Why does a 50uf 350v Capacitor only read 35pf on a Chinese tester?

[Murphyv310]

At the moment I'm only getting the odd readings with a 55 years old dual can capacitor in a Pye Piper, and what I'd term a NOS modern dual 32uf can I'd fitted to aradio  a few years ago, which hasn't been powered in a long time.  I'l try a few new high voltage types (10u/400v or similar) to see what readings I get with them.

Hi.
I think you are answering your own questions yourself! Capacitors deteriorate overtime especially if they are left unused, re-forming will often bring a capacitor back to a useful level of operation, I have found though that often the ESR never returns to it's original state.
You need to really test your Cap tester with a new capacitor to ascertain if the tester is to spec. I personally refuse to use test equipment if it is in any way faulty or inaccurate, it will be destined to the bin.
Regarding checking the values of old capacitors its far better to check them on a bridge, modern testers use a high frequency oscillator to test ESR and they also calculate the capacitance from the charge time, no good if the cap is leaky or has an ESR that is wildly out.
Perhaps you will get a better reading if you perform a long and slow reforming session, probably unlikely but worth a shot.

[Old Sparky]

I have since tested a new radial 450v electrolytic on the same tester, and the correct capacitance is displayed along with perfectly normal figures for ESR & Vloss, so the tester is OK, so reforming or replacing the 100 + 50uf can in the Pye Piper may be the way to go. I made a note somewhere that the original cathode bypass capacitor (which I've replaced with a new one), a 25uf, 25v axial type, only read a few hundred NF

[Murphyv310]

I have since tested a new radial 450v electrolytic on the same tester, and the correct capacitance is displayed along with perfectly normal figures for ESR & Vloss, so the tester is OK, so reforming or replacing the 100 + 50uf can in the Pye Piper may be the way to go.  I made a note somewhere that the original cathode bypass capacitor (which I've replaced with a new one), a 25uf, 25v axial type, only read a few hundred NF

Good stuff!
What you are seeing is the often waste of time faffing around with old or even NOS capacitors. Capacitors have been the bane of many engineers lives since radio reared its head in the early 20th century.
My experience was that capacitors vary considerable, The Mullard mustards were the best around at the time and in the main are still perfectly good.
Electrolytic caps only really got good in the mid 70's and beyond till the cheap Chinese ones were imported then standards dropped, I'm talking about in consumer gear, there were seriously good caps about in industry though way back.

[Old Sparky]

Having now proved, by substitution, that the 100 + 50uf can is totally 'shot', and the very low capaitance readings on my tester were indeed correct, I shall now obtain a new Reservoir & smoothing cap. Should have done so right at the start!