Capacitor doubled its value? - UK Vintage Radio Repair and Restoration Discussion Forum

ITAM805 · 27th Jan 2021, 3:19 pm

I have a rare Baldwin Professional guitar amp from 1968 in for service. The first cap I tested is marked 500uF but it reads 1001uF with one end lifted but the ESR is low. Replace or should be ok? Where possible I like to leave amps in original condition and this one appears to work well

G6Tanuki · 27th Jan 2021, 3:28 pm

A lot of the small capacitor-testers are confused when faced with a leaky electrolytic.

The usual way these testers measure capacitance is to connect them through a known-value resistor to a voltage-source, then measure dV/dT as the capacitor charges.

From this you can derive the capacitance - but if the capacitor is leaky the rate-of-rise-of-voltage is slowed by the leakage-resistance (which appears in parallel with the capacitor) and so the 'apparent' capacitance is larger than expected.

If you're suspicious about a capacitor, replacement is the most sensible option.

paulsherwin · 27th Jan 2021, 3:34 pm

I agree, a very high reading normally means that the capacitor is leaky.

Radio Wrangler · 27th Jan 2021, 3:36 pm

As an electrolytic capacitor depolarises, the thickness of the aluminium oxide dielectric on the metallic electrode decreases. This simultaneously decreases the working voltage (which leads to failure) and increases the capacitance. Be VERY suspicious of a capacitor when the value goes up, whether or not it is caused by capacitance meters being foxed by leakage.

I agree, change it!

David

kalee20 · 27th Jan 2021, 5:04 pm

Quote:

Originally Posted by ITAM805

The first cap I tested is marked 500uF but it reads 1001uF with one end lifted but the ESR is loW.

If the ESR is low, that's good news.

Do you mean leakage resistance (equivalent parallel resistance?)

ITAM805 · 27th Jan 2021, 5:33 pm

Thanks guys. Kalee, ESR measured with an ESR meter?

Diabolical Artificer · 28th Jan 2021, 8:24 am

Check the ripple either using a scope or DMM on AC volts.

Andy.

Maarten · 30th Jan 2021, 12:36 am

If it has been in regular use, the capacitor would be beyond rescue.

If it hasn't been switched on for years, don't switch it on to check for ripple but first try reforming the electrolytic through a 47k resistor. If it won't reform (draw below 100uA or so) in some hours, I'd replace it.

27th Jan 2021, 3:19 pm	#1
ITAM805 Nonode Join Date: Jul 2010 Location: Folkestone, Kent, UK. Posts: 2,172	Capacitor doubled its value? I have a rare Baldwin Professional guitar amp from 1968 in for service. The first cap I tested is marked 500uF but it reads 1001uF with one end lifted but the ESR is low. Replace or should be ok? Where possible I like to leave amps in original condition and this one appears to work well

27th Jan 2021, 3:28 pm	#2
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 14,007	Re: Capacitor doubled its value? A lot of the small capacitor-testers are confused when faced with a leaky electrolytic. The usual way these testers measure capacitance is to connect them through a known-value resistor to a voltage-source, then measure dV/dT as the capacitor charges. From this you can derive the capacitance - but if the capacitor is leaky the rate-of-rise-of-voltage is slowed by the leakage-resistance (which appears in parallel with the capacitor) and so the 'apparent' capacitance is larger than expected. If you're suspicious about a capacitor, replacement is the most sensible option.

27th Jan 2021, 3:34 pm	#3
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,970	Re: Capacitor doubled its value? I agree, a very high reading normally means that the capacitor is leaky.

27th Jan 2021, 3:36 pm	#4
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,902	Re: Capacitor doubled its value? As an electrolytic capacitor depolarises, the thickness of the aluminium oxide dielectric on the metallic electrode decreases. This simultaneously decreases the working voltage (which leads to failure) and increases the capacitance. Be VERY suspicious of a capacitor when the value goes up, whether or not it is caused by capacitance meters being foxed by leakage. I agree, change it! David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

27th Jan 2021, 5:33 pm	#6
ITAM805 Nonode Join Date: Jul 2010 Location: Folkestone, Kent, UK. Posts: 2,172	Re: Capacitor doubled its value? Thanks guys. Kalee, ESR measured with an ESR meter?

28th Jan 2021, 8:24 am	#7
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,670	Re: Capacitor doubled its value? Check the ripple either using a scope or DMM on AC volts. Andy. __________________ Curiosity hasn't killed this cat...so far.

30th Jan 2021, 12:36 am	#8
Maarten Dekatron Join Date: Dec 2007 Location: Haarlem, Netherlands Posts: 4,203	Re: Capacitor doubled its value? If it has been in regular use, the capacitor would be beyond rescue. If it hasn't been switched on for years, don't switch it on to check for ripple but first try reforming the electrolytic through a 47k resistor. If it won't reform (draw below 100uA or so) in some hours, I'd replace it.