Faulty Yamaha MCD-64 RAM Card

Back when I was offering the original MCD Sweet16 modification there were several MCD card owners who were having problems with their pre-modded MCD cards. As I was going to be “in there” anyway I would repair whatever the damage was whilst doing the Sweet16 modification.

The owner would describe the behaviour as “just not formatting”, “the battery keeps going flat” and the worrying “there’s a kind-of burning smell from it”.

The MCD32/64 cards are fairly well constructed with stainless steel covers so what could possibly be wrong with a card that shows no sign of physical damage?

Inside the MCD64

The photo below shows the insides of the faulty MCD64.

Note the little yellow arrow on the PCB pointing to a burnt area. This used to be the dual diode that channelled power from the synth or battery depending on which one has the highest voltage. Completely toast now.

What possibly could have caused this – as everything else on the card looks fine!

MCD Card - Burnt Diode
MCD Card – Burnt Diode

Looking at the PCB underside explains it all. Somehow the steel cover on the rear of the card has short-circuited to one of the traces on the PCB.

In this case it was the VCC rail that supplies power to the ICs on the board. This rail gets its power through the dual diode so when it shorted to the metal cover the little diode gave up soon afterwards. The PSU in the synth would have been loaded down a fair bit to achieve this.

The yellow coating on the metal cover is an adhesive layer – it appears to be some kind of paper layer impregnated with an Evo-Stik type glue. This thin adhesive layer is the only insulator between the PCB and the cover.

Here are the locations of the short on the PCB and the metal cover:

I have seen many faulty MCD cards and all the faults have been shorts between the PCB and metal cover. Somehow the glue layer insulator between the bottom of the PCB and the metal cover breaks down over time (20+ years) and the current arcs over.

Not all of the faults involved the VCC rail – it appears to be random just where the short happens. When it is a signal trace the card just will not format successfully, or it will not hold data between synth power cycles.

Repairing the card was simple enough. The Sweet16 Mod involved replacing the dual diode anyway, and when re-assembling the card there is a proper insulation layer put in place to stop this kind of thing in future.

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