Arcade Parts repair Mylar Capacitors

[CHRIS-F]

Hi Everyone, I'm new here hope I'm posting in the right place. I am new to electronics and arcade repair as a hobby, I know a little about electronics and am quite good at soldering. I recently bought a City connection game board of ebay labelled as "Working last time it was used 3 years ago, can't test due to not having an adapter" anyway I built a small adapter using a jamma 56 pin edge connector, a 36 pin edge connector and a jamma fingerboard (I wont tell you I built it back to front when I checked it" Ha! Ha! but before I got to test it I noticed that all the green coloured capacitors are cracked quite badly. so firstly does anyone know can I damage the board pluging it in if the caps are damaged will it do anymore damage or just not work? and secondly does anyone know where I can find suitable replacement caps? see attachment. so far I have learned that they are mylar film caps and some have a j in the number and some have a k, J is tolerance of 5% and k is 10% the numbers on most of them are .0015j50 how do I read these 4 digit caps and is the 50 at the end the voltage? I have found something similar on ebay but they are labelled as 2A152j. Anyone have any idea where I can find the right replacements or what type of modern cap I can use instead. Thanks in advance, Chris.

[ed12]

.0015 is the cap
50 is the voltage
with the value of .0015 it is more likely a timming cap
and not a normal bypass
bypass >noise on the bus< is .1/.01.
digikey/mouser,just about any electronic house will have them in stock
stay away from fleabay,to many knock off's comming in
and as a rule the j/k beening tolrance they can at that voltage perty much interchange

ed

[CHRIS-F]

Thanks for your reply Ed, so is the value of the caps in uf or pf or?? 0.0015uf? also what is the modern equivalent of the mylar cap? I can't find any mention of mylar caps in the likes of maplin, I am used to using polarized metal can type electrolytic caps but I know nothing of this other type. I had a quick look on the sites you suggested but couldn't find a suitable replacement, when I typed the value in, the search results only showed voltages of 500+ I know you said stay away from ebay but what makes you say that, are the cheap knock off caps way out of tolerance or do they just fail prematurely? I found these which seem to be the right type from Hong Kong http://www.ebay.co.uk/sch/i.html?_odkw=.0015+mylar+fil+cap&_osacat=0&_from=R40&_trksid=p2045573.m570.l1313.TR0.TRC0.H0.X.0015uf+mylar+film+cap&_nkw=.0015uf+mylar+film+cap&_sacat=0

Also the ones on my board seem to be 50v and the board only uses 12v I know you can use a higher voltage but how high can you go? the ebay ones seem to be 100v.
Thanks again,
Chris.

[ed12]

see if this helps u?
if not ring my bell
>Capacitors.htm<
just google the capacitors.htm part

ed

[CHRIS-F]

Thanks Ed, but I have googled till I am blue in the face and I can't for the life of me find the right caps, when the cap has the value printed with 4 digits ".0015" is that measured in Uf? can I use any type of cap polyester etc as long as the capacitance is the same? and would 100v be too high to go on a 12v board? The originals are 50v. if you or anyone can point me to a link for .0015j50 Mylar caps I would be most grateful , I thought these would be really easy to find but it appears not. Also if my board is switched on with damaged caps will it do damage to the board or just not work properly, I am assumiong these caps are bad because loads of them are cracked quite badly.

Thanks again,
Chris.

[lilshawn]

mylar, ceramic and other small caps are often measured in microfarads...even if it's 0.015uf

technically it's PICO farads but for the sake of continuity of the markings between all caps... microfards.

for simplicity's sake just google "0.0015uf" or plunk that into the mouser/digikey search and you'll find what you are looking for.

[CHRIS-F]

Thanks Ed/Lilshawn things are starting to make a bit more sense now, I'm only new to electronics but am keen to learn....... Is nf and pf the same mesurement? I am familiar with uf as I have changed quite a few bad electrolytic polarized caps in the past. Also can the mylar caps be replaced with ceramic disc types of the same value? or other types? I had a good look at my board and these caps seem to be connected mostly between gnd and pin 16 on most of the chips, such as the "hd74ls157p" which appears looking at the pinout to be 5v or vcc is that just to smooth out the power supply?

Thanks again guys.

[lilshawn]

it's kinda like the kilogram, gram, milligram "order of magnitude" aspect of things.

or even like ohm, kilo-ohm, megaohm...

a picofarad is just a tiny portion of a microfarad, a microfarad is a tiny portion of a farad... etc.

1 000 000 picofarads is in 1 microfarad

1 000 000 microfarads is in farad

so 0.015 microfarads is the same as 15 000 picofarads (or conversely 15k picofarads)

0.0015 microfarads is 1500 picofarads (or conversely 1.5k picofarads)

and so on.

the reason you find these caps on the chips is because they are being used as what's called a "bypass cap" the tiny value helps eliminate noise from the 5 volt before it enters the chip reducing the chance of a power blip causing a glitch. even switching small transistors introduces a tiny blip to the power rail for a very short amount of time.

a large value (like 10uf) would allow a small blip to go straight through but help smooth out a larger "sag" in the power. a tiny value (like 1pf or 2pf) will quickly smooth a very short blip but be no good for a larger sag because it has no capacity.

[ed12]

again grey area on voltage
in a nice real world,5volt would be 5 volt
but supply houses carry what is most likely to move out
hence most of us will use a 12volt cap in place of a 5 volt
lord 1 suppiler i deal with olny carry's them in 16 volt
the voltage is before the cap will fail,so if u put a 12volt in where a 5 volt is the cap will
not fail until u exceed 12volt,about that clear
the fard's is where u need to stick your mind
as shawn pointed out exactley
but for voltage on a bypass 12v will work just fine
if it will fit ?

ed

[CHRIS-F]

Thanks Guys, I really appreciate your help. I was originally going to replace all 91 Mylar capacitors, but I just removed the ones I could see that were visibly cracked and replaced them, the board works fine. one question though, most arcade boards I have seen have the ceramic disc caps, but mine has the Mylar type all rated at 50v, my board only needs 5v and 12v for the sound, is it possible that someone has replaced all of them in the past with what they could find? as I say my board is working fine thanks again :-) I just wondered is it usual for manufacturers in the 80's to use the green Mylar type or ceramic discs, as I may replace them all to give the board a new lease of life, I'm going to do the polarized caps in the sound section too and there are a couple of others.

Thanks
Chris.

[ed12]

it is truly a tech cost thing
if the mfg was making a million or so board's ?
then they tend to lean to green >ohm's law apply's here on tolorance<
but on less run's the disk caps are cheaper
so like i said :tech-cost:

ed

[lilshawn]

there isn't much to go wrong on mylar caps...it's basically foil wrapped in plastic film dipped in epoxy. not much to go wrong there.

electrolytic caps on the other hand are similar to a battery, where you have a liquid electrolyte, foil plates, crimped connectors, rubber seals... plenty of places where it can fail and be no good.

i wouldn't worry about them too much, i'll bet if you measured them they'd still be spot on. 

[CHRIS-F]

Thanks again folks, I've got a pretty good understanding of those caps now, I bet the cracked ones I replaced were still functioning but I couldn't check them as I don't have a capacitance meter, speaking of which any suggestions for a good meter? I don't want a really expensive one just something to check components out on a few arcade boards/power supplies etc, are the stand alone meters better than a multi meter which does capacitance? I've seen those little devices that Maplin sell that look like an old pager and you just attach your component they identifiy caps transistors resistors etc are they any good?

Second question is, I came across some disc capacitors, in my search for replacements that were 50v and 1500pf which I thought would be a good replacement for the 0.0015uf Mylar's but the tolerance on them says +80%/-20%, and the originals were j/k 5%/10%.....question is if they are just to filter out the 5v supply for the chips such as the (HD74LS157P) is the tolerance that important and also is the actual capacitance that critical? is there a formula to work out the optimal capacitance? I am ok at replacing something when I know what the old part was, but if it was missing how would I calculate or make an educated guess of capacitance to use?

Thanks again,
Chris.

[lilshawn]

it is and at the same time it's not. it just depends where the tolerance is in respect to temperature of where it's being used and the size of the cap in question.
for a 1500 pf cap, +80% and -20% put it in the range of 1200 - 2700 pF ( 0.0012 - 0.0027 uF) which in the grand scheme of things is eff all. if we were talking about microfarads, 80% would be a HUGE difference. but in a tiny value to begin with, it's not much.

if it's staying relatively room temp and the circuit doesn't produce much heat, a 80/20 is probably fine and won't fluctuate much . if it's for a high current power supply that will range between room temp and 40 degrees over room temp, you'll want a more stable cap. if you are using the value of the cap as say, a timing cap for timing a flip flop or discharge circuit, you'd want a more accurate value that is stable because changing value will cause the discharge of the cap the change and thus the timing of the circuit to change.