How to change the power output of an Icom IC-91/IC-E91

Recently I purchased an Icom IC-91A with D-Star module UT-121. It's not a bad radio, but in my opinion it has a design flaw: It gets awfully hot when running it at 5 W output power and at a certain heat sink/case temperature finally reduces the output power to a level it can survive. When manually switched to "low", the output is approx. 0.5 W. That in turn seems to be a bit too low for me.

So I searched the internet and found M1BXF's very fine instructions where the adjustment procedure is described in great  detail. In case the original source disappears, here is the same information as PDF file.

Experiences applying the above instructions

I have to emphasize that

Before changing anything, take notes ot the original settings.

These are my power measurements

N.B.: The radio detects the supply voltage and switches to one of the power levels. 5 V is the voltage from the BP-216 dry battery pack (with up converter), 7.4 V is the nominal battery voltage and 13.4 V is the nominal external power supply voltage.

Before the modification, the output was approx. 5 W in high power setting and 0.5 W in low power setting. When using the dray battery pack, power output is generally reduces to 50 mW.

Supply voltage/V Output 70 cm (H) @ Battery current Output 70 cm (L)/W @ Battery current Output 2 m (H)/W @ Battery current Output 2 m (L)/W @ Battery current
5 V 0.05 W @ 0.34 A 0.05 W @ 0.34 A 0.07 W @ 0.42 A 0.07 W @ 0.42 A
7.4 V 5.5 W @ 2.3 A 0.7 W @ 0.77 A 5 W @ 1.85 A 0.55 W @ 0.68 A
13.4 V 5.5 W @ 2.1 A 0.44 W @ 0.64 A 5 W @ 1.92 A 0.5 W @ 0.65 A

After the modification, the output power varies:
It is interesting to note that even the power output at 5 V input voltage from the dry battery pack voltage can be increased to several Watts of output power. It verified this using a lab power supply and I don't know how much current the dry battery up-converter can deliver without catastrophic failure.

Supply voltage/V Output 70 cm (H) @ Battery current Output 70 cm (L)/W @ Battery current Output 2 m (H)/W @ Battery current Output 2 m (L)/W @ Battery current
5 V 0.04 W @ 0.32 A 0.04 W @ 0.32 A 0.055 W @ 0.41 A
0.055 W @ 0.41 A
7.4 V 2.3 W @ 1.37 A 1 W @ 0.92 A 2.3 W @ 1.29 A 0.95 W @ 0.82 A
13.4 V 5.0 W @ 2.0 A 0.8 W @ 0.86 A 5 W @ 1.96 A 0.9 W @ 0.84 A

Note: I accurately set the power levels of my IC-91A using a HP 435B power meter and HP 8481A measuring head. In the US version the alignment frequencies are in the center of the US amateur radio bands which are much wider than the european ones. I assume that this is the reason why the power levels at 435 MHz is not exactly the same as at 445 Mhz. It's not so important but I consider it worthwile noting.


I now have 2.5 W resp. 1 W using the BP-217 Li-Ion battery. This combination seems to be much more suitable for my needs (yours may be different). The average operating time with one BP-217 increased drastically. Compare 2.5 W @ 1.37 A against the original 5 W @ 2.3 A. Theoretically, this increased the transmit time by 67%.

But what is much more important is that the radio stays much cooler now. It still gets warm but you don't need asbestos gloves to hold it after a few minutes of transmitting at 5 W.

The average BP-217 battery operating time increased significantly. Unfortunately, the radio draws quite a lot of current in receive mode, too. When actually receiving, my IC-91A draw approx. 165 mA in analog FM mode and 200 mA in DV mode. Depending on your setting of the "save mode", this drops to 66 mA some short time after the signal disappeared. Although it seems to be some kind of dynamic duty cycle, the receiver is switched on approx. once every second when no signal is present.


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DL1YDD, last change: 2008-12-12