Ranger RCI-69 (1st edition) blowing finals...

[Hawkeye351]

Got a Ranger RCI-69 (1st edition) on the desk today.

Reason for being on my desk:
Blowing finals not long after being replaced.

Evaluation:
The last final (RT1) shorted and next to last final (RT1) weakened, then popped.

I've replaced these mosfets once before about 2 months ago, then the user stated he had an issue with the input of his amp and it took out those new finals. I just replaced those mosfets again, keyed up for a brief second and pow, it blew the last final in the chain again. Each time I replace finals, I always reset the bias before testing it, as I did this time also. The bias sets great, but once you reattach the bias jumpers and key up, pow it blows a final, as if it sees an immediate high SWR, but my external meter shows a perfect SWR from my dummy load. When it blows a final, the radio immediately shuts off and the power supply won't come back on unless you unhook the DC supply feeding the main board.

Any advice guys/gals?

 

[brandon7861]

Dummy loads are non-reactive, so harmonics and other spurious emissions are all the same to the dummy load. It will absorb all of it without any reflection. To see no reflection (flat SWR) on a dummy load is expected.

Try the next batch of finals into a dummy load only (so there are no reflections to do damage) and have a look at the spectrum coming out. Any RF not on 27MHz will be bouncing back to the finals when the radio is connected to anything other than a dummy load. Maybe the harmonic trap is messed up or higher gain replacement parts are causing oscillations.

Edit: Should have asked first. Is it blowing on the dummy load too? Or just into a reactive load?

 

[Hawkeye351]

Dummy load, haven't tested on air yet. Checked my jumper also, no issues on my setup.

 

[brandon7861]

Check for continuity between the antenna jack and ground just to be sure none of the filter and matching caps shorted.

 

[brandon7861]

I don't have a schematic, using the dx55hp as a reference since it is dual final using 520's.

Another thing to check would be the zener diodes. They don't do much for biasing, they just protect the gate from higher voltage spikes. If somehow the driver or self-oscillation provides more than 20v to the gate, its toast (and I don't think it would even take that much since that datasheet spec is just for ESD or oops moments, not repeated spikes that turn it on hard over and over).