Burned C18 on my PA100-D

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Burned C18 on my PA100-D

Postby HB9FZG » 05 May 2016 22:06

After running the PA100-D at 40m with about 7 Watts in G1? mode, some smoke came out of the unit. The culprit was C18 burned black. Any advice for a replacement?

73,
Uwe
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Dsc00553_c18.jpg
HB9FZG
 
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Re: Burned C18 on my PA100-D

Postby 5B4AIY » 06 May 2016 06:29

Hi, Uwe,

The capacitors used in this kit for the low-pass filters are ceramic, and it looks as if this one had a poor dissipation factor, and has absorbed power and burnt out. The best replacement is to use silver mica capacitors for all the capacitors in the filters, but as they are expensive I can well understand the designers using ceramic. You should be able to find on eBay a suitable replacement. In my case I've used 1,000pF/500V silver mica and have not had any further problems. Whilst at 100W the output voltage into a matched load would only be 70V RMS, 200V p-p, you need some margin to take account of less than perfect SWR, as well as the fact that the peak output power of this amplifier can be as high as 150W, hence I would recommend 500V capacitors.

I assume that you have made 'sanity' checks of the amplifier on all bands to ensure that it operates sensibly. For example, a poor solder joint in one of my filters resulted in a much lower output power on that band compared with the others, so it is important to set the amplifier at a low gain setting and check carefully that the output power on all bands is essentially the same, any significant discrepancy needs to be investigated or else you risk damaging the filters or the output transistors. I used a swept frequency signal generator on my amplifier and found that its overall frequency response when set to 10M is reasonably flat from 160m to 10m, so the output power should be within about +/-1dB on all bands. If you find that one band is low on output, then operate the amplifier into a dummy load for a minute or so at 25W to 50W, and then in the standby state carefully check the temperature of both the coils and the capacitors associated with that band to see if they are dissipating power. If they are, you need to investigate.

73, Adrian, 5B4AIY
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Re: Burned C18 on my PA100-D

Postby HB9FZG » 13 May 2016 20:56

Hi,

Here are some feedback about this issue.

The fact I use a end feeded wire antenna, the SWR an some bands are not like 1:1. Some have 1:2 or 1:3.
To overcome burned capacitors in the future, I decided to replace all of them with mica types rated for 500V.
Unfortunately they are so much bigger, you have to solder some of them on the under side of the pcb. Also some of the pins are a bit to thick to fit in the holes. You must use a file or other abrasive tools to reduce the diameter of the pins. But after that, it looks quite nice.
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Original_vs_New.JPG
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Posts: 9
Joined: 21 Apr 2016 19:54
Location: Swizerland


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