As mentioned above, while diagnosing you can only do so much and get a probable cause that chips XYZ are duff and change them. There are indeed a lot of recapping services about. But you need a fully qualified electronics engineer with experience in diagnosing these machines which is basically nobody (aside from me I guess). Recapping is not a universal fix to every motherboard problem. I have seen them blown to bits in all kinds of weird and wonderful ways over the years. A dead board is either broken tracks for blown silicon in general. But also can be simple things like the RTC has died. But generally you will still see the floppy LED blinking so the machine is still at that point showing signs of life. In which case it would be worth the gamble in trying to fix such machines.
From when I was doing Falcon repairs it really depends on what the history of the machine is. Like who had it before, did they mess with it previously. Are the machines totally dead or at least even tried to boot up.
But also as I have mentioned before many times, the power supply seem to be killing Falcons totally. Either by putting crappy picos in the thing, wiring up the power supply backwards and killing the board or not recapping the original power supply. Generally the Videl gets killed first.
It could be anything from a dead RTC to someone's wired 5V and 12V backwards and killed the entire board. Really if you want stuff repairing you have to be prepared to spend over $1,000. It may even need every chip on the board replacing. But this also does not guarantee success as there could be multiple internal track damages or something.
For example
viewtopic.php?p=206 that guy got lucky. But the last 2 or 3 falcons I had were in a very bad state. I gave up with them because I was not comfortable to keep charging £100's for no results. Like I say they become a money pit. At some point you have to ask is it really worth spending the time and money in repairing. I stopped doing repairs a few years ago because of my RSI and lack of time.
Selling the boards on may just be a better thing to do. It is crazy because I have seen them go for like £700 on a faulty motherboard on evilbay. I personally would not pay more than £50 for a faulty falcon. As if all the main chips are busted, costs will soon ramp up to repair it. Things are even worse with the TT etc. more special chips.. but again it could be anything from my simple power connector problem to every chip on the board being blown.
But if you really get stuck and have multiple Falcon motherboards and in the UK, then I may be able to look at them briefly. But I have already got a chap patiently waiting for me to tidy up his Falcon. With my ever-increasing backlog of work I may not be able to look at them until next year. If it is something simple and obvious to fix then great, but otherwise anything else, you can quickly sink £500-£1,000 into the things. In which case I probably just would not bother. I do of course hate scrapping such machines but when you get to spend in such a large amount of cash..At which point is it not worth it anymore..