Start replacing ceramic caps now!

[357]

seeing it more and more. Ceramic caps on rectifiers in the power supply slowly shorting out and making the transformer smoking hot just in RX or pumping the voltage on the regulator to 16v.

TRC 458, ceramics on the rectifier and regulator sending the volts to the moon.

TRC 850 ceramics on the output of the transformer making it screaming hot just looking at it.

Cobra 148 ceramic in the final section causing a short and sending the base volts to the moon.

The time is now.

 

[sp5it]

Did you measure them?

 

[Tallman]

I have only rarely seen ceramic capacitors fail. Electrolytic and Mylar and Tantalum, I have seen may of those fail. In power supplies it is electrolytic capacitors used primarily. Are you "Volting the radio?"

 

[Robb]

My limited experience with working on any electronic gear has netted no failures of disc ceramic caps as of yet (last 10 years).

 

[nomadradio]

A couple of times a year.

Out of literally hundreds of radios, tube and transistor.

This does not include surge-induced failures.

Most-common failure is disc caps across the AC power line. They will serve as a demented surge protector in a lot of RCI-made base stations.

Tram D201A:

We see this a *LOT* in RCI-made base radios with various names on them.

It's amazing how many radios power up just fine after repairing only the damage in this line-filter board. We adopted the use of safety-approved disc caps, covered with the logos of UL, RU, DVE, etc. printed all over the part.

Might reduce some future liability, might not.

Just safer.

73

 

[Robb]

Nomadradio:
Aren't there aggravating things that can happen to a ceramic cap that would cause them to fail?
eg - exposure to high levels of moisture/humidity?

 

[Handy Andy]

Edited - by Author - due to Deprecation.

 

[Robb]

So; they can still conduct RF and not be a total DC short too?

 

[Handy Andy]

Yes, marginal breakdowns - the SR ratings makes them look like they're the typical insulator - isolator but then they approach their breakdown ratings - they appear as dead shorts and heat up. You can get burned by the heating they will emit when they are failing.

They are a lot (the SR rated ones) like the Varistors - only when they start to take on more hits against their rated breakdowns - they get more and more inclined to fail while in operation but show nothing wrong with them out of circuit. They are a lot like DIACS - they clamp like they should but in RF environments - they never truly reset and simply conduct both forward and reverse until melted - then you know they're gone - but until then, they will "poke and short" and make a good SWR read from just moments earlier - look like you're in the 3+ range because the tank circuit it's in - is shooting out RF and bias at the same time.

 

[Tallman]

A quick way to determine if the caps are on their way out - is to press your fingers into the traces and a slight tingle when you key up - shows me that the part may on the way out because it's not coupling right - it's using you too...

I urge everyone DO NOT PUT YOUR FINGERS INSIDE OF A KEYED RADIOS OR AMPLIFIERS! Unless you wish to get some nice RF burns or even Die.

 

[Handy Andy]

Ok, ok...good point - thought this was a CB Radio forum...

Cobra 148 - TRC - 458 - CB right?

 

[Tallman]

Ok, ok...good point - thought this was a CB Radio forum...

Cobra 148 - TRC - 458 - CB right?

And CB'ers never run amps or converted ham radios

 

[Handy Andy]

Don't worry not flaming - you make a valid point would you like for me to remove the post?

 

[Tallman]

That's up to you. I was just worried about the safety of the other members here on the board.

 

[nomadradio]

A ceramic cap will overheat and fail from RF current in excess of what it can handle. Older RCI EPT3600 radios used a ceramic disc cap just out of the final transistor, blocking DC voltage from the circuit leading to the antenna. It was prone to overheat and fail in the hands of a long-winded operator. RCI learned to use a silver-mica capacitor in that spot. It takes the final stage's RF current better with less power lost as heat.

Any part can fail for no obvious reason, that was the "2 or so per year" that we see here. As for caps that are damaged by excess voltages and current, that's just different. A spontaneous failure with no explanation is still rare for a ceramic disc.

Breakdowns from stresses that exceed the limits of the cap's rating are far more common.

Not the same thing.

73