Mixing different PSU valued capacitors

[Pacman]

First off, congrats on your forum, Exxos! You've done a great job
A treasure worth of information!

I read through your Atari ST PSU capacitor and upgrade information writeup which explained things very well. My foremost question was about why you've suggested to use different valued capacitors (the LaST upgrade: part 6 -PSU update) for when recapping the various Atari ST power supplies. Well, now I know -to further improve their existing performance!

My question is about mixing "old" (the capacitor values used originally) together with the "new" values (those suggested on your PSU page). I've bought all the values suggested, in Panasonic's FR-series (low-ESR) from a reputable dealer, but for the two 400V capacitors I couldn't get the suggested value (150uF/400V) with a similar hour/temperature rating as the rest of the capacitors, so I opted for a 120uF/400V (the original value used in this PSU) instead. Will this cause any problems? If yes, I can always get that 150uF/400V cap instead, although the hour/temperature rating will be a lot lower.
Mind you, these are 400V caps are "normal" capacitors (good quality, reputable brand though) and not the low-ESR variety.

I've attached the schematic below (Mega STe power supply: PSM 5341) with the Exxos suggested values. The 150uF/120uF capacitor in question is circled in red.

MSTe_PSU_exxos.png (263.83 KiB) Viewed 4231 times

[exxos]

First off, congrats on your forum, Exxos! You've done a great job
A treasure worth of information!

Thanks and

My question is about mixing "old" (the capacitor values used originally) together with the "new" values (those suggested on your PSU page). I've bought all the values suggested, in Panasonic's FR-series (low-ESR) from a reputable dealer, but for the two 400V capacitors I couldn't get the suggested value (150uF/400V) with a similar hour/temperature rating as the rest of the capacitors, so I opted for a 120uF/400V (the original value used in this PSU) instead. Will this cause any problems? If yes, I can always get that 150uF/400V cap instead, although the hour/temperature rating will be a lot lower.
Mind you, these are 400V caps are "normal" capacitors (good quality, reputable brand though) and not the low-ESR variety.

I assume you have not spotted the post I did on the forum https://www.exxosforum.co.uk/forum/viewt ... p?f=9&t=25 ? I explained there some reasons about different values.

I've attached the schematic below (Mega STe power supply: PSM 5341) with the Exxos suggested values. The 150uF/120uF capacitor in question is circled in red.

It is just the bulk capacitance which suppliers the energy for the transformer primary.Though it really does not matter if you use 120uF or 150uF.

I used a higher value, as capacitors do age, and the value goes down over the years, so a higher value will keep the equipment running for longer, and higher values keep the voltage more stable and ultimately do help with overall regulation. Higher values keep the stress on the capacitor lower also, IE if the capacitor dropped 50 volts under load, it would maybe drop 30 volts with a higher value. So the capacitor "charge and discharge" is less, reduces heat/stress on the capacitor and it lasts longer.

I have seen countless equipment over the years, if a value of 4uF is needed by the circuit, manufactures will probably go with 3.3uF or 4.7uF. Normally they go with lower values as cheaper in general. Though once the capacitor drops in value a fraction, the circuit becomes unstable and will start to malfunction. Often a higher value lasts longer, and keeps things more stable as well. Mostly capacitors are used in regulation of the voltage they are connected to, so no harm at all using slightly higher values in such instances.

Some may call this "overkill" but if we are going to the trouble of changing capacitors, I always go for the best I can find. Of course as said in my other thread, capacitor values, brands etc are coming and going off the market every few months, so its difficult to keep up with all the changes. It could be possible the 150uF is no longer manufactured for example.

[Pacman]

Thanks, it's nice to be here

Yes, I read both your capacitor write-ups in this part of the forum which was excellent reading! Not too complex (i.e. readable for the non-engineer) and not too general (i.e. informative for hardware hobbyists). I learnt a lot from it and I'm sure others will as well.

It is just the bulk capacitance which suppliers the energy for the transformer primary.Though it really does not matter if you use 120uF or 150uF.

Thanks. I was hoping it didn't make much of a difference, but with high voltages involved, and the possibility to kill my ST I really wanted to confirm this first.

I have seen countless equipment over the years, if a value of 4uF is needed by the circuit, manufactures will probably go with 3.3uF or 4.7uF. Normally they go with lower values as cheaper in general.

I assume cutting corners was a a high priority by Atari at the time. Fortunately I read the part where you said the Atari chipset was pretty robust, so that's good, as opposed to an inadequate PSU supplying "dirty" power to the motherboard over several decades, slowly killing it.

Some may call this "overkill" but if we are going to the trouble of changing capacitors, I always go for the best I can find. Of course as said in my other thread, capacitor values, brands etc are coming and going off the market every few months, so its difficult to keep up with all the changes. It could be possible the 150uF is no longer manufactured for example.

Yes, that may be it.
No, I wouldn't call it overkill. If you can improve on something without risking electrocution, fire etc. or a very large financial outlay I'd say go for it!
I see that recapping itself is a heated subject in various discussions, for some reason unknown to me, but since I want my Ataris to last as long as possible I want to do what I can, and I agree -when I do recap I want to get the best parts I can get hold of, within reason of course.
Looking forward to getting it done and seeing if I can notice any changes. Doing away with the ticking timebomb is in itself reason enough to get it done