IC-7300 ALC overshoot - IC7300 4m SSB ALC spikes Power overshoots

IC-7300 ALC overshoot - IC7300 4m SSB ALC spikes

Since the original round of ALC and spike messages, I replaced my transverter setup for 4m with an IC-7300.

While it is OK elsewhere, and on CW, on 4m SSB it produces overshoot spikes of 50 Watts when set for 12… I does this regardless of compression and mic settings, although it is worse with compression than without. As I used an LDMOS linear with over drive protection, the use of the 7300 can be tedious at times.

Feeding around -2V into the ALC input socket “cures” the problem. I have made an outboard -ve Voltage generator board using an NE555 oscillator and diode rectifier to plug into the rear 13 pin DIN connector of my 7300. The unit is cheap and simple to make.

Having contacted both Icom UK and the UK amp manufacturer, I was disappointed with their apparent lack of knowledge that anyone else had experienced a problem. I know of one ham who returned his amplifier on two occasions in the belief the amp was the problem, and another has tested his 7300 and finds it misbehaves exactly the same as mine.

The overshoot is easy to produce. Set the frequency to the 4m band, SSB, power 25% (12W), mic level 30 to 50, compression to 4. Speak normally into the mic and observe the output level on the bar graph power metering on the radio. Now gently tap the mic with a finger nail or screwdriver handle and observe the level.

For those outside Europe, the EU version of the IC-7300 is aligned to limit the power output to 50 Watts on 4m. My 7300 doesn’t show the spikes on 6m, where the maximum output is 100W. My LDMOS amp requires 25 Watts max drive.

It would be interesting to find out if all 7300s exhibit this problem.

A brilliant post by Dave / Frank, hope it helps someone.



Example #2

We are experiencing power spikes from our IC-7300 to an ACOM 1200S PA. Sometimes these spikes are making the PA go into protection. There is no ALC output available from the 1200S, so we used the 'standard' solution often used with success on other rigs. Schematics below.

When we connect this to the IC-7300 ALC input, we are not able to change the output power.
We checked the voltage on the ALC connector when the above circuit was connected.
To our surprise, the voltage when the pot was at max, was + 0,11 VDC.
When the unit was disconnected from the ALC input, the voltage on that connector was + 0,19 VDC.

try this.

According to the schematic in the service manual, both the ALC input (jack 10) and pin6 on the ACC connector, are the same.
This is the comment for pin 6: " 6 ALC voltage input. Input level: –4 to 0 V, Input impedance: More than 3.3 kΩ".
So, clearly rather low imp. compared to the used circuit.
I think the suggested modification will work.


this is why many of todays solid state amplifiers, such as those using MOSFET and LDMOSFET use a resistor pad (eg...Ameritron uses 35 ohm 100W resistor on the RF input side of the amplifier). The amp requires more drive power to achieve full output but offers some protection from ALC overshoot, which can be as much as 120% of rated RF output power or more(eg...a spike of 120W in a 100W HF radio).
You could just as easily build an external 6dB-8dB 'RF input pad' to protect the amplifier. This can be used for any amp that forgot to include an ALC output circuit or when using with radio that overshoots ...

in fact, this may be a better option because most solid state radios exhibit a poorer IMD of transmitter signal when running lower power. Higher power (toward max) typically is where best IMD occurs, so you will not only be protecting your expensive amplifier but also being kind and courteous to fellow hams on the band.


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