Re: Blown finals?


Good job on the “learning experience” description.

Thank you for sharing.


On Jan 14, 2019, at 3:46 PM, Peter Russell <peter.russell@...> wrote:

It's now about a month since I got my ubitx and I haven't tried to transmit as my power supply wasn't up to it.
So I built a new (5A) one.
Powered with the new PSU, I wound the bias controls fully clockwise and with my meter checking the current, pressed the key. Turning RV2 produced the expected current increase, but turning RV3, no increase at all!
Must have somehow blown one of the finals!
So I ordered 3 new IRF510's.
I changed the one I thought was at fault and tried again.
Same problem.
Must have changed the wrong one!
Changed the other one.
Same problem.
After a good deal of bad language I got round to checking voltages and resistances around the final circuits, where I realised that one of the bias circuits was behaving very strangely.
To cut a long story short, I removed RV3 from the board only to find that it had broken one of its legs and was hanging only on two ( the ground one had snapped off!).
So I've ordered some presets.
Maybe it's all those tales of woe about IRF510's that I've read on this forum that made me think that one had blown, or maybe I should have done what I was taught and followed correct fault finding procedure before jumping in with both feet.
The presets should be here tomorrow so I'll do a proper repair job and hopefully learn a useful lesson.

Peter G8FWY

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