Electrolytic Cap Question.

[DavidMil]

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

 

[MaxHeadRoom78]

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

 

[Pommie]

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.

 

[DavidMil]

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.

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

 

[DavidMil]

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

 

[Pommie]

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.

 

[tim-f]

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