yes.
but as soon as I put the mainboard on the voltage drops to -15V
there seems to be some strange interaction going on…
where have you measured the ground to the pin of the Vreg? between which points to give 0.1 ohm?
you are sure the vertical line in the middle of the board is not a crack? (my thin red arrows)
it would explain why the voltage drops when you connect the mainboard by closing the ground between the two connectors
when you measrue 25V it’s really directly between the two pins of the C22 capacitor, not the ground on the connector and the positive to the capacitor, right? it’s to be sure, because the crack seems to be on the ground and if you measure it on the connector you will measure the voltage through the resistor R3 and a transistor. (broken ground trace)
on your picture I measured from the right pin (pin1) to the big joint in the middle, but it doesn’t make any difference if I use other points connected to the same plane (is that what it is called? not sure)
ok and about the C22 voltage measurement?, you have made it really accross the capacitor, on its pins, it’s really important.
if you have used the ground at the end of the broken trace on the connector it will not work.
just did that again, it says -26V
very very very strange behaviour.
but i’m pretty sure it’s a crack on the picture.
the ground between the transformer 0V (black wire on the 3 pins connector) input and the pin 1 of the 7915 is ok? (0.1 ohm)
I’ve measured every joint that is on the earth connection, they are all 0 - 0,2 ohms
yes black wire to 7915 pin 1 too
the strange thing too is the very high input voltage.
34V is more than the max input voltage for the uPC7915 and near the max for L7915CV but the board is unloaded so…
the best will be to replace the 7915 by a third one without connecting the mainboard.
We must find why the -15V is not regulated before to connect the mainboard again.
maybe I should give up on the power-supply and try a MW RT-50C from meanwell?
There is additional circuitry on the Yamaha power supply that provides a signal for the processor reset line. See werner’s #5 post.
Therefore the Meanwell power supply will not work properly.
Are you buying the voltage regulators from a trusted supplier?
Pat
it’s very strange to get this behaviour and can’t be able to solve it with such easier schematic 
there is an electric shop that has all kinds of components, so I’d say yes.
they won’t be putting themselves out of business i guess…
Just verifying that you have good parts that you’re installing and not using pulls or seconds from other equipment.
One last thing i would suggest you can do if you have access to a variac to adjust the voltage to the power supply.
This will allow you to “soft start” the power supply.
Turn the switch on the synth before you apply any AC power from the variac.
Start with about 70% setting on the variac and measure the power supply voltages. Increase by 5% each time until the +5V and +and - 15 are stable.
Once the voltages are stable connect the power supply to the synth and retest the voltages to make sure they’re within spec.
great idea, i’ll have to find one first…
Steven,
no need to go that far - it may be much easier to get hold of a benchtop Power Supply.
Remove the PSU board from the DX7 - remove the fuses F3 and F4 and connect the benchtop PSU as per picture below - then you can play and follow the voltages …
I have a variable power-supply here now
when I bring the voltage AC down to 215V nothing really changes I still get -25V DC (and 15V and 5V)
so then I brought the voltage down until i got -15V DC (the other voltages stay consistent 15V and 5 V)
when I then measured the AC-side it was on 153V AC
That is a big difference!
what surprises me most is that both +15V and -15V use the same rectifier bridge, and for some strange reason +15V works fine and -15V doesn’t
I’m thinking maybe I can try to build in a 15V power supply and reverse the polarity
would that work?
I tried to connect the mainboard as well.
15V and -15V are fine. 5V drops, but i can adjust that.
there is another voltage that was 1,3V I believe that one drops down to 0,3V but I cannot adjust that or will it follow if I adjust the 5V?
the problem is the voltage 5V only drops as soon as the motherboard is plugged in again…
I’m still stuck…
I believe that the voltage regulator has failed and something before the voltage regulator could be damaging it.
At this point i would replace the bridge rectifier and re-test with the variac. If the -15V still doesn’t work then i would order new voltage regulators from another seller. I’m not implying the seller is bad. He may have bad stock without realizing it.
There should be no problem with the 1.3V - it will follow the 5V.
The bridge rectifier rating is 4A and 200 PIV. The small caps around the bridge rectifier are 1,000 pf or
.001 uf if you decide to replace those.
This one is a real head scratcher.
Just realized i gave you the wrong bridge rectifier specs. The bridge rectifier you want to replace is the smaller unit. It is rated at 1.5 Amp and 200 PIV. Part number is W02.
