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Yamaha RX5 Repair

The Yamaha RX5 is one of my favourite drum machines and when I acquired an example that wasn't 100% working, A Yamaha RX5 repair definitely seemed on the cards at some point in the future.

Every once in a while, I receive a mysterious parcel, sometimes from as far away as Japan. It took me a while but I eventually sussed out that my ex-frontman / singer, Rob would occasionally get the urge to buy me something. Anyway, a couple of years ago, one of those parcels contained a Yamaha RX5 drum machine (yes, you read correctly) that indeed had come all the way from Japan.

Being a die-hard Sisters of Mercy fan and like many, wanting to get those big Doctor Avalanche drum sounds, it was quite obvious where Rob was coming from. He'd given the game away, LOL! 😀

While much appreciated, the RX5 wasn’t in great cosmetic condition and outputs 10, 11 and 12 were dead. This wasn't a pad problem as I was getting MIDI. It was defiantly a problem with the outputs. Despite the issues however, this RX5 came in really useful for testing my Nebula balanced outputs jack-board for the Roland MKS-70. I made a special lead that connected four outputs from the RX5 to the audio inputs on Nebula, where the outputs from the MKS-70’s voice-boards would go. It worked like a charm but to be honest, I would have preferred to have a Yamaha RX5 in my keyboard room.

So, as you do, every once in a while, I’d check out the Yamaha RX5 action at the usual on-line places. Well, a few weeks ago, a RX5 popped up. Cosmetically in great condition it did have a fault and so the auction started at 50 GBP. To cut a long story short, I ended snapping it up for 108 GBP. On top of that, collection was only twenty-five minutes away from where I live. GEEEERATE!!!!!!

Yamaha RX5 on ebay
My patience paid off and I finally found a nice condition Yamaha RX5 for a very reasonable price.

When I went to pick up the unit, the seller showed me the fault and indeed as described, it simply wouldn’t boot properly, with garbled characters on the display and randomly active and sometimes blinking LEDs.

Having sussed out that the fault on the RX5 that Rob bought me, was down to a couple of faulty op-amps on the jack-board and that the fault on the unit I’d just bought was on the main CPU-board, I did the obvious thing and just swapped out the main-board in my machine with the one I’d just bought. Okay, so now I have one pretty good condition, working Yamaha RX5. That can now go upstairs into the studio. Yippee!

I put the main-board from the unit I’d just bought into the case of my old unit. Of course, it still didn’t boot properly.

Before switching on loads of test gear, I always do a quick visual inspection. If the item in question will switch on, then I switch it on. On this occasion something was definitely not right as I noticed that an analogue-to-digital converter was getting rather warm. In fact, after half-an-hour or so, it was about 50°C while adjacent devices were only a little over ambient temperature.

Normally associated with the DX range, the Mitsubishi M58990P 28-pin ADC can be found in many early Yamaha digital instruments from that period. Now-a-days it’s not too easy to source but I managed to find one that was unused and for a decent price.

Faulty components removed from Yamaha RX5 repair
The Yamaha RX5 main-board. To the right are the components that I removed; a M58990P and two M5218L dual op-amps.

Unfortunately, IC 12 (the M58990P) wasn't what was causing the malfunction. This ADC simply converts the analogue reading from the Click and Data sliders. While it had to be replaced anyway (because it was cooking), I knew it wouldn't fix the problem as it doesn't have anything to do with the computer side of things. This RX5 still wasn't booting. Damn!

I had to dig a little deeper and soon discovered that the FIRQ counter to the processor was only running at about half of what it should be. A lot of back-tracking and things were becoming quite challenging. The counter clue was great and in conjunction with the symptoms, it certainly pointed me in the right direction. Several logic chip changes later and I eventually got to the heart of the problem.

Interesting; the RX5 I'd just bought, actually had two seemingly unrelated problems; the ADC and some duff logic. Hmm... The guy I bought it off, threw in a GliGli RX5 USB card interface and I'm wondering if that screwed things up somehow.

All in all, my Yamaha RX5 repair took about three hours. While I was at it, I also changed the battery. After putting everything back together, reinitialising the unit and restoring factory data, I switched it on and YES, life and.... SOUND!

Mitsubishi M5218LYes, of course ICs fail. That's a big reason that I do what I do! While accepting the ADC going down and even a couple of logic gates, I have to admit to being quite surprised at the failure of the two M5218Ls. Remember that these were technically out of my first RX5. These humble single-in-line dual op-amps are used extensively in equipment manufactured during this period and are amazingly reliable, with a pretty good specification for the time. In fact, I can't remember the last time I had to change one of these, let alone two in the same machine.

Being ex-Roland, you wouldn't be blamed for thinking that I should almost have a natural affinity towards machines like those in the TR range and even the R-8 but I have to confess that the solid, punchy feel of the Yamaha RX5, was more my cup of tea than the rather traditional, rounded, perhaps even 'jazzy' sounds associated with Roland, at the time.

Yes, I know the RX5 is only 12-bit at 25kHz but just put the specs away for a moment and LISTEN! 😀