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Is this a capacitor?

T

tezza

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Hi,

I'm working on my third Lisa. The motherboard doesn't work on this one. The computer refuses to switch on. In searching for the fault I came across this component which looks a little like a diode but I think it's a capacitor. It's at C13, which also seems to suggest a cap. There is no polarity band. It's markings say 50V, Z, 104. What do you think?

mystery component.jpg

This may be the culprit as it shows low resistance (280 ohms) across it wereas the same part on a working board shows no connectivity at all. It may be causing enough of a short for the PSU not to fire.

I've never seen a cap like this (if indeed it is a cap). Can it be replaced by a different type?

I've looked for a schematic of the Lisa motherboard to check but can't find one. There are a few out there called "Lisa Motherboard" schematics but these turn out to be ones of the motherboard of the Lisa Widget drive!

Thanks in advance.

Tez
 
It's a capacitor. .1uf. Pretty standard little "deglitching" cap, it can be replaced by any other .1uf cap. Alternatively, since these are just for additional noise suppression, most all circuits will work without them. Just desolder one leg and try it..

-Ian
 
I googled 50V Z 104 and got plenty of hits for ceramic capacitors used where voltages spike (common tantalums do to handle voltage spikes at all for example).

If you took a resistance measurement while it is soldered in you could be reading a short somewhere else. 280 Ohms isnt a dead short anyway, at a full 50V the capacitor would act like an 9W heater (.18Amps) getting toasty and that doesn't look burnt to me. Unsolder the part and then check the 2 solder pads for a short (meaning something else is causing the problems).
 
Unknown_K said:
I googled 50V Z 104 and got plenty of hits for ceramic capacitors used where voltages spike
Click to expand...

This is a ceramic capacitor. It's just enclosed in a little glass envelope. They were once very common, however now the plastic coated ones are more popular (probably cheaper).

Apple used a lot of these. Look at a Lisa RAM board, and each and every RAM chip has one of these caps for deglitching.

-Ian
 
I dont know what picture you people were looking at, but thats a zener diode. I've never seen or even heard of a capacitor encased in glass.
 
luckybob said:
I dont know what picture you people were looking at, but thats a zener diode. I've never seen or even heard of a capacitor encased in glass.
Click to expand...
"There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy."
Hamlet Act 1, scene 5.
;-)
 
I'm just saying, I've seen a lot of ducks. And all the ducks Ive seen have feathers. Hell, I spent 4 years in college learning about animals! You might be correct that its not a duck but in fact a goat, but I have to admit. its the first goat thats ever looked like a duck that i've seen.
 
Yes, they're surface-mount chip capacitors encapsulated in glass. I guess it saves cost, with only having to produce SMD capacitors and just encapsulating them in a single size of package.

104 denotes the capacity: 0.1 uf
 
glitch said:
Yes, they're surface-mount chip capacitors encapsulated in glass. I guess it saves cost, with only having to produce SMD capacitors and just encapsulating them in a single size of package.
Click to expand...

I'm not 100% certain, but I think these slightly pre-date SMT. But you're right on the value--0.1µF.
 
Thanks for the quick replies guys.

@Unknown_k; it was checked witb one leg desoldered. As you say it does show some resistance. However its double on the other board shows complete resistance.

Is a standard ceramic ok as a sub?
 
Thanks for the quick replies guys.

@Unknown_k; it was checked witb one leg desoldered. As you say it does show some resistance. However its double on the other board shows complete resistance.

Is a standard ceramic ok as a sub?
 
Yes, anything around that value will work. I usually use monolithic bead capacitors, but your standard disc ceramic works...it's just larger.
 
Just to let you know the result.

No, the replacement didn't make any different. On looking closely at the board (very closely) there are tracks which almost seem to disappear due to what would seem to be a past leaky battey episode. In fact, there is no connectivity between where some of these fine tracks start and where they are suppose to end. I'm assuming they are damaged.

I could rewire them underneath the board but I've decided not the bother (for now). My masochism does have it's limits. The non-working cage/motherboard and the third Lisa shell make a good container to hold all the working spare parts in case one of the other working Lisas suddenly fall over. :)

Tez
 
You may not need to rewire them--there exist PC repair pens that allow you to draw a trace using conductive ink. Very handy thing to have around.
 
If you have 2 working units then making up another probably isn't worth your time (unless you plan on selling it). Years ago I got a massive amount of Apple Powerbook G3 wallstreet parts and ended up building 5 complete working units before I got bored and left the other parts as is.
 
Chuck(G) said:
You may not need to rewire them--there exist PC repair pens that allow you to draw a trace using conductive ink. Very handy thing to have around.
Click to expand...

If just a few traces are bad rewiring using that vellum wire is pretty simple and quick. I think those conductive inks are ok for logic but not for something that might have power going over it.
 
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