Electric piano distorted notes (Roland RD100)

[Dannyx]

Good day folks. Today, I came across something kinda special, which I haven't worked on before: a Roland RD100 electric piano. It was just sitting there in the shop covered in dust, no doubt having been ravaged and then abandoned by my oh-so-skilled co-worker. I actually remember seeing him struggling with this thing and I recall briefly witnessing the problem too: some of the notes play distorted. I'll try plugging it myself and maybe do some captures for you to hear for yourselves, but let's just say it sounds....bad.

I haven't actually dug into it myself yet, so this is mostly a generic discussion to kill some time and maybe learn something in the process, especially since I lack the most important tool to diagnose audio stuff: a scope !

I managed to grab the service manual - it's as simple as I expected: a PSU and some chips communicating back and forth. The block diagram on page 7 comes in handy to hopefully work out where the distortion may be happening. Unfortunately, most of these chips are dedicated and impossible to find, so I doubt this is fixable in the long run, but still...worth giving it a shot. If not, it could still be used as a MIDI controller I guess.....

I'd say the amp section on the "jack board" to the right is fine, since most notes play OK - if there was something wrong here, ALL notes would sound off. My money is on that "XP" chip (IC2) on the main board to be honest.

[stj]

page15
run the self test in the ram and roms

[Dannyx]

Originally Posted by stj

page15
run the self test in the ram and roms

I will - just have to dig it out of there and power it up myself. Still, if it's any of that, I don't think it's fixable, since even if I somehow found the chips, they would still need to be programmed...

[stj]

i can program them - smd will fit in an easter card
the ram is standard stuff too.

[Dannyx]

Originally Posted by stj

i can program them - smd will fit in an easter card
the ram is standard stuff too.

Yes, but I reckon you must also have the necessary file to upload TO the chip. I'm not sure they can be sourced anyway...

[Dannyx]

Ok, just in case someone is still watching this thread, I finally got a go on this piano, to see for myself what is up with it: EVERY sound it makes sounds distorted - any key you play and even the sample songs stored on it. The distortion is present on both the line outputs and the headphone outputs, so it can be anything really: looking at the block diagram, it may be caused by IC15 (the DAC), IC7, IC16, or IC1 itself, since the path seems to loop back through it somehow after the "vol" and "brilliance" pots at the top...

Can a DAC cause a distorted output because of the input it receives ? In this case, the signal for the DAC (IC15) seems to come from IC2 (the XP chip), so if that's feeding garbage to IC15, it will reproduce garbage on its output....a scope would be a handy tool to have right about now

[kaboom]

Originally Posted by Dannyx

Can a DAC cause a distorted output because of the input it receives ?

It can (missing/stuck bits), but check the supply(ies) for the DAC first. Especially the caps for the analog rails for the DAC- can make it sound harsh, "chirpy" or extremely "aliased."

Are they SMT caps?

[petehall347]

try a clean signal direct to main amp . . that should rule out the analogue bit .

[Dannyx]

Originally Posted by kaboom

Are they SMT caps?

I haven't opened the thing up myself yet to know for sure. If they're thru-hole caps, then this old dude who had his way it it before me is sure to have replaced them, since that's basically the only thing he does to electronics: replaces caps and hopes for the best, without even thinking if they have anything to do with the problem in the first place

Originally Posted by petehall347

try a clean signal direct to main amp . . that should rule out the analogue bit .

I was thinking of the same thing, but in reverse so to say: in-lieu of a scope, I was thinking of "probing" the sound output at different points after the DAC (IC15) to see where the distortion enters the signal path. I was going to use something like the line input of a small battery powered speaker, it this is at all possible with the rest of the components in-circuit not interacting in some weird way and possibly destroying something.

If I was to do what you suggested and try to inject my own external audio source into the pre-amp part, then I guess I'd solder my signal wires for L and R to the output of IC7A and IC7B on page 9 and hope for the best (before R19 and R27)

[petehall347]

just be sure there is capacitive coupling to block any dc .

[Dannyx]

Originally Posted by petehall347

just be sure there is capacitive coupling to block any dc .

So I should put a ceramic cap in series with my audio source/output ?

[petehall347]

or electrolytic low uf like 2.2uf . or film cap ..its just to block dc

[Dannyx]

Ok guys, after a ridiculous amount of time, today I decided to crack open the piano to have a look inside. As you can see, the damage done by my "skilled" co-worker (who thankfully retired in the meantime) is quite extensive: there's almost no component that hasn't been touched with the iron and he broke some traces and had to repair them using jumpers...the list goes on. I mean I was expecting it to look bad, because it IS this guy we're talking about, but not like this !

Upon seeing this, I was just about ready to stop and put it all back together, but I decided to at least do what I suggested in the discussion and that is to try and "listen" to the signal at various points to try and close in on the component that's causing it to sound rough, if at all possible: I tried it right after IC7A and IC7B (page 9 of the manual) and sadly the same thing happens: the audio is an absolute mess of pops, which tells me the problem is likely somewhere in IC15 or before it and unfortunately this is where my knowledge of audio electronics and test equipment at my disposal also stops. Even if I DID find the culprit, it all proprietary parts with no way of sourcing them, so this piano is only good as MIDI controller or spare parts ATM. Cheers.