An Aerostar pilot reported descending below the segment altitude between the IAF and FAF while flying practice RNAV and ILS approaches. Personal distractions and avionics issues were factors.

Narrative

[I] went to a nearby airport to practice approaches. I misevaluated or misread where I was on two approaches. I had thought [I] was inside the FAF and cleared to a lower altitude but I was actually between the IAF and FAF. ON RNAV 34; I didn't focus on the 1;700 FT limit to the FAF and descended below that arriving at 800 FT. I conveyed my belief to Tower they told me the altitude was 1;700 minimum. I climbed up. I'm familiar with the area and I was VMC towards the end of the descent. After climb out; I shot the ILS 12 and I descended below the minimums on that as well; but never entered IMC conditions. Terminated that approach; was given 2;000 FT; and climbed up to 2;300. I also told the Controller I was VMC; which technically was irrelevant since I was on an IFR flight plan. He probably thought I didn't know what I was doing; even though I've been to the airport flying these same approaches 10 to 20 times a year for the past 16 years. After that; I decided I didn't have the focus to continue; so I flew back to my home base. Had some avionics problems; one com radio was intermittent and I've been having some problems with the autopilot for months. Because of the autopilot problems I've shot dozens of approaches in the past few months (many in VMC; but at least 10 in IMC and some with an instructor helping with evaluating the autopilot problems); so I don't think it's lack of currency. The only thing I can think of is to focus more on the approach plates. I was distracted from some work and personal issues; coupled with some avionics issues. I've been experimenting with electronic charts. I'm going to go back to paper charts which are easier to read and study.