Yamaha DX7 Power Supply

Yamaha DX7 Mk1 Safety Warning!

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This was recently reported by one of our forum members and has the potential to be quite serious.

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Over the years, I have amassed a total of seven mk1 DX7’s.. and counting! .. don’t ask me why :lol: I tend to keep my eyes open for sub £100 bargains (not so frequent these days).

I want to draw attention to the fact that two of them have had very serious identical electrical faults, completely independently (and in different houses, so nothing to do with the electrical system in the house).

The first event happened 15 years ago and at that time I thought it was a one-off. However today exactly the same issue happened again to another DX7- so it looks like something that could happen to other people, hence the warning.

The Issue:

While the DX7 is switched OFF but plugged into the mains, the 0,022μF capacitor on the power stage (right where the mains enters the synth) may get really hot, eventually causing it to smoulder and potentially burn. This happened to my DX7 in 2003(ish). The problem originally manifested itself by way of the DX7 turning on by itself, even when the switch on the back was off. When this happened 15 years ago, I thought nothing of it, other than being a bit of a pain. Little did I know then that inside, one of the capacitors was cooking and after some time ( I think a few days later), so much smoke started coming from the capacitor, that (fortunately) my smoke detectors picked this up and alerted me to the problem. I immediately unplugged that DX7 and it hasn’t been switched on since. It’s in the attic.

Today exactly the same thing started to happen to another one of my DX7 mk1’s. It turned on by itself after a couple of minutes being plugged in, despite the switch being off at the back. Recalling the incident from the past, I immediately unplugged it, way before any serious smouldering took place.

This evening I have been doing a few ‘controlled’ experiments with it. I have noticed that the rectangular 0,022μF capacitor near the power cable entry heats up considerably when the DX7 is switched off – but cools down again when I turn it on with the switch on the back. In the switched ‘off’ position, the capacitor gets very hot and the DX7 powers up after about 2 minutes, with the capacitor continuing to heat up – exactly the same way as the issue on a different DX7 15 years before. By flicking the switch to ‘on’, the capacitor seems to cool down again and the DX7 behaves as it should.

Given that 2 out of 7 of my DX7’s have had this problem, that’s a relatively high statistic which I don’t think should go untold, so I want all DX7 mk1 owners to be aware and not leave their DX7’s plugged in while they’re away. Especially if the DX7 starts randomly turning on by itself (but please don’t assume you’re ok if your DX7 doesn’t do that…. my two didn’t either at first).

In another thread, someone else has taken a photo of the 0,022μF capacitor: viewtopic.php?t=5994

I’ll probably put a YouTube video up demonstrating the problem, soon but in the meantime you can join in the discussion on the forum here.

I hope this helps…
Steve. (iixorbiusii)

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