Plesseal Precison Caps - or not! - UK Vintage Radio Repair and Restoration Discussion Forum

Pamphonica · 14th Dec 2014, 2:01 pm

I have just been restoring a 1960s Daystrom/Heathkit Audio generator (AG9U) and wondered why it was struggling to oscillate.

I tested all the components and wondered if I was dreaming when I tried the 0.5uF 2% Plessey cap in the attached photo. It was reading 3.9uF out of circuit! I tried other cap meters and got the same reading.

The 50nF was reading 180nF and the 5nF was reading 9nF. The final two caps (silver mica 500pF and 50pF) were absolutely spot-on. So the little twin-T filter frequency-determining network was not working at all well.

These 2% "Plesseal" caps are probably the most off-spec components I have seen for a bit - 800% out (but not open or short)!

ukcol · 14th Dec 2014, 2:16 pm

Hi Jeremy,

What are you using to measure the capacitance? Simple instruments, such as a DMM, measure the current through the capacitor at a particular frequency and show leakage current as an increase in capacitance.

Pamphonica · 14th Dec 2014, 2:26 pm

Peak Atlas LCR meter, plus a chinese-made multi-component tester.
I could haul out the precision 6-digit LCR bridge but can't be bothered!

Heatercathodeshort · 14th Dec 2014, 3:26 pm

Capacitors often increase in value with age. This has been mentioned in other threads. The ones I have encountered have almost doubled in value but have not been leaky in any way. These Plessey capacitors would have been 2% or less when new. J.

Radio Wrangler · 14th Dec 2014, 4:02 pm

Water has a VERY high dielectric constant. About 80.

David

Lucien Nunes · 14th Dec 2014, 4:55 pm

If these are rebranded Wima toffees as I suspect, I wouldn't put anything past them!

MotorBikeLes · 14th Dec 2014, 6:06 pm

The early Telequipment S31 "Serviscope" used those in the timebase and elsewhere. I learned to replace them all on sight. Usually S/C or leaky; I never had need to check them for capacitance, they were in the bin too quickly.
Les.

Sideband · 14th Dec 2014, 7:43 pm

Quote:

Originally Posted by Lucien Nunes

If these are rebranded Wima toffees as I suspect, I wouldn't put anything past them!

They look suspiciously like it....change on sight...preferably don't fit them in the first place!

As has been said, most cap meters will measure leakage as a higher value capacitance. Put them on a proper bridge and their true fault will be seen. I would expect the 'loss' control to be hard over and still not be able to balance the bridge.

Pamphonica · 14th Dec 2014, 8:36 pm

There speak the voices of experience! I'll add them to my "change on sight" list.

14th Dec 2014, 2:01 pm	#1
Pamphonica Nonode Join Date: Mar 2008 Location: Virginia Water, Surrey, UK. Posts: 2,877	Plesseal Precison Caps - or not! I have just been restoring a 1960s Daystrom/Heathkit Audio generator (AG9U) and wondered why it was struggling to oscillate. I tested all the components and wondered if I was dreaming when I tried the 0.5uF 2% Plessey cap in the attached photo. It was reading 3.9uF out of circuit! I tried other cap meters and got the same reading. The 50nF was reading 180nF and the 5nF was reading 9nF. The final two caps (silver mica 500pF and 50pF) were absolutely spot-on. So the little twin-T filter frequency-determining network was not working at all well. These 2% "Plesseal" caps are probably the most off-spec components I have seen for a bit - 800% out (but not open or short)! Attached Thumbnails __________________ Jeremy, G8MLK, BVWTVM Friend, VMARS, BVWS Secretary. www.pamphonic.co.uk www.bttt.org.uk

14th Dec 2014, 2:16 pm	#2
ukcol Rest in Peace Join Date: May 2003 Location: Harlaxton, Lincolnshire, UK. Posts: 3,944	Re: Plesseal Precison Caps - or not! Hi Jeremy, What are you using to measure the capacitance? Simple instruments, such as a DMM, measure the current through the capacitor at a particular frequency and show leakage current as an increase in capacitance.

14th Dec 2014, 2:26 pm	#3
Pamphonica Nonode Join Date: Mar 2008 Location: Virginia Water, Surrey, UK. Posts: 2,877	Re: Plesseal Precison Caps - or not! Peak Atlas LCR meter, plus a chinese-made multi-component tester. I could haul out the precision 6-digit LCR bridge but can't be bothered! __________________ Jeremy, G8MLK, BVWTVM Friend, VMARS, BVWS Secretary. www.pamphonic.co.uk www.bttt.org.uk

14th Dec 2014, 3:26 pm	#4
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: Plesseal Precison Caps - or not! Capacitors often increase in value with age. This has been mentioned in other threads. The ones I have encountered have almost doubled in value but have not been leaky in any way. These Plessey capacitors would have been 2% or less when new. J.

14th Dec 2014, 4:02 pm	#5
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,866	Re: Plesseal Precison Caps - or not! Water has a VERY high dielectric constant. About 80. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

14th Dec 2014, 4:55 pm	#6
Lucien Nunes Rest in Peace Join Date: Apr 2008 Location: London, UK. Posts: 2,508	Re: Plesseal Precison Caps - or not! If these are rebranded Wima toffees as I suspect, I wouldn't put anything past them!

14th Dec 2014, 6:06 pm	#7
MotorBikeLes Nonode Join Date: Dec 2012 Location: Kirk Michael, Isle of Man Posts: 2,350	Re: Plesseal Precison Caps - or not! The early Telequipment S31 "Serviscope" used those in the timebase and elsewhere. I learned to replace them all on sight. Usually S/C or leaky; I never had need to check them for capacitance, they were in the bin too quickly. Les.

14th Dec 2014, 8:36 pm	#9
Pamphonica Nonode Join Date: Mar 2008 Location: Virginia Water, Surrey, UK. Posts: 2,877	Re: Plesseal Precison Caps - or not! There speak the voices of experience! I'll add them to my "change on sight" list. __________________ Jeremy, G8MLK, BVWTVM Friend, VMARS, BVWS Secretary. www.pamphonic.co.uk www.bttt.org.uk