Falcon PSU recap - 28uS spike

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foft
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Falcon PSU recap - 28uS spike

Post by foft »

I just recapped my second Falcon PSU using the exxos kit. The first is perfect. The second one I see a 28uS spike on the 5V line.

This is tested using the 10R resistor on 12V and the 2.2R resistor on 5V.

Has anyone seen this before?

I'll double check my work tomorrow evening in case some of the caps are not soldered properly - dry joint etc. Rather annoyingly I didn't check it on the scope before so don't know if it was already doing this...

28us_5v.jpeg
28us_5v.jpeg (67.61 KiB) Viewed 195 times

(horizontal:10uS, vertical:1V)
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exxos
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Re: Falcon PSU recap - 28uS spike

Post by exxos »

I would first check your scope wires are not close to anything like the inductors or something as they can pick up noise. Also remove the 12v resistor and see if it makes any difference.

But it's odd a PSU under constant load would spike every 35khz or so. Might be an idea to zoom in on a spike to see what's going on better. Also look at the rotating images sticky ;)
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foft
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Re: Falcon PSU recap - 28uS spike

Post by foft »

The probe isn't going past anything switched on, though perhaps it is interference.

Weird thing:
i) If I put the scope to 20MHz bandwidth limit I no longer see it.
ii) If I set the probe to 1x I also no longer see it.

So guess it could be something very fast aliasing down to the lower frequency.

I checked I can see it on another channel and also by switching the probes.

Without the 12V resistor connected I still see this on 5V.

Close up:

IMG_9924.png
IMG_9924.png (3.11 MiB) Viewed 149 times

With 20ns time base and 2v

Edit: I really should check out that sticky.
foft
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Re: Falcon PSU recap - 28uS spike

Post by foft »

Thinking about it I guess I don’t see it with 20MHz limit since that is about 100MHz.
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Re: Falcon PSU recap - 28uS spike

Post by exxos »

foft wrote: Wed Jun 22, 2022 8:23 pm Thinking about it I guess I don’t see it with 20MHz limit since that is about 100MHz.
You should be able to sample the frequency on your scope.

If you zoom in elsewhere on like 100mv / div, you should see a constant ripple on the 5v. Spikes would be ordinarily at the start and end of the waves. There's images of those spikes on my website in fact. I think your spikes are to consistent and not the spikes found when caps are failing.
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foft
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Re: Falcon PSU recap - 28uS spike

Post by foft »

I thought I'd check out any chips on board.

PC111 is an opto-coupler.

SK-8085 is an ... no idea! I can't find the data sheet, though someone posted an alternative here:
https://www.edaboard.com/threads/lookin ... -ic.29898/
So I guess its doing something like this: https://www.mouser.com/datasheet/2/149/ ... 309322.pdf
This has a built-in oscillator in the khz...

If it does pwm at this frequency that could be the cause, though whether its a failing part or the surrounding components I don't know.

I guess I need to figure out and/or find the falcon PSU to see how this beast actually works.

Perhaps I should just swap some 'it could be this' components from the working PSU.
Steve
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Re: Falcon PSU recap - 28uS spike

Post by Steve »

I think by the time you add up the price of the re-cap kit, then replacing ICs, buying them from mouser and paying the costs - you probably have paid as much as a brand new Exxos PSU which far exceeds the originals quality. Not saying this to be annoying, just trying to save you time and money :)
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Re: Falcon PSU recap - 28uS spike

Post by exxos »

foft wrote: Wed Jun 22, 2022 9:13 pm I thought I'd check out any chips on board.

PC111 is an opto-coupler.
...
Perhaps I should just swap some 'it could be this' components from the working PSU.
Before you randomly start changing stuff (untested changes on this PSU in fact) , please answer my previous questions.
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foft
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Re: Falcon PSU recap - 28uS spike

Post by foft »

Sorry I didn't mean to ignore your reply, it's definitely appreciated. I see low ripple for a while, then periodic small and large spikes. Of 73MHz according to the scope.

Nice section, close up:

IMG_9925.JPG
IMG_9925.JPG (502.73 KiB) Viewed 112 times

Small spike, close up:

IMG_9926.JPG
IMG_9926.JPG (486.42 KiB) Viewed 112 times

Large spike with measure, close up:

IMG_9927.JPG
IMG_9927.JPG (560.49 KiB) Viewed 112 times

Small spike with measure, close up:

IMG_9928.JPG
IMG_9928.JPG (572.87 KiB) Viewed 112 times

Note: different time bases, needed to zoom in to get frequency. Also flat section is flat even at 20ns, so showed longer period.

@Steve: Yes good point, though I will probably fit a CT63 anyway, so change for a pico PSU. Just trying to get this working out of interest.
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Re: Falcon PSU recap - 28uS spike

Post by exxos »

foft wrote: Wed Jun 22, 2022 10:14 pm Sorry I didn't mean to ignore your reply, its definitely appreciated. I see low ripple for a while, then periodic small and large spikes. Of 73MHz according to the scope.
Nice section, close up:
OK so your first image zoom in to like 100mV / div and you will see the ripple of the PSU. This is its operating frequency, probably around 50khz at a guess. You need to alter your time / div as well until you see something like a 50mV sinewave or something like that.

mste5v-min.png
mste5v-min.png (110.87 KiB) Viewed 95 times

73Mhz is way out of that frequency. Changing the opto would likely change the 50khz to a higher frequency, but this isn't going to do anything if your seeing 73mhz spikes.

PSU spikes look more like this.

falc5-min.png
falc5-min.png (6.19 KiB) Viewed 97 times

Yours are just to consistent. The only time I saw that was when I was running a fan to cool the load resistors, but that was about 400hz. Which makes me think its more a external cause than a PSU generated one. But difficult to say currently. It could be many things.

I would suggest you try the measurement in another room , less electronics the better. It could just be your scope probe is oscillating or something daft. You mention changing x10 / x1 changes things. So if your not seeing it on x1, then it could even be your probe. I don't know what probes you are using, but make sure you get at least a 100mhz capable one. I can only really guess why those spikes are there, but its not exactly a PSU fault I think in this case.
https://www.exxosforum.co.uk/atari/ All my hardware guides - mods - games - STOS
https://www.exxosforum.co.uk/atari/store2/ - All my hardware mods for sale - Please help support by making a purchase.
viewtopic.php?f=17&t=1585 Have you done the Mandatory Fixes ?
Just because a lot of people agree on something, doesn't make it a fact. ~exxos ~
People should find solutions to problems, not find problems with solutions.
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