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Performa/LC TDK power supply overvoltage

I was able to get the TDK manufactured power supply for a recently acquired Performa 476 working and reliably powering the computer after a fair amount of repair work, but both the +5V and +12V rails are producing somewhat above-spec voltages. The +5V rail stabilizes at 5.2V, and the 12V rail at 12.3V when powering the system. My main question for the experts on this forum is, would this level of mild overvoltage be potentially harmful to the system in the long run?

Additionally, in order to achieve this level of voltage I had to adjust the potentiometer on the low voltage side of the PSU all the way counter-clockwise (as far as it physically goes). Before this adjustment with the potentiometer in the neutral (12 o'clock) position, I had +5.4V and +12.7V respectively, which seemed too high for peace of mind. Another question for y'all is if the additional voltage regulation would cause problems in the PSU in the long run, e.g. due to additional heat generated? Am I just masking some other underlying problem by doing this?

One interesting finding is that the PSU in these machines are definitely designed to drive a SCSI HDD as well as a relatively power hungry fan (both on the 12V rail). At one point during testing I had both disconnected, which cased voltages to climb above 14V which in turn caused the PSU to start resetting rapidly (I assume due to internal overvoltage protection.) A SCSI2SD device draws way less power than a HDD, not surprisingly, which can also add to the overvoltage problems. Something to keep in mind if you are fitting such a device in your pizza box Mac...

A final tip that I wanted to leave here, for anyone else attempting to repair a TDK power supply and running into extreme overvoltage, is to test / refit / replace diodes on the low voltage side. If it was heat or cap electrolytes (or both) I don't know for sure, but I had bad traces and/or cracked solder joints on all four diodes. Simply desoldering, cleaning the legs up, and refitting them made a huge improvement on my specimen. One bodge wire was required to fix a trace. Hope this helps someone!
 

joshc

Well-known member
Just speaking from my own experiences, but I've not had a problem with stock LC PSUs and SCSI2SDs.

I'll have to go and do some measurements of the TDK ones I've got that work...

I've also powered a lot of LCs without hard drives, using their stock PSUs, so this might suggest there is something not quite right with your PSU still.
 

cadenya

Member
All of the dead TDK Psu's I've recapped still don't work. They just click click click. The only time Ive gotten a recap to work on these was when they were still working.
 

Fizzbinn

Well-known member
All of the dead TDK Psu's I've recapped still don't work. They just click click click. The only time Ive gotten a recap to work on these was when they were still working.

I recapped two dead TDK power supplies (out of original LCs) and it bought one back great, the other got better (working 5v gave me a chime!) but ...12v only measured 2v. I recently reposted my thread on this:

FYI in my testing I left out the hard drive/floppy and didn't have a problem, in that state, 5v gives you a chime and 12v a running fan. I would test the voltages (except for -5v) via probes on motherboard HD power connector.
 
Thanks for the input, y'all. It's really helpful to understand 'normal' behavior of these power supplies.

Just speaking from my own experiences, but I've not had a problem with stock LC PSUs and SCSI2SDs.

I'll have to go and do some measurements of the TDK ones I've got that work...

I've also powered a lot of LCs without hard drives, using their stock PSUs, so this might suggest there is something not quite right with your PSU still.
This is what I'm afraid of. Interestingly, after longer use the voltages eventually stabilize at 5V/12V, but there seems to be some spikes early on. I also hear some crackling from the internal speaker during boot which goes away soon after. The PSU PCB is seriously weakened, with two pads coming off solely by the suction force of my desoldering pump. I'll probably leave it as is for now, and hope it will last me a few years.

All of the dead TDK Psu's I've recapped still don't work. They just click click click. The only time Ive gotten a recap to work on these was when they were still working.
I assume the clicking is when they're powering the computer? Without load, these PSUs will invariably click (continuously restart) and not give you reliably voltage readings.
As I touched upon above, a thread on stackexchange.com helped point me to the low voltage side diodes as a potential source of overvoltage under load. You could try measuring the diodes in-circuit to check for shorts or other weird behavior (after discharging the circuit of course.)
 

dan.dem

Well-known member
While I wouldn't worry about 2.5-4% over voltage it looks alarming that you encounter more than 14V when reducing the load to the PS. This means that the regulation doesn't work - or only within a narrow range of loads. I often read that a (very) minimal load is required for the PSUs to start/work, I doubt, the behavior you're encountering is the way it should be.

Since I have 3 defective Mac PSUs (the TDK from an early 1991 LC died first - after 18 years), I'm contemplating about a repair attempt. But I'm finding it often turns out difficult since often enough the initially failing components take out the regulation circuit (chip) as well. This makes a repair a fairly complicated process, including the exchange of components difficult to obtain.
 

belzrebuth

Well-known member
I've had similar problems with some of those slim power supplies and it was always the capacitors and passives that their legs got so corroded that they broke off.
I've seen diodes and resistors which looked okay but one or both the legs got broken off instantly just by gently pushing them with a screwdriver.
A small and dense wirebrush or a fiberglass pen is handy when dealing with stuff like that.
I gently scrape all the component legs I can reach to remove the corrosion and determine if they're still okay.
 
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