Resolving an Icom IC-7000 Power Meter Problem

Recently my Icom IC-7000 died during transmit. The result was a totally-dead appearance, which I resolved as detailed in this recent post. After repairing it with new parts, the radio powers on, but all is not well.

The radio now exhibits a strange symptom related to the Po (power output) and ALC meters. In SSB mode, pressing the mic key shows about half-scale deflection of the power meter with no modulation and regardless of the RF Power setting. This should be zero. With modulation, some meter activity over the static level could be seen, but never full deflection. Further, in FM or RTTY mode, the power meter would show about 80% deflection when RF Power was set to 100%. This should show full-scale. Lastly, in SSB mode the ALC meter would show full deflection with no modulation if the RF Power was set over about 40%, and zero if it was set under that level. ALC should mirror the modulation input, regardless of power.

At first, this seemed reminiscent of the self-oscillation problem that could occur in the 756 and 746 radios, where the RX line wasn’t fully pulled to ground during transmit, causing similar behavior with power deflection during transmit with no modulation. However, I ruled this out by looking at the current draw on the power supply. In SSB mode with no modulation, the radio would only pull an additional two amps or so, despite the meter showing about 50W output. The power draw would fluctuate as expected with voice peaks, even though the power meter did not show any activity. In FM or RTTY mode, the power supply would show about 22A draw even though the radio claimed it was putting out less than 80% power.

Another very interesting manifestation was that the radio wouldn’t drive an external tuner. Even when connected to a dummy load, the radio would kick off a tune cycle, the tuner would achieve a satisfactory result, but the radio would kick the TUNE indicator off after it was done. My guess was that the confusing power output indications to the CPU yielded a “not a good match” determination by the radio itself.

All of this led me to think that something in the power metering or ALC circuits was not right. The forward and reflected power is sampled on the PA board at the antenna connector and fed to IC960 where it is amplified and fed to the CPU on the main unit via the HFOR and HREF lines. Since the radio was showing zero SWR deflection, and since the ALC and Power meters were based on the HFOR line, I focused there.

There is a check point in the ALC signal on the main board – CP1601. I measured 1.7VDC here during transmit regardless of modulation input or the RF Power setting. This, to me, seemed to be the problem: basically a static invalid feedback signal to the CPU, which it interpreted as power output when there was none in SSB mode, and potentially less than full output in FM or RTTY mode, when there was plenty.

There are a number of capacitors and resistors around IC960 before the HFOR line leaves for the main board, which I tried to test in-circuit. However, I couldn’t get anything like reasonable values for these without taking them out.

But, there’s a 300 ohm resistor in the HFOR line, R960. I tested this in-circuit and it seemed like a dead short. To further test it, I disconnected the ribbon cables going to the main board, which should free up one side of it and measured again. Still a dead short. So, I pulled it out and replaced it with as close of a temporary resistor as I had locally: 200 ohm. To my delight, the radio started behaving normally! SSB with no modulation showed no power deflection, and modulation made it bounce as expected. RTTY and FM showed full-scale at 100% and otherwise mirrored the RF Power setting as I backed it down, as expected

After this change, I measure differing voltages at CP1601 depending on drive and power output, which is what I would expect. The radio also happily drives the tuner, and seems to measure the proper SWR when fed with 25 and 100 ohm loads. I expect it is either not achieving full output (or over-driving the PA) due to the wrong-value resistor I installed. After replacing the temporary unit with the proper value I will test power output to make sure it’s behaving properly

Category(s): Hardware, Radio
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