I have an Indy with a Nidec supply. The speaker has been slowly getting noisy with "digital" sounds like beeps, buzzing, etc. that varies with CPU/GPU operations. Even moving the mouse pointer around the desktop is audiable. I suspect the electrolytic caps in the PS are going and the power supply rails are getting pulled all over the place, but I've not yet verified this with a scope. Could somepoint point me to the schematic for the power supply? Any suggestions on which caps are the ones to replace first?
Noise on Indy Speaker
- Thread starter Jamieson
- Start date
You'll never find the schematics but I do rebuild/Test Indy NIDEC power supplies . I have a posting on IRIXnet (https://forums.irixnet.org/thread-3332.html). I have quite a number of Indy power supplies ready to be purchased that have all had their electrolytic caps replaced, including their main filter capacitors, and I give a 90 day warranty and free shipping in the continental US. Asking price is $250.
Each power supply has been tested for six hours on an Indy, reloading IRIX. You're welcome to buy multiples. If your power supply is in good physical shape I'll also give you a discount if you ship your old power supply back to me after purchase of a rebuilt unit.
If you really want your power supply rebuilt and you do not wish to swap it out I can do that just inquire on pricing and timeline. I do have parts right now to do the work.
All caps were replaced with carefully researched models from the exact same manufacturer and family lineage. Most of the caps are going to be Nichicon and are the direct replacements for the obsolete models originally in the PSU. The main filter caps may be from another company due to pandemic stock issues while I was purchasing supplies. All the rest of the caps will be Nichicon and Chemcon United to directly replace same with same.
Each power supply has been tested for six hours on an Indy, reloading IRIX. You're welcome to buy multiples. If your power supply is in good physical shape I'll also give you a discount if you ship your old power supply back to me after purchase of a rebuilt unit.
If you really want your power supply rebuilt and you do not wish to swap it out I can do that just inquire on pricing and timeline. I do have parts right now to do the work.
All caps were replaced with carefully researched models from the exact same manufacturer and family lineage. Most of the caps are going to be Nichicon and are the direct replacements for the obsolete models originally in the PSU. The main filter caps may be from another company due to pandemic stock issues while I was purchasing supplies. All the rest of the caps will be Nichicon and Chemcon United to directly replace same with same.
I have not seen any schematics on the power supplies (Nidec or Sony), but I have information on the ins and outs of them: https://forums.sgi.sh/index.php?threads/indy-power-supply-information.14/ which may be helpful if you want to test.
I don't doubt that the capacitors are going -- they pretty much are for most SGIs given the age -- but it could be worth measuring to see if it is the cause of your audio problem. The noise would probably be present at the audio amplifier on the motherboard, as the speaker in the power supply is only an unamplified pass-thru.
I would at least replace the large line side capacitors and output capacitors in the supply, but if you are in there you might as well do the rest. They are all too old now.
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Good to know. I had assumed that the speaker amp was on the power supply board. I'll poke around on the motherboard with my scope and see if I can find some power rails that look nasty.
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I'm poking around on the Indy main board looking for the audio amplifier that drives the speaker. I'm finding lots of the basic digital stuff like 7400 series and the like. Still haven't found the audio chip. Any suggestions where to look? Is it back by the audio jacks?
Edit: Duh. Found it. U53. 3W single channel audio amp. Datasheet recommends a 200uF and 0.1uF cap on the power pin 2.
Edit: Duh. Found it. U53. 3W single channel audio amp. Datasheet recommends a 200uF and 0.1uF cap on the power pin 2.
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There you go! The big multi-pin tabbed packages (esp. TDA series) are classic single chip audio amps 
My Indy - that I just upgraded with a ZuluSCSI - has just started making noise thru the speaker, especially with the transient draw when accessing the SCSI bus. I've never recapped the thing, and with the recent XZ graphics upgrade might have started to strain those old caps in the supply.
@Jamieson , did you ever end up fixing this? Can you update with your experiences?
@Jamieson , did you ever end up fixing this? Can you update with your experiences?
I'm prepping my Indy for sale and want to get this speaker noise figured out first. With an oscilloscope I can see extra noise on the +12V and +5V rails when I do stuff (like move the mouse, etc.) and at the same time I can hear the noise in the built in speaker. Interestingly when listening with the headphone jack there is NO noise; it only seems to affect the built in speaker. Anyway, I've tried adding some extra caps (various cominations 0.1uF through 220uF) soldered directly to the power pins of the amplifier chip, no change. Will probably recap the Nidec PSU next to see if that helps.
Recapped the Nidec PSU, no change, the internal speaker is still making lots of noise. Looks like the audio amplifier chip TDA7056A is powered from a dedicated +8.0V rail, which is derived from the +12V rail through a linear regulator. That SHOULD provide clean power to the audio chip, will need to verify again on the scope to be sure. Noise could also be coming in the audio signal or the DC volume control inputs to the audio chip too. Does a schematic exist for the Indy main board?
Only portions of Iris hardware has schematics in bitsavers...the rest were purposely destroyed by SGI when they went bankrupt during its IP sale to prevent having to pay to research & relicense the IP, as some of what they themselves had was licensed from some other entities. Instead of tracking them all down to create a new licensed entity...they shred, burnt, destroyed EVERYTHING relates to MIPS SGI hardware. Useless asking, long since given up.
It's more than likely you have a bad cap on your mainboard, likely a bad tantalum cap near the regulator you're talking about, also the -5V line if often used for audio as well. Sputtering and rippling can be an early sign of failure in those caps, specifically. It will NOT show up as ESR in a Tantalum cap. The only way to see the failure to is the use an oscilloscope to view the output real-time through the cap..it will look very dirty. it' easier to just shotgun the cap replacement given they are easy to access and low cost of the parts.
That would be my guess.
With an oscilloscope I can see the noise in the main +5V rail, simply moving the mouse results in static on the internal speaker and voltage spikes on the rail. I recapped all but two electrolytic caps on the Nidec PSU (there are two caps buried in hardened goo). I'll try replacing those soon, and will also look around at the mainboard tantalum caps.
I replaced the last two electrolytic caps in the Nidec PSU. No change. The speaker is still very noisy and I can still see the noise ~100mVpp on the +5V rail when moving the mouse, booting up, etc. The Indy runs fine however, just buzzing squeaks and chirps from the internal speaker.
So it turns out the noise on the +5V rail was coupling into the mono amplifier TDA7056A on pin 5, DC volume control. I soldered a 100k ohm resistor from pin 5 to pin 4 (GND) and that quieted down the spurrious speaker noise. System sounds (startup chime, opening folders, etc.) play as usual just a slight bit quieter. Now with this fixed and a new super quiet fan in the PSU Indy is whisper quiet.
So it turns out the noise on the +5V rail was coupling into the mono amplifier TDA7056A on pin 5, DC volume control. I soldered a 100k ohm resistor from pin 5 to pin 4 (GND) and that quieted down the spurrious speaker noise. System sounds (startup chime, opening folders, etc.) play as usual just a slight bit quieter. Now with this fixed and a new super quiet fan in the PSU Indy is whisper quiet.
