Yamaha DX7 gets hot

[Luka Ray]

Hello everyone! I'm very new to this forum but I wanted to share this. So a month ago I bought a Yamaha DX7 MK1. After using it for a month, I noticed that the synth gets quite hot to the touch after 30 minutes of use. It only gets hot where the algorithms are printed. I've opened it up the day I bought it, and the battery and capacitors all looked fine. Yesterday I opened it up again after using it for about an hour to see what really is causing the heat. Apparently the heat sink is getting very hot and the bridge rectifier on the same board of the heat sink gets very hot aswell. It even gets so hot that I can't keep my finger on it. Of course knowing that it's almost an 37 year old synth, I was a little bit worried when I bought it, but the synth seems to keep running just fine when using it for like an hour. I'm wondering what causes this, because usually the capacitors dry out at this age and I thought that they were the ones causing the heat. Thanks in advance!

[Tyco72]

Hello. Such temperatures are quite commons. The temperature point for "can't hold long a finger on it" is around 50°C. For a semiconductor it is actually not so much, compared to the max working temperature limit of over 100°C, but it is not good for electrolytic capacitors, exposed to the same temperature.

Anyway, I would try to drop the temperature also for the semiconductors. Is the rectifier fixed on the heatsink? Which other components are fixed on it?
If there is space you could apply with silicon additional heatsinks, to reduce temperature. But if the produced heat is too much and there is not enough air circulation to the extern, it may not help much. You could add a small fan in the air intakes on the case, if there are.

[Luka Ray]

Hey Tyco! Thanks for your response! So the only components that are fixed onto the heat sink are the voltage regulators. The bridge rectifier and the capacitors are all located on the same board as the heat sink (see image). There are also no air intakes at all. I thought about adding another heat sink into the synth, but there isn't any space to let it sit anywhere.

[Tyco72]

Hi, thank you for the photo. On the heatsink I don't see possibility to add easily additional surface. On the rectifier you could fix a small heat sick. There are many which can fit. Like some types made for the TO220 format. You can cut them, if necessary.
The problem is that the missing of air intakes doesn't allow the internal air temperature to drop. The only way would be to make holes in the case, but I can't say if the situation is so dramatic to require it. On a collection object like this, a such modding would decrease the market value (and damage the aesthetic of the back side).

[Luka Ray]

Yeah I agree with you on that. Adding an aditional heat sink for the rectifier could be someting I can consider. Like you said, drilling holes into the case could fix the problem aswell, but I'm not sure if I want to. In the end the synth still works perfectly but I was just worried about it getting hot. Thank you very much for the reply!

[Luka Ray]

So here I am again +6 months later. I decided to recap the Power Board with 105°C rated capacitors, but it seems that the S4VB bridge rectifier still gets very hot. After a few minutes I can't even hold my finger onto it. Also the big heatsink where the TO220 transistors are mounted to, gets hot after 10 minutes aswell, which could be normal because it is doing its job. I can use the synth as normal without any problems, but I'm just concerned about it getting hot after a while. Thanks in advance for any advice!

[synthomatic]

I bought a DX7 MK1 recently and have the same problem. Maybe this is an issue with these synths. Despite getting hot it still operates well.

[synthomatic]

I know that this is an old post but I was wondering if you fixed the heat problem in the end and has your DX7 run ok since you raised the issue?

[madtheory]

Finger is not an adequate test

Check voltage into regulator(s) and check ripple before and after rectifier. Then you can see if you need new silicon or caps. The photo is not adequate to see bulging and that is not definitive anyway. You would need to remove caps to properly test them. They could be leaking on the bottom and the glue is hiding it.

Does service manual state the operating temp? Check datasheets for the silicon on that issue. Assuming you have a scope and a thermometer and understand linear PSUs. They are easy, plenty of good web tutorials about them.

[synzoneau]

This is completely normal for the Mk I DX7. The hotter the heatsink the more heat is being removed from the voltage regulators attached to it. I've seen this not working properly the regulator overheats, the 5v one for example and the synth stops running