Generally an 1800uF 25V electrolytic cap in good order would be expected to have an ESR of between 0.05 and 0.1 Ohms. Any higher than 0.1 Ohms would be very suspicious.
There are generally two types of ESR meter out there, one type uses a 100kHz sine wave test signal, the other type (such as the Anatek Blue meter, designed by Bob Parker) uses a pulse method. Which I think is a better method for assessing the ESR of SMPS capacitors, because they are in circuits with current pulses, not 100kHz sine waves.
Interestingly, the original Bob Parker design ESR meter kit, is still available from Altronics, I regard these as the World's best ever ESR meter:
ESR Meter Kit. Check website for full details at Altronics.com.au
www.altronics.com.au
( not everyone might agree, but keep in mind I am a very harsh judge of everything, including test equipment and seldom easily impressed )
Apart from the ESR, the capacitor should also be checked on a capacitance meter and checked for excessive electrical leakage at it max operating voltage for a full health checkup.
One interesting thing about electrolytics, they can just leak out a little electrolyte, or if they have leaked in the past, sometimes it is not obvious. However if they have leaked in the past, the electrolyte soaks into the surface of the rubber bung on the base, and makes the surface conductive and if you check with the Ohm meter, you can detect electrical leakage by pushing the probe tips into the surface of the rubber bung. There should be zero electrical leakage.
One other thing, if a particular brand and age and type (uF and voltage rating) of electrolytic had leaked electrolyte on a vintage pcb, it is highly likely that its same aged bothers and sisters on the pcb, will do the same thing in the next few coming years. This can be a clue that it might be wise to change all of the ones of the same type, if one has chosen to leak electrolyte.
