Electrolytic Cap Question.

I recently bought an old Percom Floppy drive on ebay. Two of the larger (3300uF) caps were swollen and leaking,
so I decide to change all the electrolytic caps. Now I have a problem. One of the 33uF 16-volt caps appears to have
one end connected to ground and the other end connected to a 6821 PIA reset (pin 34). What doesn't make
any sense to me, is that the silk screen is showing the + side going to ground! When I pulled the cap out, I found,
under the green coating, another + sign in copper on the other end from the silk screen + sign! I've included a picture
to show you what I'm looking at. Sadly, I don't remember which way the cap was orientated, so I'm open to suggestions!
I'm guessing that the silk screen is wrong!

David

Power it briefly and measure across the cap terminals for polarity.
The left hand end appears to be GND.

Most (inverted - active low) reset circuits have a capacitor to ground and a resistor to Vcc to produce a short reset pulse at power on. Normally, the cap would not be an electrolytic but a small ceramic (1uF or so). How big is the cap you removed and can you identify the value of the resistor charging it?

Mike.
Edit, sorry, I see you stated 33uF.
Edit2, can you confirm that the left hand end of the cap is connected to ground - continuity to pin 1 of IC.

Pommie said:

Most (inverted - active low) reset circuits have a capacitor to ground and a resistor to Vcc to produce a short reset pulse at power on. Normally, the cap would not be an electrolytic but a small ceramic (1uF or so). How big is the cap you removed and can you identify the value of the resistor charging it?

Mike.
Edit, sorry, I see you stated 33uF.
Edit2, can you confirm that the left hand end of the cap is connected to ground - continuity to pin 1 of IC.

Click to expand...

On the right-hand side of the parallel combination of R2 (18K) and CR3 (1N914) you can see that
there is a connection to a large trace on the board. This is a 5 volt rail that goes back to one of the
7805 regulators and ties to most of the 74LSxx chips (pin 14) on the board. The left-hand side of
the cap ties to large trace that bolts to the case of the floppy drive. See 2nd pic...



Sorry about the upside-down orientation...
David

Sorry I forgot to add that pin one of the 6821 chip is indeed tied to ground and pin 20 of the same
chip is tied to the right-hand side of R2 (5-volt rail).

David

The parallel combination of R2 and CR3 is to ensure the capacitor quickly discharges when power is disconnected. The 18k 33uF combination gives a time constant of almost 0.6 Seconds (must check my maths) which is a very long reset pulse. Maybe there is some component that needs a long settling time - might be the mechanics needs time to get up to speed. I would just put back the original (unless known dead) using the copper + sign for orientation.

Mike.

You might consider a 33uf film capacitor so you won't have to worry about polarity.