Ceramic disc capacitor failures

[agardiner]

I have had 3 separate instances in the last month of radio's with failed ceramic disc capacitors. I had always thought that these components were super reliable, and to be honest I cannot remember ever changing one in my 35+ years in the trade. (with the exception of high voltage ones in TVs)

All 3 items were low voltage battery sets. In all cases, the failure mode was it basically changing into a resistor; typically around 35 ohms. In one radio, a Zenith Transoceanic, I had to replace 4 disc capacitors which had all turned into resistors.

They were of course never designed to be in service this long, but it does seem that some components that previously didn't come under suspicion are now starting to cause some interesting faults.

Had anyone else come across this?

[ms660]

Yes, me, back in the day those little thin ones in transistor circuits used to fail leaky or short circuit and the ones in higher voltage circuits used to also fail, usually with them (higher voltage) the giveaway was often a blob of once molten metal protruding from the encapsulation.

Lawrence.

[Cruisin Marine]

The black one's in Grundig gear do that, I remember them failing back in the early 90's

[Terry_VK5TM]

Yes, in small Philips portable tv's (forget the model), picture used to slowly lose colour and if you squeezed a particular ceramic cap, it would come back.

Only seemed to affect one particular model for some reason.

[turretslug]

There seemed to be a rash of untrustworthy high-capacitance/low-voltage disc ceramics, made around late '60s/early '70s? Marconi test gear (and no doubt some others) suffered a spate of them. One time, I bought a job lot of components that included a bag of several dozen NOS 100nF 30V ceramic disc caps that were very similar to those that I had evicted en masse from a TF2002- every single one was grossly leaky, even using a 1.5V ohmmeter! I wondered if there was some sort of metal migration through an excessively willing dielectric, maybe a sort of "crystal garden" effect.

About 30 years ago, I also became very mistrustful of the-then seemingly wonderful miniature multi-layer ceramics- again, high-capacitance/low-voltage types- I seemed to be seeing multitudes of faulty logic and audio/video processing cards where one capacitor among several dozen in parallel that were decoupling supplies would go very low resistance, shutting down carcase PSU. Connecting a heftier but current-limited PSU across the supply terminals soon revealed the sweaty culprit, though this course of action earned me a Krakatoa-grade bollocking from my supervisor at the time, I suppose he thought I could have fused tracks on a now-obsolete part.

I gather (though I stand to be corrected) that there was something of a learning curve with multilayer ceramics in that their low ESR could be self-destructive if they were too directly connected across low-impedance supply sources, there was a similar industry experience with tantalum bead capacitors that wasn't helped by manufacturer thrifting. Nowadays, I'm quite happy to use multilayer ceramics, but always with an eye to possible situational caveats.

[unitelex]

Originally Posted by agardiner

I have had 3 separate instances in the last month of radio's with failed ceramic disc capacitors. I had always thought that these components were super reliable, and to be honest I cannot remember ever changing one in my 35+ years in the trade. (with the exception of high voltage ones in TVs)

All 3 items were low voltage battery sets. In all cases, the failure mode was it basically changing into a resistor; typically around 35 ohms. In one radio, a Zenith Transoceanic, I had to replace 4 disc capacitors which had all turned into resistors.

They were of course never designed to be in service this long, but it does seem that some components that previously didn't come under suspicion are now starting to cause some interesting faults.

Had anyone else come across this?

I don't remember any failed disc ceramics. I have had a similar experience though over the last few weeks with failed tubular (dog bone) ceramics, normally I would never suspect these. Two instances, one was in an audio circuit on the grid of a valve and it was breaking down causing crackling and distortion. Another was in a VHF tuner head, no local oscillator, replaced it and the LO started working.

Chris

[FrankB]

I have seldom seen a failed disk ceramic capacitor unless it was in the HV section, and then they usually had a burned spot on them or they exploded.
Of course, the sets coming into the U.S. may have had different component specifications than the sets in the U.K.

I do seem to remember a couple going wonky from heat in tuning circuits in a radio and one in a TV set. A close look (IIRC) at the leads showed the ceramic coating had not fully sealed the capacitor. I suspect maybe moisture caused the failure through corroding inside the capacitor. Possibly lead to ceramic failure?

[Radio Wrangler]

The ones we used in professional test gear seemed to be very reliable, both decoupling grade and high stability tuning grades. The decouplers had a proportion dead on arrival out the box, but beyond that, once fitted we had very little trouble.

This changed with surface mount types. As well as dead off the reel ones, a significant proportion of decoupler type ones would go short circuit over a period of hours to months.

This seemed to be a matter of moisture ingress through microscopic cracks causing migration of metallisation through a process like electroplating. The cracking may have been related to thermal shock in soldering and cooling. This is much more severe for the tiny thermal masses of SMT parts. Years later, working in the avionics industry, using flexible-termination SMT larger parts to survive board flexing, things seemed more reliable again. We hit these with elevated humidity and severe vibration testing (vibration levels of a faulty helicopter!) and didn't have any trouble.

David

[agardiner]

Interesting responses so far. It would seem on balance that I have been lucky up to now! Still, it is not really surprising that some sill go faulty after all this time!

[CambridgeWorks]

Around 1980, the Pye Telecom MF6AM mobile vhf tx/rx started developing random intermittent tx no power faults. Ron Swain, a great, methodical workshop engineer at Nottingham depot eventually proved it was due to leakage of one or more 4n7(yellow colour) Philips ceramic plate capacitors in a dc coupled series modulator stage at fairly high impedance.
He had the confidence to challenge service hq at Cambridge and stood by his findings, supported by his manager. It was said that over half a million were in stores and would be disposed of!
The failures happened at random, if at all. Even years after production. Some never failed.
For a while, new capacitors had what looked like a conformal coating.
Rob

[Jon_G4MDC]

Arbury Road ~1980. Opening MF6AMs spraying them and putting them back together.
It's what student engineers are for!

[G6Tanuki]

I've had a few of the flat circular types encapsulated in a matt-brown biscuit-coloured cement go leaky: one that was a detector/AGC line decoupler in a transistor radio was a real pain to track down since its leakiness seemingly was synchronised to the phase of the moon, the instantaneous value of the Dow-Jones Index and the number of days to the dog's birthday.

I've also had them go microphonic - which is sort-of understandable since there's not that much difference between a piezoelectric transducer and a flat ceramic capacitor.

The cylindrical NP0/N750 types, OTOH, seem boringly reliable.

[McMurdo]

I've really seen very few failures in my career as a repair person. A little brown one went leaky in a NAD amplifier once but I thought it had got too hot. I've had several of the axial yellow types in the same make of amplifier go leaky or even short, usually the high capacitance type eg 0.22uF or 0.47uF.

Maybe a few SMD caps over the years going short.

I was given a bag of the brown disc ceramics from somewhere and they were obviously rejects as some of them had missed the dipping process and had bits of the disc exposed. Those all went in the bin before they had time to fail!

[agardiner]

Originally Posted by G6Tanuki

I've had a few of the flat circular types encapsulated in a matt-brown biscuit-coloured cement go leaky: one that was a detector/AGC line decoupler in a transistor radio was a real pain to track down since its leakiness seemingly was synchronised to the phase of the moon, the instantaneous value of the Dow-Jones Index and the number of days to the dog's birthday.

So true!

[MotorBikeLes]

I have two different experiences. My first NordMende portable radio, a Transita Spezial. Where I lived, in a lowish valley, whenever an aircraft flew over, I would get that familiar rising and falling sound. But then it started happening, and instead of disappearing, it continued on occasions. I then notices that colder or foggy mornings were a factor. i was a bit (a lot) green back then (more than 50 years ago) but I formulated a plan of action. I wrapped the radio in a plastic bag, and left it in the 'fridge overnight. Next morning, unpacked it, refitted batteries, and sure enough the problem was there. I then touched various components to warm them up, and a disc ceramic from one connection on the VHF tuner to ground advertised itself to my finger. Bingo.
Some years later, I was reconditioning Grundig panels, and found a few problems with tuning panels which I traced to black 0.1U ceramics. Not discs, but little rectangular things. With an Avo on high resistance scale, leaks were found. These caps had a matte surface, and I concluded they were adsorbing moisture. I replaced then with red Wima caps and had no more trouble. On later models, I found the same components, now blue, and with a shiny coating, maybe epoxy or similar. None of those gave trouble. Similar to that sited by Robinshack earlier. Later panels changed, like me, to red plastic Wima devices.
Les.