Capacitors in old amplifiers

[SAG Electronics]

I have been working on a few old amplifiers requiring new caps. I am curious, as I am a neophyte, why modern polar caps have the negative marked and old ones have the positive marked. I have been replacing caps and the ohm test method makes me think that I have installed them backward. The resistance slides down to 0. This all has me perplexed and frustrated as I have matched negative to negative and positive to positive. Good thing my clients are friends and allow me to take my time. Any education is appreciated.

[The Dude]

As to marking convention, I have no idea. If you think you installed them backwards, look at the schematic and verify. If no schematic, measure the voltage at the point the cap attaches and glean polarity from measurements. You say, the resistance "slides down to 0. Is that in circuit? By zero, do you mean 0 ohms or infinite ohms? A good cap out out of circuit should charge until the meter shows infinite resistance- the same reading as the test leads connected to nothing.

[xtian]

why modern polar caps have the negative marked and old ones have the positive marked

Great question! I have noticed that, too.

[J M Fahey]

Just thinking aloud: marking positive requires a "+" sign.
It may be missed or , Murphy´s Law standing, be on the opposite side.
*Some* caps printed a continuous line of"+" signs all around or at least 3 or 4 of them, so at least one is always visible..

While marking the negative just takes printing a thin unobtrusive line all around.
That said, I also have seen a few caps with a black band all around and tiny "-" signs embedded.

[Mick Bailey]

In addition modern wire-ended radial caps usually have different lead lengths to identify the polarity - the positive lead is longer. Except if it's non-polarized where the leads are different but the polarity doesn't matter.

[SAG Electronics]

Thanks for your replies. I have followed the schematics to the best of my ability and have installed all the radial and axial capacitors correctly. I will look into the interaction with later sections of the circuit and see if that gives me any more information regarding my readings. Above I said "slide toward zero" meaning the opposite of infinity. Still wondering why i get these readings. Also why the darn amp is still blowing fuses as soon as I put the power tube in.

[g1]

Are you using a needle type meter? If so, it should move toward zero while charging, then go back up toward the infinity side. And make sure it is discharged before checking.
Check known good new caps that are not connected to anything. That will give you an idea of how it should look.

And just to screw everything up, some meters need the polarity of the leads reversed for resistance measurement of caps.

[Enzo]

Your meter is charging the cap as it measures..Read the resistance and watch the reading slide. Now quick, reverse the probes. Now you should find a small voltage stored in teh cap. It may read as a "negative resistance" until it crosses zero.