JX-3P power supply repair

[Envergure]

Hi everyone,

I am working on repairing my Dad's Roland JX-3P. I have determined that the two fuses for the +15V and -15V rails are blown, and two opposite diodes in the bridge rectifier have failed short. I cannot find any other damage that could have precipitated this. The best theory I can come up with is that since there are no components for inrush current limiting, the bridge has degraded from inrush spikes over its 34-year life. The fact that two opposite diodes failed is strange, though.

(My job includes failure analysis of power supplies... but I've never looked at anything this old in my young career, nor have I worked on audio equipment, so there's a lot I'm not sure about.)

Anyway... I am posting to ask how I should repair this. If I keep the original power supply, I will just have to replace the diodes and fuses and I think everything else is fine. Should I replace the three big vintage electrolytic capacitors or leave them? They say "3N25" on them, which I'm guessing is a date code meaning 1983, November 25. All the other electrolytic caps have what looks like a 1996 date code, so it must have been "re-capacitored" at some point. I have heard that, because modern capacitors have lower ESR, they can cause problems in older equipment that may have been relying on the higher ESR (for inrush limiting, for example).

On the other hand, if I replace it with a modern switching power supply, it will improve the efficiency and possibly be more reliable than a 34-year-old monster linear power supply. I am concerned about two things: (1) I don't know how much current each rail needs to supply--I would have to guess based on the ratings of the old PSU's components. (2) I doubt that some random SMPS I might order online would be quiet enough for audio applications. How quiet does it need to be?

Thanks,
Chris

[Rasputin]

You clearly have a level of expertise that exceeds my own, however, my opinion is that unless you enjoy such tinkering (possibly you do, if it is related to your day job) then you're over-analyzing the situation.

Just recap anything suspect, replace the rectifier bridge and fuses and get the stock power supply running. You'd want to at least get it working in its stock configuration to rule out any other problems before fiddling with anything else, and also if you get the original PSU working then you can sell it should you replace it with a different power supply in the future.

I wouldn't worry about modern caps. I've used low-ESR caps in all sorts of vintage stuff and nothing has blown up or failed yet. Just my anecdotal experience.

How often is this JX3P going to be powered up to make it worth using a switching power supply instead? If this was a studio mixing board that is left on 18 hours a day, every day for years then I'm sure it would make a dent in the electricity bill, but a synth you might only use a couple hours a month on average just doesn't seem that important to revamp on that level -- unless you simply enjoy the challenge.

I can't tell you about the specifics of current requirements in the JX3P or anything, I'm just looking at this from a pragmatic/hobbyist perspective.

[Mooger5]

^ What he said.
That diode failure might have been casual.
Anyhow you could bypass each diode with a small value capacitor (100n or 10n) to absorb the spikes.
If you´re going to replace the PSU caps look for 110ºC rating, instead of 85...

[jxalex]

Yes, no problem indeed with these newer capacitors. Also to be safe then You can use with a step even higher voltage marked on these, just for higher lifespan.

Check the old 1983 year capacitors. Are they shorted inside? Perhaps thats the reason why the fuses blow? The capacitors really go old so that they cause short Circuit at the end sometimes.
Next just get the Power supply running and are the current consumtion levels in their specifications in
synthesizer? So, more capacitors shorted?

If I were You then I really would use the linear Power supplys whereever I can and really
would not rush to change for a SMPS as there are more details to go wrong, also SMPS introduces tons of noise up to megahertz region, currents at the chassis -- all of which requires almost hamradio operator class decent grounding for equipment, in order to get rid of that trouble! Also rails are not at all so Clean and cant stand sudden current changes, higher demand to Components, shorter lifespan compared to linear regulator, sometimes high pitched whining comes to audio path... etc... Even if the SMPS are more efficient I would use them as a last resort for items which makes audio... or if the linear regulator gets too hot and inefficient for this actual solution.
I talk about the 99.995% SMPS modules which are in Equipment or for sale. Thats exactly how they are built as
they need to filter the things by sending half of the mains to chassis and with high frequencies.
They run somewhat cooler and are more efficient, but dont expect them to live with the same 30..40 year lifespan as the old linear Power supply with all elements at 50..60Hz and LM78xx LM79xx regulators.
If there is really need for several voltage rails and they must provide several ampers then
compromise would be using DC-DC modules (even by using 78S40 Circuit) for a 5V rail, and keeping the 15V rails with linear regulators and still using the 50..60Hz transformer. (Thats how I made my Korg M1R Power supply).

[Envergure]

The old caps are fine. They're just old.

Thanks for the replies, guys. I will just repair the existing supply. I might replace the good diode bridge too, just in case it is also deteriorating.