Involved with the PA, I see it is very difficult to access/repair. And maybe expensive if transistors are damaged by overvoltage. The Manual tells you to keep an eye on the 24V because overvoltage would destroy finals and drivers. I allways used the radio as receiver and removed the 24V fuse. I wonder wether I ever transmitted with it.
The PS is quite good in regulation, Q01 is a courrent source and it is somehow limited in A. But if regulator transistor Q1 shortens the PA can get 42V and get destroyed inmediately.
The crowbar shortens to ground the 24V in case of overvoltage. There are many examples of it. It needs a SCR, one or two zeners (one 24V to trigger and another maybe 3V or 2 silicon diodes) to protect the SCR base, two resistors, and a good fuse. In case of overvoltage the 24V zener lets voltage to SCR gate that shortens positive 24 line to ground. To avoid damage of SCR gate it must be protected from overvoltage to ground, with another zener or a couple of diodes (1.2V) enought as triggering occurs maybe with 0.7V?
There are two posibility.
1.- Easy one. The crowbar in PS output. Easy access to 24V, use of radio 24V fuse. Problem when crowbar acts, Q1 upwards in PS may get destroyed before the fuse blow
2,. More elaborate. The crowbar SCR anode connected to? the input of Q1? and the zener to Q1 output. In this case the SCR shortens the capacitor C01+C02, so it needs a real BIG capacity of intensity, but protects the PS and the PA. Main radio fuse probably will blow. Maybe it is possible to fit a fast fuse between main capacitor (36000uF) and crowbar/Q1
All this because I think a damage in PS can be repair by an average OM, but change of drivers and finals, besides expensive, needs some tools and qualifications
73 EA1FC
