Ibanez DM2000 Digital Delay Problem

[Maggit]

Hi there, newbie to this site.

I have an old Ibanez DM2000 rack delay that I've had for years and I recently got it back out and hooked it up. I forgot about the noise problem it has. The dry and bypass signal is perfectly clean but the delay signal has this fuzz/distortion/noise on it that I can't seem to get rid of. I've traced the delay signal through the circuit and haven't been able to pinpoint where the signal picks up that noise and I'm starting to get frustrated. I have a service manual, schematic and all that but I would appreciate and help with ideas of where the source of the noise could be coming from.

Everything works great in the unit, all functions buttons, etc. work fine, just this noise on the delay signal.

I can attach a clip of the sound or anything else to help

Thanks for the help!

[cjlectronics]

When I was a Roland Authorized Service Center, I'd get keyboards and other digital effects in for repair with the same issue. Roland's response was that the D/A or A/D was going bad but recommended to swap the whole board out instead of performing a component swap. I'm not saying this is your problem but distortion in a digital circuit can be caused by many things. Some as simple as re-seating a PLCC in its carrier to more difficult of an internal fault in an IC. Have you tried simple things like pushing on the PCB while the noise is heard to see if you can isolate the problem?

[Maggit]

Thanks for the reply. The more I've been looking at I I've though the same thing that's it's in the DAC or ADC process. The chips don't have mounts, they are soldered right to the board and I've already swapped out the DAC so the ADC is next .The more I'm listening to it, the noise isn't so much of a distortion but sounds more like a square digital wave on top of the analog wave, that kind of noise. Like the digital signal is leaking out and mixing with the analog signal Does that make sense? When I play chords it sounds like distortion .

[Jazz P Bass]

Can you post the Service Manual?
All that I have is the DM1000.

That said, do ANY of the controls have ANY affect whatsoever on the noise?
(If any controls do change the noise, that is a hint that the source may be before that control)
(it also may lead to that control needing a small squirt of DeOxit)

Have you checked the power supply rails?
There are different +12 & -12Vdc filter capacitors for certain banks of ic's.
Consult the service manual to see which ic's are what.
I would test right at the ic.
The 8 pin ic's power pins are:
Pin 4= -12Vdc
Pin 8=+12Vdc.


There is a +5Vdc supply rail also.

On the + & - 12 rails, look for the rail voltages to be relatively the same Vdc, sans polarity.
Then flip your meter to read Volts AC & test again.
You want a very low ripple voltage on the ic power pins. (ideally, zero volts/ac).

The same testing applies to the +5Vdc rail.

[Maggit]

Thanks for the great info. Here is the DM2000 Service manual.

Ibanez DM2000 Service Manual.pdf

I've tried tapping on the board and I notice some volume changes when I touch around IC 140 on the digital schematic. I've already swapped IC 137 which is the DAC and I have a replacement for IC 135 which is the ADC and also IC 136, just in case. I do hear some kind of noise change when I turn the Feedback knob and have an audio probe on E4 or CP2. It gets worse as I turn the feedback up but I don't think I hear it when I'm listening to the signal coming out of MIX OUT.

Also, I've traced the signal with my audio probe from the input through the pcb following the schematics and I've found where the signal first goes bad. On the Digital board, Coming out of IC 140 into R99, the dry analog signal sounds fine on one end, but on the other that goes to IC 136 and D15, the signal is very muffled and I hear the digital noise at that point. I am an extreme newbie when it comes to this electrical testing so I'm confused as to why the signal on the other end of that resistor is already affected even though it hasn't gone through the delay process yet, or has it? looks like it goes in both directions and I read about modulated SAR DAC circuits? and how they are setup this way. But anyway, that's where I first hear some bad noise and I've tested the resistor and it measures 2.999K ohms. I'm not sure if this is relevant or not but from this point and beyond it goes through the digital delay system and somewhere in there the noise is happening.

I'll give it a whirl testing all of your suggestions on the voltages and capacitors, etc. I hope I'm doing it right. This has just become frustrating as the unit works perfectly except for this annoying bug.

Thanks again for the help! I'll keep you posted and any other suggestions you have form reading the manual would be appreciated.

[Maggit]

Ok so I checked the voltages at the IC's and everything seems to be normal. They all read at 14.xx + or - and right around 8 and 5 on that rail. For AC, some readings were just about zero but some kept jumping all over the place but that could be because I had the modulation on, I think... Is that bad that it was jumping around?

Anyway, I traced the signal through the circuits again and I really think it's the ADC/DAC process causing it. The sounds goes from clean dry to delay with noise instantly at one point and I can't find a spot where the delay is clean at all. So I guess my next step is to replace the ADC chip since I already did the DAC chip.

[g1]

Any of the RAM stand out as being hotter than the others?

[Maggit]

Funny that you say that. I don't know about one being hotter but one of them has the writing mostly faded and some corrosion or burnt marks at the solder points. I'll check for heat again but could what I saw mean something?

[g1]

Are they not socketed? There are 11 of them?
If it looks like that one has seen some trauma that would certainly be an issue.

[Maggit]

Not socketed and looks there are 12. The one I'm looking at is 124. You can see the writing is faded and the material at the pins in the middle. Looks like I might need a replacement

[g1]

Try cleaning up that crud at those pins and make sure that the trace going under there is ok.
If that doesn't help, it does seem odd that only 1 has lost part of it's lettering and it may have overheated.
Bad RAM chips are a fairly common fault in digital gear in my experience, at least of that era. And depending which chip in the RAM chain, 1 bad chip can result in mild to horrible noise issues.

[Maggit]

Ok great, thanks for the tip. I'll try cleaning it and if that doesn't work I'll try replacing it.

[mozz]

Have one of these on my bench right now. You should change the filter caps, 2200uf. There are also 10 pieces of 470uf which are easy to get to. This was drawing 60 watts while it calls for 19watts max. With a cap change i am down to 14 watts, much better.

[Maggit]

That's great info, thanks. So just change them even if they look ok? Sorry I'm kind of dumb with this but where would I measure the watt output?

[Maggit]

So some good news. I measured the current draw and it came to about 18 watts, so not bad but should I still consider changing out the caps? Also, I did replace that RAM chip and there is a drastic improvement in the noise problem. I can still hear it during the feedback decay with each repeat but the noise isn't saturating the signal like it was before. I can hear the guitar signal very cleanly now while the noise is only in the background during the delay cycles. It's not distracting at all anymore like it was before but still just annoying. I can put some sound samples on here and see if anyone can recognize where the noise might be coming from.

So I'm debating whether to live with it as it is now, or to try replacing the ADC chip like I was planning on doing. I have the replacement chip and I already replaced the DAC, just thinking that changing out this one would clean it up more. Thoughts?

Thanks again for all the help up until now.