IIGS - chirp sound when I shut off power supply

[djc6]

I have a IIGS ROM3 that makes a chirp sound from the power supply when I switch it off. The chirp occurs even when power connector disconnected from logic board (no load). Otherwise, everything works fine. Its a Dyna Comp LR62786 power supply shown here:

https://stayornay.com/mac84tv/repair...e-iigs-power-s...

I've opened it up and don't notice any buldging capacitors. I'm OK with doing the soldering work, where I struggle is ordering the correct parts. For example on the above parts list, I can't even find item #3 100uF capacitor with 7mm pitch that meets the space requirements listed. I can't find a capacitor kit for this power supply either.

Any tips on finding the source of the noise?

[cruff]

It could possibly be the windings on a transformer causing it. If that is the case you may have a hard time preventing it from happening. As long as the unit is working otherwise, you may have to just learn to ignore it. My IIgs makes a quiet chirp when it is powered off, but it is barely noticeable.

[Dwight Elvey]

It is likely from the frequency of the switching shifting to the audible range as the voltage drops off. It may also be caused by the switchers core going into and out of saturation as the voltage drops off. You need to remember that as the voltage is bleeding down from that stored in capacitors the operation of the supply doesn't just stop. It often goes into erratic operation, trying to maintain the regulated output.
It is not an indicator of needing capacitors replaced. It is more likely an indication of poor power supply design. If it is from saturation of the core, it may mean that the supply may fail under brown-out conditions ( under voltage AC input ). In normal usage it is not likely to be an issue.
It clearly isn't an indication that you need to replace every capacitor. In fact, it might be that the large input capacitor is working too well compared to that that keeps the regulator running.
Dwight

[djc6]

Thank you both for the response! Is it generally safe to use the IIGS power supply as-is until it fails? This is my first Apple II machine; I do have a collection of Commodore 64s - it seems the C64 original power supplies CAN damage logic board.

My IIGS power supply doesn't have the notorious RIFA safety capacitors - mine are WIMA MP3 series. I'm hoping the WIMAs don't fail as spectacularly.

[bladamson]

One of my IIgs'en does this too. The noise is just the machine telling you that it loves you, like a cat does. ;3 It's never caused any trouble. I wouldn't worry about it. It's an Astec switching unit, a far better power supply like those old C= junkbricks, so the voltage won't slowly rise over time like the C= units do.

Do check the voltages while the machine is under load though. I did have a IIgs supply that was getting weak under load and I am pretty sure was causing some memory corruption in an 8meg memory expansion card (although the rest of the system seemed fine). Replacing the PSU with a different old un-rebuilt one fixed the RAM issues, oddly enough.

[iz8dwf]

It is likely from the frequency of the switching shifting to the audible range as the voltage drops off. It may also be caused by the switchers core going into and out of saturation as the voltage drops off. You need to remember that as the voltage is bleeding down from that stored in capacitors the operation of the supply doesn't just stop. It often goes into erratic operation, trying to maintain the regulated output.
It is not an indicator of needing capacitors replaced. It is more likely an indication of poor power supply design. If it is from saturation of the core, it may mean that the supply may fail under brown-out conditions ( under voltage AC input ). In normal usage it is not likely to be an issue.
It clearly isn't an indication that you need to replace every capacitor. In fact, it might be that the large input capacitor is working too well compared to that that keeps the regulator running.
Dwight

+1
As Dwight wrote, it might be the switching frequency dropping into audible range. I've heard it on other supplies. This one, by the look of it, seems an old design with inductive feedback from secondary to primary side, which has generally poor regulation.
Unless you see bulging or leaking capacitors, you better leave them alone.
You should however replace the polyester RIFA capacitors (the rectangular golden ones), they almost always crack and start burning sooner or later, if you are in a 220-240V mains country.

HTH
Frank IZ8DWF

[djc6]

You should however replace the polyester RIFA capacitors (the rectangular golden ones), they almost always crack and start burning sooner or later, if you are in a 220-240V mains country.

My safety capacitors are not RIFA brand, they are WIMA MP3 series. They are grey and not rectangular golden. I live in USA 120V. I was debating leaving these alone if the failure mechanism is different than RIFA branded ones.

Picture of power supply internals: https://www.dropbox.com/s/f3vegi2dlo...rview.jpg?dl=0

None of the other capacitors are bulging or leaking, on the logic board or the power supply. Everything looks exceptionally clean and spotless on this IIGS.

[rittwage]

I think mine does this too. Also, my Apple RGB display makes a pretty loud "brrraaht" sound when you power it on cold.

Both work fine... just part of the experience. They've done it since I got them in the 90's.

[cruff]

I think mine does this too. Also, my Apple RGB display makes a pretty loud "brrraaht" sound when you power it on cold.

I believe that is a built in degaussing coil used to prevent persistent magnetization which distorts the color purity.

[GiGaBiTe]

I've seen failing line capacitors cause power supplies to whine, chirp or make ticking noises.

Here's an example on a power supply I repaired a bit over a year ago.

https://www.youtube.com/watch?v=lgwUadgIM8k

Both line capacitors were VERY bad, even though they physically looked fine. One capacitor looked like a diac or direct short, and the other only had like 300 pF of capacitance with 1 kohm of ESR. It was incredible the PSU even turned on, but it could barely run the internal fan. Apply any load to it and it would shut off.

I've opened it up and don't notice any buldging capacitors. I'm OK with doing the soldering work, where I struggle is ordering the correct parts. For example on the above parts list, I can't even find item #3 100uF capacitor with 7mm pitch that meets the space requirements listed. I can't find a capacitor kit for this power supply either.

Any tips on finding the source of the noise?

I would recommend getting a capacitor tester like this one:

https://www.ebay.com/itm/143857934813

And testing all of the capacitors, especially the line capacitor. You'll need to remove them from the PCB *and* make sure they're discharged to test them. Testing capacitors in-circuit will skew the results. Making sure they're discharged is also important to avoid being shocked and damaging the tester. You can stick a multimeter across the leads of the capacitor while in-circuit to see if it has a residual charge. If it does, either wait until it self discharges, or use a fat resistor across the leads to bleed the charge. Just be careful it stays discharged, because capacitors can self recharge from residual energy left in the plates of the capacitor, sometimes to high voltages.

As for replacements, unfortunately it is not always possible to get an exact replacement, especially on 30+ year old equipment. Capacitor technology has changed quite a bit in the past three decades and a modern equivalent to an old technology is generally going to be smaller. There are some exceptions, like bipolar or high frequency capacitors, which are going to remain on the larger side due to how they're constructed internally.