Roland D-50 noise issue with PCM sounds, SYNTHE sounds fine

[frisc0]

It's been a while, but I've ordered a can of freeze spray that will come in soon. Now how to start? Should I work from 'functional parts' like all the reverb RAM ICs first, going to the other RAM elements, is there a logic way to do these diagnostics? Not used freeze spray before, so all tips are welcome, thanks in advance!

[frisc0]

So here's the 'heat image' of the reverb DRAM ic's... can I conclude from this image that IC1 and IC6 are faulty?

[frisc0]

Time lapse...

[YetAnotherTechie]

what is the timeframe between pictures. You said the keyboard worked at boot and then stopped working. If you freeze the rams after the fault reappears, does the keyboard  work normally again?

[frisc0]

Timeframe between pictures is around 10-15 seconds.
Just tried again after freezing these reverb RAM ICs the issue with PCM sounds is still there unfortunately.
Is the warming pattern shown on the pictures normal?

I've freeze-sprayed the ROM ICs as well, but they turn black again very equally and slowly, these ICs hardly warm up.

Also tried the other ICs, all memory ICs, glue logic etc, but no effect in play and no visible remarkable heating.

Should I try to check for pcb cracks on both sides of the board with the freeze-spray?

[YetAnotherTechie]

If i understand correctly, from power off it's 10 seconds to get to the first pic then another 10 for the second etc? If that's the case i don't think it's bad. And i would expect the refreeze to improve the situation. Have you tried freezing half the board, then the other half, etc? have you tried to do a reset without turning the power off? that could probably discriminate between a heat up problem and a corruption somwhere. Is you clock stable? have you found correct dumps of the ROMs?

[frisc0]

Okay, I think a break through: I tried again freezing ROM IC A more thorough, not only the top of the IC but also underneath it and meanwhile played a patch that normally sounds cracking. And from power-on the patch sounded clear for 8 to 10 key presses, where it normally starts cracking and breaking up after 2-4 key presses! So the freezing actually did improve the situation albeit very temporarily. I have repeated this a few times and the improvement is consistent with every thorough freeze.

So it looks like ROM IC A is faulty, do you agree?

(I've also tried soldering-through the IC-'legs' again, but without any improvement.)

Now I have found the ROM image files I think the solution would be either 'burning' a new ROM IC using the correct ROM image or try finding a replacement original ROM IC. Both options are not going to be easy...

However, just checked the pins of the original PCM ROM IC A and compared it with the pins of this 2mbit 32-pins EPROM and they seem to match, the M27C2001 https://pdf1.alldatasheet.com/datasheet-pdf/view/228430/STMICROELECTRONICS/M27C2001.html.

Would this EPROM be a good replacement for the original ROM IC?

[YetAnotherTechie]

I had a look and that chip won't work as replacement, without modifying the board, in fact, most (e)proms won't work because the CE is active high on your circuit, i've only seen that on much smaller eproms, or some ram. The easiest way would be to add an inverter and use a common chip, i would use a AT27C020 and two MC74VHC1GT04 or one NL27WZ04 and permanently ground pin 22. you should also check that it is not a problem with the pcb under the rom. Remove the rom and inspect it the board, install a socket if there is space.

UPDATE: i found http://www.synfo.nl/servicemanuals/Roland/D-50_D-550_SERVICE_SUPPLEMENTARY.pdf, there it's mentioned that some boards support a 4Mbit chip instead of 2x2Mbit, if that's the case, and even it it isn't, it will be very easy for you to install one larger AT27C040 rom and do small modification to the board using that document, from there it was also made clear that you can use normal/available eproms, with /CE permanently grounded.

[frisc0]

Wow, that's a great find! Thanks for taking the time to help finding a solution!

My mainboard is version -02 instead of -04, so no jumper for the 4mbit ROM version, but as you suggest, /CE and /OE can be fixed to ground permanently (as the jumper would do).

Funny to see that back then (1988) they already had availability issues for the 2mbit ROMs and adviced to use the 4mbit version 'to save replacement ROM in stock'...

Again unexplored territory for me, this fix, but a very interesting and educational one. To be continued....

[jorevf]

Hi,

Any progress in your jurney to bring your D50 to full working order ?
I’ve just got a D50 with similar issue to look at..

Regards,
Freddie