Ensoniq DP4 - won't boot up

[audiopete]

Hi I have a broken Ensoniq DP4 I scored real cheap but now have discovered the nightmare of servicing Ensoniq gear.

It will not start up. Voltages all check out, screen displays "button # *" when you push front panel buttons, replaced caps to fix dirty power supply and tried all the factory reset procedures.

I noticed the oscillator for the Sony A/Ds seems to be dead. I see life with the other 2 oscillators on the main board with an oscilloscope, but nothing from the 73.728 MHz crystal by the Sony ICs. Could this be locking up the whole unit? Pretty hard navigating this unit with no schems....

***EDIT*** OK never mind the oscillator...lol you can't see 73.728 MHz on a 20 MHz scope...I will assume that it is fine for now. long day...

Any other tips?

Thank you very much!

audiopete

[The Dude]

Unfortunately, you'll find very little info/help on this unless things have changed. I used to do warranty work for Ensoniq. They did not offer schematics or much in the way of tech assistance. They just sent you new boards.

[The Dude]

This is a total "stab", but the "button # *" message makes me think it's booting up in test mode. Possibly you have a button that's "stuck" or somehow making contact all of the time causing the unit to boot in test mode rather than "normal" mode. I'd check all of your switches making sure they're actually measuring normally open like they should and look for spills that may be causing continuity where it shouldn't be.

Also check the battery and surrounding area if it has leaked.

[Jazz P Bass]

Push comes to shove contact this guy: Home

[audiopete]

Thanks (The) Dude! I will look into all that tomorrow. It makes good sense to me that it could be stuck in test mode due to the problem(s) you mention. I read some of the horror stories for service centers around here from some former Ensoniq techs...anyone got a main board to swap? I prefer to find the problem and fix this unit....especially if it's just a switch. Finding a replacement might be a whole nother matter though.

The battery is a lithium CR2302 in a holder, and was changed before I got it. I am going to remove the battery holder and check the board underneath for damage and the other side, as well. Perhaps something was damaged when installing the holder. There's some pretty poor solder jockeys out there.

Thank you very much and i'll post my findings.

[audiopete]

Thanks Jazz P Bass. I saw him in my internet travels, so good to know there is hope.

[Enzo]

Seems to me the Ensoniq way was for the display board to go into diagnostic mode when it lost communication with the rest of the system. In other words to throw the panel into test, you unplugged the cable to the main board. The power supply would come in on a different connector. Any chance of that?

[audiopete]

I will have to check Enzo. At first glance, there are two ribbon cables and I assumed one was display/button communication and the other is analog audio signal to the front panel level knobs. I will check over the cables and connector solders and go from there. Thanks for the tip! I will post all my findings in a couple hours.

[audiopete]

Okay I checked continuity on all switches and they check out. Didn't find any damage.

Cables test good.

I started checking some of the ICs around the cable on the display board assuming they were part of the communication network. I found that the LM339 comparator has a couple inputs at 1.6V and do not register on the logic probe. I changed the chip but no change. Does this seem odd in a logic circuit? Next to that IC is a 7407N hex buffer which seems to look fine with either a high or low on each pin.

I know there are no schems but is this logic circuit much differnet than other documented circuits in other units?

[The Dude]

I did find the service manual here:

http://zaptechnic.free.fr/DP-4_INFOS...e%20Manual.pdf

No schematics, but some test procedures and other info that might help.

[audiopete]

Thanks I have that one. Basically it is all a flow chart leading to "replace main board...replace display board....replace power supply board...."

[Enzo]

So, at least that tells us what board to look on.

[audiopete]

If it is a communication error like you suggest, Enzo, then I need to find out where. The flow chart says replace the display board so I should investigate that board first. Digital electronics is newer for me than analog or tube but I do want to learn.

[Jazz P Bass]

The first post indicated that "Button #" is displayed when you push a button.
But what is displayed at boot up?
A blank screen?

[audiopete]

Yes, a blank screen until you press any button, then the "button #11" or whatever would display.