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ibmapc
May 18th, 2012, 07:32 PM
I have three IBM CGA Boards. Two of them were not working. In fact, when inserted in a system, the power supply would not come on. So, I figured they must have a short. So I got out My ohm meter and started checking for shorts on +5v, -5v, +12v and -12v. The +12v was shorted with zero resistance. So I started following the trace and found a three legged tantalum capacitor ( C8 ) at the top edge of the board near the connectors for the light pen and the RF modulator which is also used for the composite monitor on the 5155. When this Cap was clipped off, the short went away. So, I found the culprit!! Both boards had the same problem, so I figure this must be a frequent failure point. I'm thinking that this capacitor is not needed unless a Light Pen or RF Modulator is used (The composite monitor doesn't use + 12v).

Please let me know if I am correct or not. Will these boards function ok with the cap at C8 removed? I can't tell if the +12v goes anywhere else on the board.

Regards,

Greg

Chuck(G)
May 18th, 2012, 07:41 PM
Well yes, tantalum caps get old and short (or explode) not infrequently. It's probably just used as a decoupling cap, so an aluminum electrolytic would probably be just fine in its place.

pearce_jj
May 18th, 2012, 10:41 PM
Three pin though?

MikeS
May 19th, 2012, 12:00 AM
Three pin though?The third lead is there to prevent inserting it backwards; 2 leads are fine, just observe proper polarity.

pearce_jj
May 19th, 2012, 12:10 AM
But aren't tants non-polarised? Could be a dual-cap?

MikeS
May 19th, 2012, 12:33 AM
But aren't tants non-polarised? Could be a dual-cap?Well, there are indeed non-polarized tantalums and electrolytics but they are really two capacitors back to back and only have two leads.

The ones you're likely to see are most definitely polarized, that's why they make three-legged ones so that it doesn't matter which way you insert them:

http://i.stack.imgur.com/oqEyK.png

pearce_jj
May 19th, 2012, 01:03 AM
Many thanks for posting that.

ibmapc
May 19th, 2012, 10:17 AM
Thanks everyone for your replies. However, no one has answered my question.



Please let me know if I am correct or not. Will these boards function ok with the cap at C8 removed? I can't tell if the +12v goes anywhere else on the board.



Maybe I'm just lazy, not wanting to ge the soldering iron out. Or, maybe I'm afraid of doing more damage since my soldering skills are sub par due to shakey hands and poor eyesight.

MikeS
May 19th, 2012, 10:42 AM
Thanks everyone for your replies. However, no one has answered my question.Only you can answer that, empirically.

Since every penny counts when you're mass-producing an item you can be pretty certain that there's a good reason for that cap to be there, while, on the other hand, good design practices also require over-designing slightly to allow for component tolerances and aging, variable external factors like noise coming from other boards etc.

So, the answer is: probably.

But if other decoupling and filter caps have weakened or if the power supply and other things connected there are generating extra noise or transients then you might well experience the occasional mysterious glitch, even if nothing on that board actually uses the 12V.

So, part 2 of the answer: You should replace it as a precaution to avoid possibly losing hair, your temper, etc. at some future time.

ibmapc
May 19th, 2012, 10:58 AM
So, the answer is: probably......
......So, part 2 of the answer: You should replace it as a precaution to avoid possibly losing hair, your temper, etc. at some future time.

Thanks Mike, I guess you gave the answer that I was afraid of. I've ordered a few of the replacement capacitors from Mouser and will replace them to keep the boards as close to original condition as possible.

Do you know if +12v is used by the CGA board? Or is it only used by the Light Pen or the RF modulator? Since I don't have either of these to connect to the board, I was thinking that the missing capacitor would not be needed.

MikeS
May 19th, 2012, 11:36 AM
Thanks Mike, I guess you gave the answer that I was afraid of. I've ordered a few of the replacement capacitors from Mouser and will replace them to keep the boards as close to original condition as possible.If you're really worried about possibly causing damage why not just leave a note inside the computer, maybe with a new cap taped to it, and wait and see, so if it does get flakey down the road at least you'll be reminded and it might be worth while then; it won't cause any damage if you leave it out. Those caps are also used elsewhere so it's good to have a few on hand in anticipation of the tell-tale bang.


Do you know if +12v is used by the CGA board? I don't think so, but I don't have a schematic in front of me; maybe once modem7 gets up he can confirm it while having his Sunday morning tea.

I was thinking that the missing capacitor would not be needed.Probably not, but you still have the connector and pcb traces to act as antennas to pick up noise, which is what the cap is meant to get rid of.

Chuck(G)
May 19th, 2012, 11:58 AM
Easy enough to check.

The +12 uses a 0.047 F bypass and a 10 F decoupling cap. It's fed to P1 (4-pin header for a modulator) and P2 (6 pin header for a light pen). So, unless you're using either device, it's a no-op.

ibmapc
May 19th, 2012, 12:30 PM
Probably not, but you still have the connector and pcb traces to act as antennas to pick up noise, which is what the cap is meant to get rid of.

Thanks Mike, I hadn't considered the antenna phenomenon. I thought the capacitor was there just to absorb surges and noise when the attached accessory is turned on or off.


Easy enough to check.

The +12 uses a 0.047 F bypass and a 10 F decoupling cap. It's fed to P1 (4-pin header for a modulator) and P2 (6 pin header for a light pen). So, unless you're using either device, it's a no-op.

Thanks to you also Chuck, That's the same conclusion I came to with my visual and ohm meter inspection of the card.