Let us know! 
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Ahaha yes. I did build a 3rd PSU and the issue was still there. (+12v rail not powering and kicking when powered off) l am sure it is due to my first modules block, either the VCO or the wavefolder. What I am seeing is exactly the same as what is described in the modwiggler post, so I have high hopes.
I replaced the two diodes sitting between the regulator outputs and ground on the frequency central FC power board with 1n5817 diodes as per the modwiggler post.
Before swapping to the Schottky diodes, it wouldn’t power on first time 80-90% of the time.
I’ve since turned it on and fully off a bunch of times and it’s come up first go every time.
It seems to work? I’m very optimistic.
I’ll let you know if I have any recurrences of the issue.
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Sounds great.
Btw, any differences between 1n5817s and 1n5817 ?
I cant seem to find anything online so I guess there isnt. The ones I ordered are 1n5817 20V… If my PSU explodes , I will let you know that there is a difference.
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Sorry, they were 1n5817 diodes, I was just using the s to make it plural (I’ve edited the post above…)
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So it worked for me as well ! Happy days.
The PSU, that was giving me problems each time I would start it, turned ON first time. I tried 3 times over the past 3 hours and it worked 100%.
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@Bpbby Thats good news, would be good to know in a few days if this fix is still working, I am sure a fair few of us will be making the modification
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It worked every single time so far.
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At http://musicfromouterspace.com/analogsynth_new/WALLWARTSUPPLY/WALLWARTSUPPLY.php, Ray Wilson wrote:
Diodes CR3, CR4 and CR5, CR6 are protection diodes recommended in National Semiconductor Application Note AN-182 for protecting three terminal regulators from unwanted current paths that the output capacitors can inadvertently cause under certain conditions.
So I took a look at AN-182 (this version’s revised 2013, after Wilson wrote the above apparently). It does talk about two diodes per regulator, but one is to block current from a capacitor connected between ground and the regulator ground pin, which doesn’t look like it’s relevant for the MFOS circuit.
But there is also this:
When a positive regulator (except for the LM117) is loaded to a negative supply, the problem of start-up can be doubly bad. First, there is the problem of the safe-area protection as mentioned earlier. Secondly, the internal circuitry cannot supply much output current when the output pin is driven more negative than the ground pin of the regulator. Even with low input voltages, some positive regulators will not start when loaded by 50 mA to a negative supply. Clamping the output to ground with a germanium or Schottky diode usually solves this problem.
Note this refers only to positive regulators. “Negative regulators, because of different internal circuitry, do not suffer from this problem.”
Anyway, I was looking for information on whether that diode needs to be a rectifier like 1N4004, and instead found something that specifically recommends a Schottky. So that’s good.
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Having recently added a third PSU and front panel power display LEDs to my main case, I decided it was a good time to take a look at this. The top two rows have MFOS-based wall wart supplies and the third row has an FC Power. Before installing the latter I switched out the 1N4004 from the +12 V rail to ground for a 1N5817.* I’ve cycled the power many times since then and on each of the top two rows, the +12 V supply fails to start up roughly 1/3 to 1/2 the time. The -12 V supplies always start up. On the bottom row both sides start up every time. This is all completely consistent with the issue mentioned above. So one of these days I am going to switch out the diodes in the other two supplies.
* I did not change the 1N4004 on the -12 V side; AN-182 says it’s not necessary.
Added: Did a video
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Thanks I’ve been struggling to understand the behaviour but your video is very clearly explained
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I just fixed one of my two FC power supplies by soldering a 1N5817 on the back of the PCB. It seemed impossible to remove the original diode because it is too tightly packed with the other components and I couldn’t get a grasp on it with my pliers to remove it, but all seems to work well. Thanks for the great video explaining the reasoning!
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