PA100-D - 10m O/P Filter Power Dissipation

[5B4AIY]

I have been assisting some amateurs who have noticed that C27/220pF capacitor in the 10m output filter gets excessively hot and eventually fails.

C27 Discolouration due to excessive dissipation.

The above image shows this effect in my amplifier after tuning the amplifier into a dummy load for a short time. I have discussed this with Matti, and we agree that it was probably not the best idea to use a single capacitor in this filter position. The amplifier produces a certain amount of harmonic energy, and the circulating currents at this point in the filter are quite significant.

The capacitor's dissipation factor is such that it will dissipate a significant amount of power, which results in overheating. We suggest you examine your amplifier and operate it into a dummy load on 10m at an output power of around 80W - 100W for 20 or 30 seconds and then test the temperature of C27. If it shows signs of a significant temperature rise we suggest replacing this capacitor with two 100pF/100V silver-mica capacitors.

You will notice that there in fact two sets of pads at this circuit position. Since access is somewhat restricted on the top of the board, after removing the original capacitor, solder in the first 100pF capacitor from the underside of the board, and then insert the second on the top side and solder in place. In fact the output filter's frequency response will be slightly improved if two 100pF capacitors are used.

Adrian Ryan, 5B4AIY

[5B4AIY]

I have now replaced C27 with two 100pF 500V silver mica capacitors. With these in place operating the amplifier for an extended period of time into a dummy load at 100W output they remain cool, and showed not the slightest sign of heating. If you are experiencing this problem, then I would strongly suggest replacing C27. There are two pairs of through holes on the printed circuit, and these are spaced at exactly the correct distance to accomdoate these silver mica capacitors. As access to the top of the board is very restricted, after removing the original component, use solder wick to clean out the holes, and then insert the bottom capacitor first. Solder the capacitor to the pads on the bottom of the board to avoid damaging the relays on the top. Clip the excess leads. Then mount the top capacitor, and complete the work by soldering it in place on the bottom of the board and clipping the excess leads.

C27a - 100pF/500V

C27b 100pF/500V