engineer and all I could think of were the hours of work that must have gone into this. "Mike, this is awesome!"

If I'm brutally honest with myself, then I must confess to feeling slightly envious. I really wanted to take on this project myself but you simply can't do everything and at the time of writing, there was no way I could commit to this. Again, Mike did well to make the time for such an ambitious endeavour.

Anyway, a few days later and I decided to pull out Peter's SDS6. Oh boy. Memories of trying to resurrect this came flooding back. 😒 Not good but I'm glad we decided to keep this.

Before attempting to fit Mike's replacement CPU board for the Simmons SDS6, I decided to check the PSU voltages. Everything was fine so let's go.

In his ebay listing, Mike warned that there may be slight differences in mounting between various SDS6s, something I'm used to and not just with Simmons gear. Mike's board wouldn't quite 'drop' into Peter's SDS6 and I would suggest that if Mike's going to make more of these, perhaps enlarging the mounting holes might be a consideration. As it turned out, I drilled out one of them from about 3.5mm, to 4.5mm. After that, the board fitted without requiring any nudging or force.

The power on reset (POR) circuit in the original SDS6 looks as if it was an afterthought. Electronically similar to that in the SDS7, the POR circuit in the SDS6 is on a separate and very small PCB that's secured to the centre screw on the CPU board. Four cores of ribbon cable connect power, 0V, unregulated +8V and RESET to the CPU board in anything but an elegant manner.

Mike tidily incorporated the POR on to his design, a pin being made available for the connection to the pre-regulated voltage from the PSU. I ended up removing this pin and wiring the connection directly to the board but from underneath.

By the looks of it, Mike chose to include some type of battery charge monitor on his board. Comprising very small SMDs, there's nothing I can identify but D7, Q1 and U25 PCB references are a bit of a giveaway for a diode, a transistor and an IC. A status LED is also present but strangely, it isn't a surface mount type. A key above the battery pack offers reference to what the LED would indicate.

Initially, I found the inclusion of this circuit somewhat curious for two reasons:

• I’ve never seen a SDS6 with issues resulting from component failure. Every faulty SDS6 I’ve ever seen has suffered a battery leak. It seems strange therefore, to continue using rechargeable batteries.
• There is no way to observe the battery charge status LED without opening up the SDS6.

The first point can be immediately dismissed. Instead of the original Ni-Cd batteries, Mike's design uses nickel metal hydride (NiMH) types which shouldn't leak! Excellent decision, Mike.

So let's continue...

After the afore mentioned and partly expected minor adjustment, the board slipped in nicely, well almost. On this particular SDS6 (#178), there was very little gap between the power supply PCB and the new board. Perhaps Mike could have got away with shaving off a few millimetres.

I'd already connected unregulated +8V from the PSU to the POR circuit directly. All other connections had been secured, checked and double checked, so let's switch on and see what happens...

Wow! The SDS6 powered straight up and seemed to boot. The battery status LED was on so I figured I'd have a cup of tea and leave the batteries to charge. Good so far.

While I had my tea, I thought it a good opportunity to familiarise myself with the operation of the SDS6. After all, it's been a few years since I've seen one of these. Hmm... I think I might need more than just the one cup of tea. Those batteries and that SDS6 user manual aren't going to happen quickly.

After my break, I came back to SDS6 #178. I switched off, closed the lid and powered up.

Cycling through several functions, everything seemed to work just fine. I then connected my test SDS7 and proceeded to enter a simple pattern. The once totally dead SDS6 played the SDS7 just fine. I didn't test the hi-hat open / closed function and neither did I test the tape sync but the fact that this SDS6 is triggering a SDS7 seemed quite a result to me.

CONCLUSIONS

Yes, my admiration for what Mike Walden has done should hopefully be obvious by now but somehow it doesn't seem enough? Mike's board is amazing and loaded into SDS6 #178, it works. I've also previously mentioned that I fully appreciate the effort that can go into a project like this, especially when there are so little resources available. In fact, I now need to ask my readers whose native tongue is not British English, to forgive me as slip into a little colloquialism and refer to Mike’s work as stonking (an adjective used to emphasize something impressive, exciting, or very large).

I was a bit concerned about the continued use of rechargeable batteries but  NiMH is a good compromise and I fully endorse Mike's decision to do so.

Mike had five boards made and I don't know if he'll be making anymore. At the time of writing, the four that he put up on ebay had all gone.

There are nine link options which in theory, will allow this board to work in non-MIDI SDS6s. Mike told me that the link options emulate the modifications that appear on the schematics advising on non-MIDI to MIDI conversion. A nice and very forward-looking touch, Mike. Unfortunately, Mike doesn't have the firmware for these early SDS6 versions and on top of that, it would be good if he could validate his board. In fact, I'm aware that Mike's put a call out to borrow a non-MIDI SDS6, just to confirm operation. In his ebay listing, he also advised that those interested in this board for use in a non-MIDI SDS6 should contact him prior to purchase. If Mike's request for a guinea pig non-MIDI SDS6 is successful, then I'm assuming that this isn't the end of the story.

With less than two hundred SDS6s made and more than forty years on, one wonders how many are still in existence. I do however, hope that Mike decides to release a few more boards.

Many will be aware that I'm ex-Simmons and once in a while, Dave and I actually manage to catch up with each other. Dave consistently expresses his delight at the work I and others are doing to keep what he started back when it was really exciting, alive and kicking decades later.

NOTE:

• This replacement CPU board for the Simmons SDS6 is NOT my board. I had no hand in the design, manufacture or testing of the main item that's featured in this post.
• I do NOT sell these boards. Neither do I offer any technical support for these boards.
• Initially, Mike had five boards made. I have no idea if he'll be making any more.
• Whilst the pricing of Mike’s board was amazingly acceptable, I do not know if he’ll maintain that, should he release more boards.
• It would be most impractical for Mike to cater for all possible variations in SDS6 design. Hence, if Mike makes more of these boards, then I think it only fair to point out that purchasing and installing one is done entirely at your own risk.
• I won’t be publishing or distributing Mike’s contact details so I guess that the only way to keep up to date on further developments, would be via the Simmons Facebook group quoted earlier.

Just before Christmas, a customer brought me a loft-find Simmons SDS9. It was in pretty good cosmetic condition but didn't boot properly. With the start of the New Year, this Simmons SDS9 repair became my first of 2025.

Having been told that this was a 'loft-find', I figured that the battery should be the first thing I should check out. Yep, it was totally dead and the usual white stuff had started accumulating around it.

The customer got lucky though and there wasn't too much damage. Once I'd cleaned the area around the battery and replaced the original Ni-Cd device with a lithium CR2032, I reinitialised the unit as I powered up for the first time.

Okay, good news... it booted. We'll see how much of this SDS9 works but first a few words on that battery change...

I normally replace a Ni-Cd with a 3.6V lithium battery when doing the Ni-Cd / lithium conversion thing. On this occasion, I didn't. The reason I went for a more conventional solution on this Simmons SDS9 repair, is that since the main-board is secured to the top-case, components up, there's not enough room to squeeze in a CR123 or ½AA battery holder and battery. Neither did I want to mount the battery to the bottom case as it would have to be unplugged for maintenance, thus forcing the SDS9 to lose all of its pre-sets or kits as they used to known. Makes sense, I guess... drums, kits...

IMPORTANT - You CANNOT just replace the Ni-Cd with a lithium battery. Remember that the Ni-Cd is charged when the unit is powered on. If you don't modify the circuit, the same will happen with the lithium battery and that's something you really don't want to do!

With battery surgery complete, I then discovered that all the switches had issues and were either sticking, double triggering or not triggering at all. To be able to access various functions, that was the second thing that needed sorting out.

After removing the switches from the board, they can be easily dismantled and the metal spring contact and terminal contacts can be cleaned. Yep, they were certainly grubby!

The SDS9 didn't use encoders for data entry. Instead, analogue voltages were derived via potentiometers so while I was at it, I cleaned them too.

After reassembly, the switches worked perfectly and the integral buttons looked just a little newer. Other issues with this SDS9 soon became apparent, however:

• The boot time was much longer than it should be (I actually noticed that before) and once in a while, it would freeze kind of mid-boot.
• There was more than the usual amount of hum on the audio outputs.
• The low-tom wasn’t triggering from the pad input but was working fine via button-tap and MIDI and the low-tom sound was playing in demo mode.
• The rim and intermittently, the high, mid and low toms were self-triggering, like machine gun style. The frequency (tempo, if you like), of the self-triggering would reduce over several minutes and eventually it would die down to nothing.
• Intermittently, triggering the snare / rim would also trigger the low-tom albeit with a low velocity. Triggering the low-tom didn't trigger either the snare or the rim, however. This thing was one-way and would also settle down after several minutes.

The customer told me that he reckoned this SDS9 had been in his loft for at least fifteen years, if not nearer twenty. With cold and damp winters and humid summers over here in the UK, I was surprised that the case hadn't at least, started to rust. I wasn't surprised that there were problems with the electronics, though.

So, I decided to replace all the electrolytic capacitors on the power supply. Actually, while I was at it and since there weren't too many electrolytics on the main-board, I changed them, too. If looked after and regularly used, the recap might not have been necessary but remember that this was a loft-find.

Now that the unit was booting as it should, I started to look at the other problems.

The self-triggering gave me the impression that capacitors were charging / discharging and so I checked out the schematics for some caps which I thought could be responsible.

Do consider this, however...

The term 'IC' stands for 'integrated circuit' and we can assume (as if we didn't know already), that most chips (ICs) comprise various components, including capacitors. So, while checking out the discrete capacitors, one shouldn't dismiss ICs.

Indeed, there are lots of (ceramic) 1nF capacitors all over the place and so I changed them, like all of them, not just those associated with the self-triggering channels.

On the pad trigger inputs, there's a simple RC pull-down network attached to the inverting input of the first op-amp. Virtually all SDS9s I've seen however, have the resistor and capacitor on the mid-tom pad trigger input, wired the opposite way around to those on all the other channels. Instead of being 'RC' it's 'CR' if you know what I mean. I can't remember why this was done but space is tight here, if you want to wire them 'correctly' so perhaps the reason was that simple... space. Anyway, when I get a SDS9 in, I always put this right.

After swapping out a couple of TL084s (yes, they can and do indeed go screwy as well) , I replaced IC19, the 4051 multiplexer that passes triggers from the 8031 processor to the channel trigger inputs.

By now it was time to reattach the main-PCB to the power supply and jack-board and fire up this thing.

The SDS9 was now booting as expected and consistently. The excessive hum on the outputs was gone. The self-triggering on the rim had gone too but the toms were still giving me a headache and I still had that weird snare / rim to low-tom crosstalk. 😒

There was one more component that often causes problems but which I was hopeful, I didn't have to change and that's the 4067 (IC50). Like the 4051, this infamous multiplexer is also a common source of issues in all kinds of gear from the same era as the Simmons SDS9. The reason I didn't want to change it was simply because unlike the 4051, it's quite difficult to procure now-a-days.

Scoping the trigger lines going into the 4067, I noticed that triggering either the snare or rim, also produced a pulse on Pin 4 (the low-tom channel) of the 4067. This confirmed a crosstalk issue between the snare / rim and the low-tom. Oh dear! There wasn't anything weird going on between the TL084 trigger outputs and the 4067 inputs. The connections were good so this suggested that there was indeed a problem with the multiplexer itself and I really didn't want that to be the case. 😡

After having checked for short-circuits, I tested all the other channel inputs into the 4067 and everything was clean.

As well as multiplexing the channel triggers, the 4067 also multiplexes the voltages off the potentiometers.  If you have a SDS9 with potentiometers that are starting to act a bit weird, there's a good chance that the 4067 will be the root of the problem.

So back to the Simmons SDS9 repair...

Wow! That's more like it! Swapping out the 4067 resolved the self-triggering issues and the snare / rim to low-tom trigger crosstalk.

Now that I had a 'silent' SDS9, I looked at the duff pad trigger input on the low-tom. That was down to a broken trace between one of those 1nF capacitors and the associated TL084.

With a normal boot period, no self-triggering, all pad inputs working and no channel crosstalk, all seemed good but this Simmons SDS9 repair wasn't quite over just yet...

There was a really weird thing going on with this Simmons SDS9 repair. The high and mid toms didn't sound right. It seemed like there was noise missing from the high-tom and the pitch of the mid-tom was higher than I thought it should be.

I went into program mode and levelled the toms so that they all sounded identical. This wasn't a simple case of pushing over the pots to one extreme, though. Indeed the tone pitch of the mid-tom had to be tweaked so as to be the same as the high and low toms. The filter pitch of the high-tom also had to be tweaked so as to be similar to the mid and low toms. In fact, the lack of 'noise' on the high-tom was actually down to the high-tom filter not opening all the way. To be honest, I did have a hunch about that.

Each of the SDS9 tom signals passes through a CEM 3394 and it was common practice to socket these ICs. This was good because I was able to swap them around and quickly eliminate them as the cause of the high and mid tom issues. I was now able to focus my efforts further upstream.

Each CEM3394s is fed from a pair of LM13600 dual operational transconductance amplifiers (OTA). This makes conventional fault diagnostics a bit challenging as you can't monitor currents on an oscilloscope. The required technique involves a lengthy process of elimination, until such point that you're nearer 100% sure that the respective OTA is faulty. So, that's exactly what I did but... I wasn't convinced.

I sent a WhatsApp message to my friend and Simmons Vintage Technical Network colleague, Michael Buchner, telling him about my Simmons SDS9 repair and how it's almost 100% working now, except for these two annoying issues. He got straight back to me asking if by chance, the firmware was version 6.0. "Yes, it was." I confirmed. He immediately recommended that I downgrade to version 3.0. Seriously? I didn't understand how a firmware bug could manifest itself by producing such an obvious couple of symptoms.

Well, believe it or not, downgrading the firmware fixed the weird tom issues. The bad filter response on the high-tom and tuning error on the mid-tom were suddenly gone! Perhaps there was a calibration error in version 6.0 firmware. Who know, who cares! Michael's recommendation worked!!

I was at Simmons during the SDS9 period but I must confess to not having any recollection of this issue.

WOW!! Thanks, Michael.

So let's see what got fixed with this Simmons SDS9 repair:

• Battery leakage cleaned up and SDS9 converted to lithium battery back-up.
• Bad switches and potentiometers cleaned.
• Slow boot up fixed.
• Excessive hum fixed.
• Rim and toms self-triggering fixed.
• Low-tom pad trigger input not working fixed.
• Snare / rim to low tom crosstalk fixed.
• Bad calibration of filter pitch on high-tom fixed
• Bad calibration of tone pitch on mid-tom fixed.
• Sad customer is now a happy customer! 😎

To be honest, I really quite enjoyed working on this one.

Despite having been used extensively by many named artists, the Simmons SDS9 never acquired the same legendary status as other Simmons systems like the SDSV and SDS7. It's a shame because it's actually very good.

I guess Simmons decided to continue with the SDS7 idea of mixing digital samples with analogue synthesis but being a bit choosy how they did that, thus making the SDS9 more affordable.

The kick is generated in software. At the other extreme of technology of the time, the toms are classic (Simmons) analogue generated. Well, that's not quite true. They're digitally generated but pass through analogue filters which changes the sound and feel considerably. The snare and rims are sample-based making the SDS9's sound sources, as diverse as they could possibly be in 1985.

The end result was really very good and as I said, the SDS9 is quite an underrated e-kit. The sounds are classically 'Simmons' with punch, analogue 'fatness' and yet retaining clarity and definition.

SDS9 die-hards will notice that this example has blue knob caps on the volume controls as well as on the parameter controls. To the best of my knowledge, everything is original so how those knobs ended up not being grey like the pad input sensitivity controls (and like other SDS9s), I don't know.

A LITTLE SIMMONS SDS9 STORY

Back in the summer of 1987, I took a week's break with a whole bunch of friends, to the Isle of Wight. Halfway through our week, we all drove up to Hammersmith as having been involved with Def Leppard, I didn't just have tickets but I also had backstage passes. As soon as the show ended, we made our way backstage. Rick Allen clocked me, pulled me aside and asked if it sounded okay out front.

"Hang on a second. Rick Allen, the drummer from Def Leppard is asking me if it sounded okay?" WTF?!?!?!?

How do you answer a question like that from Rick Allen? The man who had been through so much but who was determined to carry on and carry on he did!

Anyway, Rick was using a SDS9 and WOW! What a show. I'll never forget it.

ON A VERY PERSONAL NOTE...

For me, Def Leppard was not just another rock band. My connection with the band went way back, prior to my time at Simmons. In fact, I was a Phil Collen fan when he was in Girl and for a brief period in like 1981 (or '82), I had Pete Willis' famous white Hamer guitar as, while a student, a mate of mine lived in a flat above the Def Leppard crew.

The guitar went up for sale for only 350 GBP. I say "only 350 GBP" but as a student in the early eighties, I just didn't have that kind of money. The other guitarist in the band however, did and Neil bought that amazing guitar.

Neil had multiple sclerosis and sadly passed away many, many years ago. As for Pete Willis' Hamer, who knows what happened to that! 😢

As the eighties progressed, I have to admit that Def Leppard was very influential in the development of my own musical and production styles, my guitar sound and playing technique and even my song writing.

Today I look at the pictures, watch the videos and play those records and a sense of that ol' wide-eyed magic washes over me, as it did back in the day, forcing a nostalgic tear to run down my cheek.

Today, a simple Simmons SDS9 repair can seriously take me back and if I ever get around to writing an autobiography, Def Leppard would have to feature in it.

UPDATE - 5th March 2025

When the customer came to collect, we got talking and I mentioned how Simmons made available a ZIF socket set for the SDS9, the idea being that swapping out EPROMs would be quick and simple. It comprised three ZIF sockets and a plastic EPROM compartment cover which replaced the original steel factory cover.  The replacement cover was raised so as to fit comfortably over the EPROMs.

Well, Simon thought it was an awesome idea. Only problem was that the kits are now considerably more rare than a nice condition SDS9.

Anyway, I thought I'd help him out as best as I could and fitted some rather smart ZIF sockets.

UPDATE - 30th March 2025

Late last night I put the finishing touches on the second SDS9 I had in this year and which I briefly mentioned earlier in this post.

The customer asked for a full service including the following:

• Replace all electrolytic capacitors.
• Supply ROM A EPROM with factory 'RIM A'.
• Bass drum trigger input not working.

Not exactly an extensive list and after a thorough examination, I advised the customer that the electrolytic capacitors didn't actually need replacing and that I couldn't find anything wrong with the bass drum trigger input.

Adrian had also given me a SDS1000 which he said (also) had a non-functioning bass drum trigger input. I wonder if things got a bit confused. What I did discover on the SDS9 however, was that the rim trigger input wasn't working and that the entire rim channel wasn't producing any sound.

I quickly sussed out that the non-functional rim trigger input was down to a wrong value capacitor. It took me a little longer to sort out the rim sound.

A little known fact about the SDS9 is that the rim technically has two voicings which can be output simultaneously and from different physical outputs.

• the 'processed' rim is mixed with the snare and is output via the tip pole on the SDS9 snare audio output.
• the unprocessed rim is available on the ring pole of the snare audio output.

So what do I mean by processed and unprocessed?

Well, the processed rim passes through the CEM3372 with the snare but the unprocessed rim is basically the raw sample, directly off the EPROMs.

Anyway, there was raw rim being output so the problem was either with the CEM3372, the TL084 that supplies the CV or both. Yep, you guessed it, both! 😕

I managed to convince Simon, the customer of the first SDS9, that I should correct the colour layout of his unit's knob caps. The knob caps on the second SDS9 were seriously faded, like the grey caps were brown and the blue caps were almost green. Hence, both units got a mild cosmetic make-over.