SE/30 simasimac - beyond recapping

[tomlee59]

One question I have to ask though. The specs for cap C6 are 1 uF 50V, the closest I could find at the local electronic store (RadioShack) is 1 uF 35V, I was wondering if there could be a problem of premature failure under stress. I had a C6 at 1 uF 35V on a different board run fine for several years but I wanted to ask.

I don't know offhand what C6 is connected to, but if it's just a power supply bypass cap, it only needs a voltage rating "sufficiently beyond" the nominal supply voltage. A 50% buffer is fine. So, if it's across a 12V supply, a 20V rating is plenty. If there are spikes large enough to take out the cap, it's curtains for the semiconductors, so that's not a concern.

[A higher voltage rating, by the way, does not always mean greater robustness. If the capacitor is used in a way that ESR is important (an example is C1 in the original compacts), then the heat generated by the dissipation there can matter much more than the risk of dielectric breakdown. It ain't so simple.]

 

[Unknown_K]

http://cgi.ebay.com/SMD-Electrolytic-Capacitor-Chip-E-Cap-50V-1uF-15-pc-/320461206985

Pretty cheap and the correct spec.

 

[Concorde1993]

[A higher voltage rating, by the way, does not always mean greater robustness. If the capacitor is used in a way that ESR is important (an example is C1 in the original compacts), then the heat generated by the dissipation there can matter much more than the risk of dielectric breakdown. It ain't so simple.]

That's why I've got a Kensington System Saver cooling fan for the Plus, Tomlee (and the fact that I don't keep it on for more than an hour). But that is interesting to note, nevertheless.

 

[techknight]

Yea, capacitors do some very strange things.

for example, I had a capacitor that read pretty darn good in ESR, and the capacitance was still within its 10% tolerance its rated for, but still be bad. it was part of a filter circuit and the ESR needed to be about an ohm or two less than what it was.

just that minute of a difference caused the circuit to totally not operate. (in case you were wondering, it was the SMPS of an old zenith projection tv).

I had to do a double and triple-take on that capacitor. Replaced it anyway, and the power supply came right on.

 

[tomlee59]

Switchers can certainly be twitchy beasts, can't they? Congratulations on figuring out the Zenith problem -- that's some mighty fine troubleshootin', my friend.

I once came across a problem that was the inverse of yours -- a prototype supply worked great with cheap caps, and the company went into production with more expensive ones without doing any electrical tests (it was "obvious" that it would work even better). Turned out that the higher ESR of the cheap caps was needed to stabilize a poorly-designed feedback loop. The better caps, with much lower ESR, caused the supplies to oscillate into oblivion. Took a lot of head-scratching to figure that one out.

 

[techknight]

Thats what I am good at most of the time, is troubleshooting.

The rest of it, umm... not to certain. but troubleshooting is keen to me like smell is to blood hounds.

I can usually figure out where a problem lies in an electronic board within minutes. but it takes me an hour or more to pinpoint which exact components.

But then you get that weird crap when you get into CRT circuits. which throws me off quite alot. but not too bad. its easier for me to fix plasma than a CRT setup. sad but true.

on a side note, i think the hardest thing i ever had to troubleshoot that had me chasing my tail for hours, is a failed opto-isolator in power supply circuitry. thats the one thing thats drove me up the wall. but i got keen to those real quick. Second worse thing is troubleshooting an infocus projector power supply with NO schematics/service data available. some of them made me chase my tail as well, but i figured it out eventually. now i know those circuits like the back of my hand.

Then comes the power supplies that have no service/schematic data available, AND the controllers dont have public datasheets. WEEEEEEe yea... nothing to go off of for troubleshooting. I only have a 1% success rate on those. any time the FET goes, it takes the controller out in most cases and with no service info available or controller info available, troubleshooting is impossible. I can work without schematics as long as any IC circuits have datasheets/pinouts. but i CANNOT work without schematics AND no data for on-board components.

I had a philips projection unit, PTV805 i think it was. the cold side controller IC had failed, and i didnt feel like paying $30 to $50 bucks for a new one at the time. SO i built an equivalent circuit using discrete logic components, op amp/comparator, and some resistors on a small protoboard from ratshack. wired it in place, and it is still working as of this day.

at least when datasheets are available, if you have resistors burned and dont know what they are/were, at least you got something to go off of to find out what that circuit is a function of, and get a ballpark idea. I have done this ALOT. sometimes ive had to use basic ohms law to figure out what burned out crap was.