Yamaha DX7 - 5V rail @ 7V

[Peacefrog]

Hi all,

I purchased a Yamaha DX7 a few years back which hasn't got much use. After pulling it out from the shelf yesterday, it was working fine when I went to bed - but this morning, it doesn't boot ;(

The first thing I checked was the power supply - alarmingly, while the +/- 15V regulators seem fine, the 5V rail is at (edit:) a fluctuating ~5.3V at the regulator and *7V* when I measure at both the power supply board connectors and IC VDD, so I'm assuming this could be the problem (although God knows if anything's been toasted even after it's resolved).

My first instinct is obviously to replace the 7805, but is there anything else I should look at (schematics are here: https://homepages.abdn.ac.uk/d.j.benson/pages/dx7/manuals/dx7-circuit.pdf)? The caps seem fine, although I'm a bit suspicious of the zener, and I've checked for shorts in case something else was pulling the rail up (that said - does this seem possible given the difference between the regulator and VDD at the IC sockets)?

One other thing - there's only voltage present when the power supply is connected to the rest of the board, which obviously isn't ideal in this scenario. Is there a right way to add a load to allow for measurement?

Neil

[Manul]

One other thing - there's only voltage present when the power supply is connected to the rest of the board, which obviously isn't ideal in this scenario. Is there a right way to add a load to allow for measurement?

That is strange, there should be voltage output. Is there voltage from bridge rectifier (input of 7805)?

You definitely should try to test power supply alone (with no load) to better isolate problem (it is even possible that problem is not a power supply at all).

[BreakingOhmsLaw]

Check the mains voltage selector on the rear. Might be set to 110.

[Peacefrog]

Thanks! Few things:

- It's a US model, so no voltage selector

- I might be measuring wrong, but I don't get a voltage at any of the regulator input pins (measuring DC between them and chassis ground) without the connector plugged in. Maybe it's time to check for bad joints?

[Manul]

So maybe there is no connection to chasis when you unplug power supply? There is 5 volt supply ground on the connector (named E in the schematic), use that.

[Peacefrog]

Thanks - knew I'd missed something. If I jumper the pin to the chassis, I now see 19V on pin 1 of the 5V regulator, and *12V* on the output.

[Manul]

Wow, thats really high input voltage for a 5 volt regulator. It must dissipate a lot of power under load. And also zenner and 220 ohm resistor. That somehow seems wrong (or really bad design). What is input voltage of +-15 volt regulators?

[Manul]

And if I read correctly, bridge rectifier filter capacitor is rated 16 volts, so it is definitly overvolting there.

Voltage should be nowhere near 19V, should be around 8-12V.

Are you sure you measured correctly? Try directly on 4700uF capacitor if you can.

[Peacefrog]

Yup - I unclipped the board and I'm getting ~18V at the cap. I'm not sure why, though?

The other thing here - there a potentiometer next to IC1. I thought it might be a trimmer, but it's not clear it's adjusting the voltage as I'd expect - anyone familiar with it?

[Manul]

What I can think of:

1. It is 110V transformer powered from 230V mains.
2. It was replaced with wrong transformer
3. Inter winding short on primary side

About potentiometer, I can not find it, can you point?

[Manul]

What you measure on these two 1000uF caps of +-15V regulators input?

[Peacefrog]

So it now works, but I'm not sure how.

The potentiometer is marked on the board as being for 5V adjustment - but when turned, the 5V rail would jump to 16V intermittently. It's also an odd arrangement - apart from the center pin, the other two connections are made by braid. On top of this, it looks like someone may have been in here before and replaced the 7805.

Figuring that I had nothing to lose, I hit the 7805 connections and the braid with my iron, then remeasured - at which point I could steadily adjust the rail to 5.2V (but no lower). Reconnecting the rest of the boards, it now boots successfully, albeit:

- 18V input to the regulator doesn't seem like a great idea
- 5.2V appears too high
- I don't entirely trust the trimmer

Perhaps the obvious thing to do here therefore is replace the trimmer so that at least the digital parts aren't over-voltaged, but I'm a bit concerned about the lifespan of the regulator. That said, there's a thread in this very forum along these lines here: https://www.eevblog.com/forum/repair/yamaha-dx7-keyboard-overheating-29v-going-into-7815/.

Thanks for all your help so far, anyway - any suggestions before I swap the trimmer and move on?

[Peacefrog]

Oh. Trying to swap in the trimmer, it seems - there's actually a *crack* in the power supply board right by the regulator, patched with lots of solder. This may explain a few things, so it might be time to go to eBay for a replacement...

[Manul]

Well, crack may be repaired, possibly by soldering small piece of wire, it should not be a big problem.

Potentiometer seems to not be on the schematic you provided, so it is hard to comment, but I would probably choose some fixed resistors and remove that trimmer.

5.2V may be ok, because there will be some voltage drop on the wires under current.

Some 7805 will have a better internal protection, some worse. This power supply seems to be designed badly, with so much unnecessary power dissipation on these poor linear regulators. Hope they have good heatsink.

[Manul]

What they did with that trimmer (newer revision of the board?) is to increase 7805 output just a little. Because alone it will produce ~5V and including voltage drops on the connectors, wires and PCB traces it may easily drop to 4.9 or 4.8V which was found to be a on the edge of too low. But old trimmer may be unstable, so it would be more reliable to have selected fixed resistor there.

[Haenk]

Btw. there is a "HV" model of the 78 (and likely 79) series, HV for "high voltage". A 78HV05 will be fine with the measured input voltage, maybe that was the part originally used. However EOL for a long time, and unobtainable.

(That's the same stupid construction with my REVOX B790, firing way more than 45V at a 78HV24 - no direct replacement possible, this part is Unobtainium for many years now, repair kits include just a standard version, which will be run way out of specs...)

[helius]

A lot of these instruments use linear power regulators, because they are cleaner than switchers (or at least were considered cleaner when the instrument was designed). The big danger is that the regulator's pass transistor can fail short, which happens pretty often, especially if it has overheated by being improperly heat-sinked. The Ensoniq Fizmo is infamous for being a ticking time bomb for this reason, and it also affects other instruments. In theory you could protect the device from regulator failure using a zener or thyristor, but they never bothered. Change linear regulators and ensure they have proper sil-pads and clamps.

[Peacefrog]

Haenk - you got me thinking; a quick search through Mouser shows that the ST parts are apparently tolerant to 35V (https://www.mouser.com/datasheet/2/389/cd00000444-1795274.pdf), so I might switch one in.