Topics

PCT55 - AC95 - Smoke seen - Checking capaitors


TimG
 

Hi,
In my previous topic "PC Turn 55 - Info for a newbie", J G kindly advised that the smoke seen from my AC95 module may come from a degraded capacitor.  I have removed the AC95 from the PCT55 and looked for evidence of damage.

Please see 2 photos attached.

I was focused on checking the large cylindrical electrolytic capacitors, but seeing no indications of burning.  I then noticed discolouration of a smaller bead capacitor, perhaps it's a Tantalum pearl electrolytic capacitor.  There is also a bead of metal(?) evident on the discoloured capacitor.

Can anyone help me identify the value of the replacement capacitor?  Do you have an AC95 handy to take a photo of that capacitor and its markings?
I suspect that the Emco electronic schematic for the AC95 is not in the public domain, has anyone identified the component values?

Tim


atsel02.ell@...
 

On Mon, Jun 8, 2020 at 03:16 AM, TimG wrote:

Hi,
In my previous topic "PC Turn 55 - Info for a newbie", J G kindly advised that the smoke seen from my AC95 module may come from a degraded capacitor.  I have removed the AC95 from the PCT55 and looked for evidence of damage.

Please see 2 photos attached.

I was focused on checking the large cylindrical electrolytic capacitors, but seeing no indications of burning.  I then noticed discolouration of a smaller bead capacitor, perhaps it's a Tantalum pearl electrolytic capacitor.  There is also a bead of metal(?) evident on the discoloured capacitor.

Can anyone help me identify the value of the replacement capacitor?  Do you have an AC95 handy to take a photo of that capacitor and its markings?
I suspect that the Emco electronic schematic for the AC95 is not in the public domain, has anyone identified the component values?

Tim


 Hi tim 
I have an AC 95 board from a PC MILL 125 but it is also broken and I cant get it until my school opens again.
This are the only photos I have of it.


TimG
 

atsel02,
Thank you for the photos of your AC95, it would be great to get the capacitor values when you can get close to the hardware again.
Keep safe, thank you


J G
 

Difficult to tell from here but that does look like power circuitry as suspected. In my experience the tantalums often fail short circuit ( from overheat / melting) while the polyprops and polystyrenes often fail open (from total destruction) - all mainly due to age degradation or prolonged overstressing from marginal cicuit design.
This means the tants can be more likely to cause damage to preceding circuit elements such as power transistors, regulators, fuses ... when they go. Not always tho and it at least its clear where to look.
Just FYI, I find electrolytics of this machine's era quite reliable. There is a widespread well known problem with modern electrolytics from "certain sources" due to poor manufacture - esp in high stress applications. I've had to replace them on numerous motherboards, LCD monitors, appliances etc. They don't usually fail catastrophically (smoke !) - just become quietly high impedance.
In your case, depending on the circuit function involved - eg de-coupling, the value may not be all that critical.
Regards JG