[Amps] Chinese Caps Defective @ RF

[mike kendall]

Jim,
 
   Isn't Y5T that is listed a class II ceramic?  The NPO/N750/N1500 are all class I ceramics that won't age like the class II and IV do.  Also, with the lesser classes, when you look at the curves the percentage of change is more significant with heat.  With a higher ESR there would presumably be more heating on 10 meters.  I'm guessing it heated to the point where it exploded.  I would suspect the Y5T is seriously derated on RF at 10 meters.
  HES is still making door knobs adn there are somne American companies out there making some good high voltage RF caps.
 
73,
Mike


--- On Sat, 3/24/12, Jim Barber <audioguy at q.com> wrote:


From: Jim Barber <audioguy at q.com>
Subject: Re: [Amps] Chinese Caps Defective @ RF
To: "Jim W7RY" <jimw7ry at gmail.com>
Cc: "AMPS" <amps at contesting.com>
Date: Saturday, March 24, 2012, 12:17 AM


I bought 20 of those on eBay and just hipot tested them TODAY, assuming 
they're the same thing. They break down around 4000 to 5000 VDC, even 
with no RF current on them. No warning, no leakage current slowly 
creeping up, just "bang" and then they're bad. My BVT is current limited 
to 200uA max, so that was a surprise.

Incredible and no joke: The very day I finish my Hipot/BVT and test the 
capacitors, there's an email about them three or four hours later. Scary.

73,
Jim N7CXI


On 3/23/2012 11:00 PM, Jim W7RY wrote:
> Well I had another Chinese cap blow up on me in my 8877 amplifier. I had 3ea
> 1000PF 12KV caps in parallel for the plate blocking caps. I was tuning the
> amp on 10 meters and KERBLEWEY!
>
> I have successfully used 2ea 1000PF 12KV in parallel plate blocking caps
> (just like Alpha does) in my 3CX800 amplifier and they lasted years in RTTY
> contesting. I replaced them a couple of years ago with doorknobs.
>
> One of them shorted (just for a minute ). That blew up the 100K resistor
> that I had across the output of the tank for keeping the voltage from the
> antenna. No big deal.. BUT was the big deal, is the Jennings RJ1A relay
> welded the common to the NO contacts. I was transmitting into a dummy load.
> It also blew the ceramic off of the 25 ohm 25 watt surge resistor that is on
> the WD7S power supply board. The resistor survived but I replaced it anyway.
> No other damage other than that.
>
> The effected caps are shown here:
>
> http://www.ebay.com/itm/10-15KV-1000PF-High-Voltage-Ceramic-Disc-Capacitor-Y5T-/150781333576?pt=LH_DefaultDomain_0&hash=item231b449048
>
> I don't think this is the vendor where I bought my caps but they are the
> same blue caps.
>
> I also had a .01 6KV Chinese cap blow up at the base of a TL-922 plate
> choke. Replaced with a Vishay/Sprague/Cerra-Mite .01 6KV and all is well
> (Mouser P/N 75-564R60GAS10 ). But at almost $6.00 a copy, I was trying to
> save a few bucks... How did that work out?
>
> I'm still using the Chinese caps in the 8877 amp to buy-pass the HV in the
> power supply and on the back of the RF deck where the high voltage connects
> (little to no RF here). But I'm using a Visay/Sprague/Cerra-Mite .01 6KV cap
> at the base of the plate choke.
>
> Bottom line... Be careful when using these caps in a high RF environment.
> 73
> Jim W7RY
>
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