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I use my FT-980 mainly as a receiver. I have INRAD Xtal 455kHz filters.

So I decided long ago to remove the 24V fuse. I heard alarms about PA destroyed by 24PS failure destroying PA transistors by overvoltage.

But I have never seen a real failure. So I dont know how needed is to install a 24 crowbar. It is simple and easy to connect it at the 24V fuse output. But I have never heard in "serious" forum about the need for the crowbar, o even about overvoltage problems.

Has anybody any notice about it? Or installed a crowbar?

73 EA1FC

Hello dear OMs,

After a repair I documented in other thread, I tested it and "best input voltage" was 200V (wired for 234V) for clean 100W out. I mentioned there too about the advantage of lowering voltage to increase degrees of conduction similar to reduce capacitors, the idea is to lengthen the conduction time and reduce intensity as heath os proportional to I square.

I studied the possibility of adding a resistor in series, before capacitors, but it went to over 100W resistor. This radio as it comes is extremely bad in efficiency, some 540VA in for 100W out.

But besides all above, when I bought the radio near 35 years ago people were talking about the overvoltage and destruction.

I have many PS and no one has a crowbar but anyway, you never know. As this radio was in the expensive side at that time, maybe someone had to find some real or unreal faults.

What works quite well it is the 13.5V PS. When my tantalun capacitors in PA went shortcircuit, the 13.5V PS protection cut voltage. Otherwise extensive damage in the RF unit, in fact the best site to check that fault is in the TX13.5V in RF unit.

Anyway it is a very good receiver. It is the way I use it, and the reason why the 24V fuse is out.

Thanks for the idea on reducing capacity, thanks for the PDF

?Hallo F5RCT
I dont really undestand the constant courrent circuit Q01, even if I have known it since FP-707. I know what it does, but I dont know how it does it. BTW Yaesu has always used too much filtering capacity, FP-707 uses 0.1mF, what means litle angle of conduction and high peak intensity. Trafo suffering. So it is a good solution to reduce capacity as long as PS works. In the FT-980 I found best solution was to wire it for 234VAC and feed it with 200VAC, heath reduced a lot. I used a VariAC but if I used the radio as TX I would place an autotrafo 230/200VAC,? this PS maybe is not best Yaesu design.

I think the first capacitor in 13.5V PS Q01 must be an schematic error, of course it must be placed before the FET.

But all these Yaesu PS's? have one capacitor after the FET, and as I said, I dont know how it works, but it worked very well in the FT-980 TX13.5V short circuit failure (PA tantalun capacitors). Voltage went down to 3V and no damage in RF unit placed between PS and PA.

I mentioned the crowbar, because I have seen it in LA of over 1kW, with transistors at 50V that die easily by overvoltage. I had this problem in one LA (4 transistors blown by 69V) and the LA is in the way to be changed linear PS for a 48V switched PS, it has a design fault and I dont trust it anymore.

73 EA1FC

Sorry mistake, FP-707 uses 0.1F after rectifiers, not 0.1mF as I said.

Hola EA1FC !

To understand the purpose of this constant current source, first you will see that the rectifier diodes and the capacitors provide a negative supply (more negative than the emitter of power transistor).
In combination with the JFET it behaves like a negative current source that can pull the base current more down as the GND output terminal.
It gives the possibility for Q02, as driver,? and the power transistor on heat sink to conduct at its maximum current (close to saturation)
By this way the voltage drop on the power transistor is minimized. ...and the voltage drop on teh regulation system too !

The voltage regulation is done by the transistor Q03 and the zener as reference.
To get the best voltage stability, the zener is supplied? by the resistor R02. Since the output voltage is constant, the zener current is constant, and zener voltage too !
But to start the power supply the transistor Q03 has to conduct ! This is not possible without the JFET current source because Q03 is referenced at the emitter of Q02.

Based on my experience as an electronic engineer, I was faced with solving instability in closed loop systems.