Ensoniq DP4+ Repair Report - Page 1
Forum section: electronics / repair

author: stefan_k133
author_rank: Newbie
timestamp: May 03, 2021, 10:29:15 am
content: Several Weeks ago I bought an Ensoniq DP4+ Effects Processor. It is a nice and flexible machine, that has 4independent Ensoniq Effect Chips on board and you can route the audio signals in many ways.The first version of this processor (DP4) was sold in 1992 and was built as a through-hole board, thesuccessor (DP4+) has some minor additions and is mostly built with SMD Parts. That version came out in1995. If some of you remember, that was the time where many electrolytic capacitors were built with lowquality chemicals (capacitor plague). And as I will describe later that is the root problem with thesemachines.So I have written this report to tell my story and help others that have similar problems with these devices.The machine was advertised with "distortion on some channels". As naive as I was I thought that could bescratchy input/output pots in the signal path and some cleaning should be enough. So I contacted the sellerand bought the machine.In my opinion the machine has some positives: You can get the full schematics of the "Plus" version, and onmost of the boards they used mostly the same components (e.g. many opamps are NJM4565, with some TL072, andthere are only 3 types of SMD Caps on all boards). So if you order enough parts, you can use them in manyplaces.After I got the device, I opened it and checked the inside. The first thing I noticed was that some of theprevious owners inserted a jack on the backside to remote switch the "Mute"-Button on the front.So I connected my phone to the input and switched the device on. On my headphones the audio sounded OK, asexpected only the pots were scratchy. I cleaned the pots, removed the mod, and started to play with it. Theprocessor can be driven by up to four sources (it has 4 input jacks, that is 4 mono signals). My first testused just one source (mono, using just on jack or stereo using two). As soon as I switched it to more than onesource, the output got distorted. Ouch.Another thing I noticed was that the signal level led for channel 3 was red (too high). So I took another lookat the audio board (the machine has 4 boards: audio, digital, frontend and psu) - this time with mymicroscope. And there I could see leaked electrolytics and the damage it has caused.So I ordered new caps, opamps and some 0805 caps and resistors, and began to clean and replace theboards. After replacing the input-opamp on channel 3 the signal level was OK, but now I was greeted with theerror message "Bad DC Offset(s)". More searching, more following the signal path. I inserted a sine wavesignal from my function generator and checked several points with my scope. The last opamp right before theADC looked suspicious. After replacing the 4565 the error was gone. But I still got distortion.What was weird was that the same signal was fine until I switched to more than one source - but with thesignal still on the same input jacks. More thinking, more head scratching, more staring on the schematics...Then I started looking at the output stage, right after the Phillips DAC (the famous TDA1541A). There was onlyone opamp for both channels (L and R), each channel carrying two signals (later switched to 2 channels). Ireplaced the opamp and some caps and resistors in the feedback loop, without succes.The mystery was that the 1-source configuration worked. My assumption was that the signal on channel 1 comesout of the L-Channel of the DAC, because that channel looked connected to channel 1 on the output. So whatpuzzled me was that the same signal got distored when changing the config... Where did that come from? Butafter some more checks I realized that that was not true: in this configuration the signal comes out auf theR-Channel of the DAC and is later routed to the channel 1 jack on the back side. After this insight I was surethat the problem was in the left output channel of the DAC.The next assumption was that the problem could be in the digital signals that the DAC got from the digitalboard. Between the connector and the DAC is only one 74HC86D XOR-Gate, and one 74HC04AD inverter nearby. Bothchips are located near one of the leaking caps. So I decided to replace both of them - but no luck.The signal on the defective channel looked kind of funny: My signal was stretched out near the zerocrossing. One of the algorithms in the device can generate a sine wave signal, so in order to concentrate onthe output stage I used this signal instead of my own on all 4 outputs. And there the problem was better tosee (sorry - I forgot to take pictures): The top of the wave was ok, but then there was a shift of about 0.5Vdownward near the zero crossing...I started to fear that there could be a fault on the digital board, because nothing on the analog sidehelped. It looked like the digital data was transmitted correct to the DAC, so where else could the errorbe. I checked the Firmware EEPROMs, but the checksum was OK...But before starting on the digital side I looked up more information on the DAC. And then I found the crucialinformation on some page: there are 7 bypass caps on each side of the chip, and each of those is used whensome bit is set on the DAC. When one of those is shorted, that could explain the shift in voltage....I replaced the whole row of caps on this side of the chip - and it worked. Distortion gone!!The bypass-caps on this side of the DAC were placed near the voltage regulators on the board, with some of theleaking smd-caps between. So it looked like heat and electrolytics in combination damaged one of those capsand caused the shift.Mission accomplished. Maybe this machine can now work another 20+ years...

author: blatboy
author_rank: Unknown
timestamp: April 02, 2022, 06:06:03 pm
content: Thank you so much for posting this awesome info. Deeply appreciated.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: April 02, 2022, 06:38:23 pm
content: I will thank also.I saw one a DP4 while ago with the many leaking caps damaging the PCB.the main reason I declined the repair was the number of vias affected by the leaking electrolyte.A time bomb, total gamble.

author: blatboy
author_rank: Unknown
timestamp: May 06, 2022, 03:22:29 pm
content: Just curious... the highly sought after Panasonic FC SMD caps ("low ESD") are all on backorder at mouser. I'm assuming "general purpose" caps are fine for this, yes? Should I go "high temp" for the one on the PSU right by the hot regulators?Link to list at mouser

author: vintasonic
author_rank: Newbie
timestamp: June 24, 2022, 11:08:29 pm
content: Saw this post because I was repairing one. Just wanted to add that it was noisy - digital noise that varied when I moved the power wires in the front (ORG, RED, BRN). I tied the wires to the metal bracket and most of it went away. I put some copper shielding on the bundle which helped. But it really cleaned up after I tightened up the screws that hold the Processor and Audio boards to the chassis. Pretty much dead quiet after that, so it was as much a grounding issue as anything else.Someone had already re-capped the power supply, so that ruled that out, which I was suspecting before I opened it. There was no leakage or signs of damage from any other caps