Yamaha SPX900 5V Drops and mainboard shuts off - Page 1
Forum section: electronics / beginners

author: ArtyMorty
author_rank: Newbie
timestamp: May 09, 2023, 03:35:53 am
content: Hello, new here.I'm a musician that tries to fix my own stuff with some success but I'm really stumped atm.I've learned some a little bit over the years from places like this forum. Thanks a bunch!I purchased a used Yamaha SPX900 multi effects unit for cheap and I'd love to get it working 100%The problem is that the SPX900 runs perfectly for about an hour or 2 then I hear the relay click and the display shuts off but PSU still has power.So far I've changed the battery because the display read "Low Battery". Warning is gone now but still shuts off.I tested the voltages on one of the connectors from the transformer to the DC circuit board and it drops from 4.9 to 1.2 when the mainboard shuts off.The other connectors read the same voltages either way.Since I'm an amateur I have no idea if this is the actual transformer or caps etc. I pulled the 4 big caps and tested them via volt meter and 9volt battery and they seem fine.Here are some pics. If anyone could give me some advice I'd really appreciate it.Thanks.

author: Kim Christensen
author_rank: Super Contributor
timestamp: May 09, 2023, 04:58:02 am
content: Looks like themanual/schematicsare available online.Here a snippet of the power supply schematic. (DC side is the same for all versions)Check the +15V, -15V, +5V on CN501 and CN502. See if any of those are going down. If so, check the corresponding input to the regulator for that voltage. (IC501, IC502, & IC 503)I suspect that it's the processor turning it off for some reason. Check out pin 8 of IC236 when the unit shuts down.

author: ArtyMorty
author_rank: Newbie
timestamp: May 10, 2023, 03:53:11 am
content: Thanks for the tips.Unfortunately I've had this thing on all day and it shut off for a few seconds then turned back on and has been on all day.This is so unpredictable its frustrating to test. I've taken notes so far with the unit on so I'm just waiting till it shuts off.One thing that I noticed when changing the battery was a pair of red wires solder side of the MB. I thought maybe it was factory but upon close inspection its a mod.Maybe there's been an attempt to fix this very problem in the past. I don't know if I should unsolder those wires.One thing I forgot to mention was that, even though the display shuts off and the effects don't work when that happens, the input meter led still lights up and shows signal.Anyway, once this thing shuts off I'll post back with those numbers you asked for.Here are some pics of the red wire mods.

author: Kim Christensen
author_rank: Super Contributor
timestamp: May 10, 2023, 04:27:56 am
content: Quote from: ArtyMorty on May 10, 2023, 03:53:11 amI'd leave those wires alone. They kind of look like a legit factory mod the way they've been glued down.The only thing that looks a bit suspect is the trace cut next to the shorter jumper. I can't really see, and it's probably OK, but just make sure that no bits of foil are bridging between where they've cut it away and the other trace they nicked while doing so:

author: Kim Christensen
author_rank: Super Contributor
timestamp: May 10, 2023, 05:29:32 pm
content: What type of test equipment do you have? VARIC? Oscilloscope? etc?Because it's still possible you have failing power supply capacitors (C505/C511) and there is high ripple across them. It could be that the ripple is bad enough that the dips are almost causing the regulator (IC503) to drop out (It needs 2V minimum across it). So the line voltage might be just high enough that it doesn't, but when the line voltage dips, your unit goes into reset. So it appears random.So you could check the ripple across C505/C511 (Or the + & - on the S4VB20 bridge which is more accessible) with a scope.If you don't have a scope, you could use a VARIAC to lower the line voltage and see if it resets when you bring it down to 110Vac or so. If it resets that points toward bad C505/C511 or maybe even a open diode in the bridge rectifier. (D505) An open diode or caps with high ESR would be obvious on the scope.You can also use your voltmeter, but first you need to know how it handles measuring AC in the presence of DC. Set it to AC and measure a 9V battery. It should read zero.If not, you won't be able to use it directly this way. (You'd have to put a small capacitor (0.1uF or so) in series with one lead to measure AC in the presence of DC)First you'd measure the DC voltage across C505/C511. Then you'd measure the AC voltage across C505/C511. The AC reading will give you the average/RMS ripple value which we can use to guestimate the Peak ripple which we can subtract from the DC value. This is a very rough guestimate but can be useful if that's all you've got to work with.

author: ArtyMorty
author_rank: Newbie
timestamp: May 12, 2023, 04:13:25 am
content: Quote from: Kim Christensen on May 10, 2023, 05:29:32 pmThanks so much Kim for all of your help / info.I only have a multimeter for now - the one in my pics lol! So far I've been able to repair 2 Alesis Midiverb II's and a Midiverb 4 by changing the power caps only. I bought them cheap in great shape and was able to bring them back to life and now I have some cool vintage effects in my rack.Ok, I flipped the MB over on the SPX900 to take a closer look at the red wire/trace and there is no bridging. Actually the cut trace is not exposed it just looked that way I think because of lighting. There is shiny brown glue over the trace and I think it reflects light at certain angles and looks copper-ish.While I had the board flipped over I wiggled the capacitors to see if there was any loose ones and C163 was loose on one leg and the solder joint was moving with the leg. I pulled that capacitor and replaced it with a Chongx Vent Low ESR 2200uF 16v -40+105c (E-Projects). So far I've had the SPX900 on all day in a rack with other equipment and it hasn't shut off.When the SPX900 was shutting off before the PSU still had power but the MB would loose power. Maybe that cap is for the 5v on the board?I think in the future I'd like to replace all of the caps on all of these old effects units but I want to make sure that I get good capacitors for the audio signal paths.Anyway, I greatly appreciate your help Kim and if the SPX900 goes down again I'll try the other stuff you suggested. This has been a satisfying experience and I saved money by fixing these things myself. Not sure if I'd enjoy replacing/soldering any of the tiny stuff but its a start.Cheers!