Help: DX7 with high voltages in power supply

[okBcLU5]

Hey everyone,
I have a 120V DX7 that heats up a lot. I checked the power supply, and all the output voltages are fine but the regulators are generating a lot of heat.

Checking a bit more, the regulators are receiving some pretty high input voltages. For example.. there is +29V going to the 7815 15V regulator. Has anyone experienced this before? Can this be the result of any damaged component that I might be able to repair?

 

[happyrat1]

29 VDC Peak or 29 VDC RMS?

What is the AC output of the stepdown transformer?

It may just be bad design.

I'm assuming this is an old fashioned full wave rectifier bridge supply instead of a modern switching design?

Given the age, there's a distinct possibility that the electrolytic filter caps are drying out. Use an ESR meter to determine if they are still within spec.

Also any thermal compound used on the regulator heatsinks may be drying out and becoming ineffective as well.

That's pretty much all I can say without seeing a schematic or knowing what test gear you have available.

Gary

 

[okBcLU5]

29 VDC Peak or 29 VDC RMS?

What is the AC output of the stepdown transformer?

It may just be bad design.

I'm assuming this is an old fashioned full wave rectifier bridge supply instead of a modern switching design?

Given the age, there's a distinct possibility that the electrolytic filter caps are drying out. Use an ESR meter to determine if they are still within spec.

Also any thermal compound used on the regulator heatsinks may be drying out and becoming ineffective as well.

That's pretty much all I can say without seeing a schematic or knowing what test gear you have available.

Gary

Hey Gary, thanks for the reply. 29VDC was measured going into the 15V regulator, that's DC not peak or RMS. There was a ripple voltage as well, but it was small.. about .4V.

Yes, this is an old fashioned supply.. Schematic is on Page 5 here: https://homepages.abdn.ac.uk/mth192/pages/dx7/manuals/dx7-circuit.pdf
https://homepages.abdn.ac.uk/mth192/pages/dx7/manuals/dx7-circuit.pdf

I have most test equipment available, decent scope and multimeter.

 

[happyrat1]

Check where you're measuring from. That's a +/- 15V regulated supply. Output from the rectifier SHOULD be about 30 VDC across the output terminals of the bridge. It should actually be a bit higher.

What are the voltages from each output to the ground?

Gary

 

[happyrat1]

BTW, if you don't already own an ESR meter then get one. It's your best friend when troubleshooting old electronics.

Gary

 

[happyrat1]

btw, 0.4 VAC ripple on that circuit with all that filtering looks very bad to me. It should be closer to about 0.1 VAC

It may not be a bad idea to replace the electrolytic filter caps before one finally starts leaking.

Gary

 

[okBcLU5]

btw, 0.4 VAC ripple on that circuit with all that filtering looks very bad to me. It should be closer to about 0.1 VAC

It may not be a bad idea to replace the electrolytic filter caps before one finally starts leaking.

Gary

I'll be happy to replace all the electrolytic caps once I'm confident there's no other problems with the keyboard. All the power supply output voltages measure as they should. +15V, -15V and 5V are all within .1V.

 

[happyrat1]

Other than the high ripple at the bridge side I don't really see any other problems with the power supply from what you've told me.

If you're uncomfortable with the heat on the regulators you could always shoehorn in larger heatsinks. Manufacturers always design these things with a specific MTBF in mind and the DX-7 is WAY past EOL.

But the ripple on the bridge side is definitely way too high. The caps are on their last legs and typically the first things to go in vintage hardware. Start sourcing your supply now.

Gary