Ensoniq DP4 - won't boot up

[rosscodiscotrain]

So the sage continues here.....

- discovered that the interface chip for the CPU to the display board that I replaced before was bad and messing up CPU Enable signal. I changed it again and some of my bad clocks are now crisp and clear.
- still a bad , smeared WCLK signal from the A/D converter. changing the converter did not fix it. I next suspect the crystal oscillator circuit on the CX20018 so I've ordered the tiny load caps, coupling cap and inductor and I am going to change out those components. In comparing to my working unit, the crystal circuit clock signal seems to be fuzzier than the good one, so perhaps it isn't in spec anymore. If this doesn't fix it, the next thing I am going to do is pull and test every 74 series logic chip. With a socket, if the chip tests good, it can go back in. I picked up a TOP2008 USB eprom programmer which also tests all logic chips. I've gone through some of my stock and I have found bad parts, so it's worth it. When I fix this one I can put the last version firmware in both units via 2 new burned eproms each.

Hello there, it's been 6 years! Did you fix it?

I have a DP4 with a screen which shows random characters, half screen, faded pixels and rock solid BUTTON messages and all sorts of other stuff across the course of a week. I stepped down from 110 to 100v as I bought it from Japan, which did seem to do me some favours. I don't have a scope or a logic probe but I have ordered the CMOS RAM chip, as I've read elsewhere that it can fail. I'm hoping this works but am welcome to other ideas. Of course I changed the battery first

[Enzo]

Before you replace any CMOS, check the battery that keeps the CMOC alive. And did you reinitialize the unit?

[rosscodiscotrain]

Hi Enzo, thanks for the reply. I did check and change the battery before doing anything else, yes. I have since replaced that CMOS RAM with no success. To top things off, my wife sneaked up on me while I was probing it with a multimeter and I ended up shorting something, blowing two fuses. Having replaced the fuses I'm now devastated, the front panel doesn't even turn on any more, no power to that section. I've tested some voltages and I now have nothing from unbanded side of CR71 to +C83 as the first line of the service manual PSU test indicates. I've tested some others however and got some correct voltages, I'm going to go through the rest of it today. Right now, with no schematic for this machine online, and no other secondary machine to compare with, I'm at a real loss for what to do.

EDIT: I have now checked all the voltages, and the only problems I have are C258 and CR71. I can only guess I have perhaps shorted these two, since they are so close to each other on the board. I'll go and buy all components from that little area of the board and see if I can make any progress now. Perhaps once I've fixed this issue I can somehow fix the screen problem I started out with

[g1]

Right now, with no schematic for this machine online, and no other secondary machine to compare with, I'm at a real loss for what to do.

Is is different from the DP4+ schematics? https://www.synthxl.com/wp-content/u...Schematics.pdf

[rosscodiscotrain]

Is is different from the DP4+ schematics? https://www.synthxl.com/wp-content/u...Schematics.pdf

It seems very different when I look at the two boards physically. I can only imagine the schematics vary pretty widely. I'm going to take a closer look a that section I just tripped this weekend. At best replace a cap or two, at worst have to somehow un rivet the voltage regs. WIll plug it all back together and get the multimeter on it when no one is home

[rosscodiscotrain]

So I've got it back up and running, but screen is still dodgy. Someone in a Facebook group has told me about a battery level chip that can be damaged if the battery is left low for too long. I'll investigate further and get back to this page with my results.

[rosscodiscotrain]

So I got to the point where I had changed a few chips, whatever was socketed, and the thing still has display issues. I ended up downloading an iPad app called iDP4. This lets you talk to the machine in a much more open interface, it doesn't need a working screen to function. It's a really good way to edit patches, I'm over the moon with it.