O/C alarm from the PA100-D when switching bands

[va2dv]

Hello group,
I using the PA100D with an Icom IC-705. The 705 is set at 5w.
The amplifier is in automatic band switching mode.
When I change the operating band on the radio, let's say 40m to 20m, the amplifier does detect the frequency change but I got a O/C alarm and must recycle the amp with the power button. It's a bit annoying since I am using this setup remote.
I know it is probably a very simple thing but cannot find the root cause.

[5B4AIY]

Hi,

This is a strange one. I am also using an IC-705 with my PA-100D perfectly satisfactorily. When you say you are using an automatic band detection mode, I presume that you mean the Frequency Sense mode, as the IC-705 does not have a RS-232 port from which to pick up band changes. You also do not say what gain setting you are using on the PA-100D, nor do you mention what mode you are using, CW, SSB, DATA, FT8?

The Over-Current alarm is a non-maskable primary fault indicator, and it means that for some reason the O/C latch was tripped by a current spike in excess of about 25A. This trip is virtually instantaneous. So let's see if we can determine what is provoking this.

The first step is to set the IC-705 output power to zero, set the PA-100D gain to G1, the lowest gain, and then change bands, and key the transceiver. Does it still trip? If not then set the output power to 1% and change bands again. Keep on increasing the power up to no more than 3 or 4 watts.

Remember, the PA-100D has a fairly high power gain, for a bipolar transistor amplifier, of about 15dB, and it is very easy to over-drive it and provoke an over-current alarm. Since the IC-705, even on its internal battery pack can still provide 5W, it is better to operate the PA-100D on as low a gain setting as possible.

Regards,
Adrian, 5B4AIY

[va2dv]

Hi Adrian,

First, many thanks for the reply. I went thru the testing process yesterday.
It seems that the occurence does decrease when using very low power but it still present.
As a exemple, even in SSB, with the IC-705 set at 1w and the PA100-D set at G1, I just key the microphone with no audio and it trigger the O/C alarm.
It's strange because I can see a short spike of 120w on the amplifier display. I am puzzled.

I've tried a few thing like turning off the compression on the 705, introducing a TX delay...with no correction of the problem.
My only discovery is that it only happen on 14 MHz when coming from another band.

I am not using any tuner as my antenna is fully resonnant on the bands I use. I have several chokes and ferrites on every possible leads.

Any additional suggestions ?

[5B4AIY]

PA100-Main-Board-Schematic-2 LPF & SWR Section.pdf

(72.22 KiB) Downloaded 4 times

Very strange! So, please confirm that this only occurs when you switch to 14MHz from another band. Let me have a think about this.

I am wondering if a 20m low-pass filter fault could cause this. If this only occurs when switching from another band to 14MHz, and does not occur on any other band, then this suggests it is connected with the 20m filter, as this is the only thing different. However since the 20m filter is also used for 17m, it should occur on this band as well.

Open the amplifier and carefully examine the filter, especially the capacitors. If the amplifier is fitted with the original capacitors then these could be somewhat lossy. If any look discoloured, dark brown, or cracked then they need to be replaced. The best capacitors are silver-mica.

I had to replace some capacitors in my amplifier because they were dissipating power and became warm.

Another possibility is a stuck relay, or faulty relay driver. Using the schematic I am attaching, with the amplifier disconnected from everything including the power supply, check with a multimeter that at the normally open contact of RL1B it is an open circuit to ground. If not there is a relay stuck closed.

Next, with the amplifier powered up, but disconnected from everything else, check that one and only one relay of the group RL4, RL5, RL6, RL7, RL8, and RL9 is actually enabled as you manually select bands. If the driver is faulty, then it is possible that more than one relay is energised.

Let me know what you find.

73, Adrian, 5B4AIY