Egnater Renegade 65 Circuit Help

[Mason]

I've got an Egnater Renegade 65 that I'm working on and had a quick question regarding the schematic.  There are two small capacitors in the power supply section, both 0.02uf. What is the purpose of these two caps? They aren't actually on the PCB and there's no space for them either.

Here's the schematic, https://el34world.com/charts/Schematics/files/Egnater/Egnater_renegadesch_8_09.pdf. You can see the caps I'm referencing because they both have purple boxes around them.

[shooter]

in the digital realm they are quite common, typical purpose spike suppression usually from fast switching.  I believe they like to be a special type, poly? 

[silverfox]

in the digital realm they are quite common, typical purpose spike suppression usually from fast switching.  I believe they like to be a special type, poly?

Don't know if you gandered at the schematic but yeah, digital in this design. The PS spikes could cause problems but there are already spike surpressors across the rectifiers. Either over kill or to deal with an actual design glitch that arose. Has me wondering if the spike is induced in the HV winding through the 12 supply. Yeah, lot of spike sources in this design I think.


silverfox.

[shooter]

 
yep, seen all the plastic, well most.  IIRC, there was a rule of thumb that you put 1 cap per 10 chips.  If you ever ran into  "bizarre" timing problems 8/10 it was the spike suppression caps either not doing their job, or creating OT for me 

[sluckey]

Back in the days of hi-speed TTL chips it was common to see a .1µF directly on every DIP. When CMOS rolled around the number of de-spiking caps went way down.

[Tony Bones]

It looks to me as though the designer was fond of paralleling big electrolytics with small film or ceramic caps because electrolytics are "slow" (high ESR and ESL.)  The little caps are fast and handle the high frequency stuff that the big electrolytics just can't handle.

That's the story anyway. The fact is, since switching supplies became the norm, modern electrolytics are pretty fast by necessity, so paralleling them with small caps is probably not as helpful as it once was. But there are still a lot of old engineers around.

When laying out digital PCBs, I will put a small ceramic cap near every power pin (if there's room.) That's not so much because the big bulk capacitors aren't fast enough, but because the power traces have some inductance. The little caps near the chips bypass that inductance and can prevent a lot of spikes and dips in the PS and ground.

[shooter]

Just built this PS for an audio build,  lots of spike caps and that's just the secondary