Korg Poly-61 synthesizer RAM issue  - Page 1
Forum section: electronics / repair

author: belzrebuth
author_rank: Frequent Contributor
timestamp: March 13, 2018, 10:37:11 am
content: I don't know if anyone is familiar with this particular synthesizer but I though I'd post anyway.The problem is that the synth can't retain program edits inoddnumbered banks even while it's powered on. (it should keep program parameter values to ram(?) while it's on )For example if I try to edit a program say program 11 (eg set filter cutoff to 50) and then save that patch and then go to program 12 and return to 11 my filter cutoff value is returned to whatever value it was before me editing it.The synth can store patch changes in even numbered banks (eg 22, 41, 63 etc) as it should and even after powering it off and on again.So this is not a ram battery issue.I thought I'd open up and see if there's maybe a faulty logic chip around the ram area or maybe half of a dual chip that may be responsible for the odd bank address locations but I don't see anything like that in there.The schematics for this are in general barely readable but the circuit around the ram chip is clear enough.I'm afraid there's not much except from the ram chip itself or the IC30 (main cpu) because the ram chip inputs are fed straight from IC30.I have no idea how the main cpu chip or this particular ram chip work but I really hope there's something I'm missing here because either of these chips is harder to find and the CPU chip is expensive too.I attached a piece of the schematic.IC 32 is the ram chip.It's a M58981P-45 and I can't seem to find a datasheet for that either:(

author: helius
author_rank: Super Contributor
timestamp: March 13, 2018, 10:40:31 am
content: Could be a board short between /WE and one of the address lines?

author: belzrebuth
author_rank: Frequent Contributor
timestamp: March 13, 2018, 11:06:09 am
content: If there was a short the /WE should be kept high you say when the odd address lines are high?Makes sense that ; I'll check and report back.

author: belzrebuth
author_rank: Frequent Contributor
timestamp: March 13, 2018, 11:57:28 am
content: /WE is always high for some reason but the even banks are working okay so I don't really know../WE signal does not go even when I move the write enable switch to disable.

author: stj
author_rank: Super Contributor
timestamp: March 13, 2018, 02:23:21 pm
content: what are you testing with?only a scope or logic-probe would be fast enough to see that signal.

author: belzrebuth
author_rank: Frequent Contributor
timestamp: March 13, 2018, 03:05:45 pm
content: an analog scope (tek 2235).I don't have a DSO or a logic probe..

author: Rasz
author_rank: Super Contributor
timestamp: March 13, 2018, 04:04:24 pm
content: sram is bog standard 2114 1KB x 4, C64 uses same chip as color ram ;-)first google resulthttps://www.princeton.edu/~mae412/HANDOUTS/Datasheets/2114.pdf$5 from chinabut I would start by checking address lines, every other setting not saving could suggest A0 stuck low/hi for example, you would have to check all address lines since A0 can be anywhere in this design (doesnt have to be same as on the chip).Trivial with logic analyser, somewhat tedious with scope.can be sram, can be microcontroller, 8049 is masked rom so cant just whack any random ebay part, thankfully someone already dumped it and even made a replacement modhttp://www.tauntek.com/Poly61.htm

author: belzrebuth
author_rank: Frequent Contributor
timestamp: March 13, 2018, 04:49:00 pm
content: Quote from: Rasz on March 13, 2018, 04:04:24 pmoops!I'll check the datasheet!I really hope it's a bad trace or something, I really don't feel like waiting a month or so for a new chip.I'll check the address lines..

author: daybyter
author_rank: Frequent Contributor
timestamp: March 14, 2018, 04:05:30 am
content: You could ask in one of the retro computers then? I.e. lemon64 .

author: ap
author_rank: Frequent Contributor
timestamp: March 14, 2018, 05:43:44 am
content: Even with an analog scope, when probing address lines you should see them toggle, at least a short blip. Go into slow time sweep mode (1s/div or so) , turn the display bright and when you press store you should see the related line toggle (does not necessarily have to be A0, even though that would be logic). I am afraid, since the schematic is so simple, that if its not the trace (sometimes on these KORG synths given the PCB is of cheap quality, they have cracks, have even seen it on ground reference planes) its either the CPU or the SRAM that needs replacement.I dont understand though, why at this stage you have not yet ruled out the traces. Thats just ohms measurements, and since your problem is stable, any interruption would be stable too.

author: Synthtech
author_rank: Regular Contributor
timestamp: March 14, 2018, 10:49:21 am
content: Where are you located? I have several scrapped Poly 61’s in my store room. There were two different versions of that machine with different boards and CPU though.

author: belzrebuth
author_rank: Frequent Contributor
timestamp: March 14, 2018, 11:18:27 am
content: @ap I haven't ruled out the traces because I've already fixed a dead voice which was partially due to a bad trace.It took me a long time to find it because the fault was intermittent (a crack probably)The trace looked immaculate; no signs of damage whatsoever but it was in a suspect path (vca adj) and that's why I eventually found it.The voice board is single sided so it's easier to check the traces.The CPU board suffered light battery leak but all the traces around that area are repaired now.I haven't really checked *all* the traces near the ram chip but I buzzed every ram chip pin and they sure go where they need to be..I'll try slower sweep time and see.@Synthtech I'm in Greece so thanks for your offer but we're too much apart:)

author: belzrebuth
author_rank: Frequent Contributor
timestamp: March 17, 2018, 12:43:50 am
content: It was the ram chip that was faultY..I borrowed another P-61 from a friend and when I replaced the ram chip with his the synth worked fine.I reloaded the factory patches, turned it off, let it for unplugged for a while and when I came back all the patches were there.I've ordered the chip from ebay; hope I get a genuine/working replacement.

author: james_s
author_rank: Super Contributor
timestamp: March 18, 2018, 09:44:16 pm
content: That doesn't surprise me in the least. A lot of 70s-80s Atari arcade games used 2114 RAM chips and I've seen quite a few bad ones. It's usually the first thing I check on boards that won't boot.