2m High Power QRO Linear Test

It was time to test (on air) the club's 2 * 4CX250B Linear that has been undergoing refurbishment and repair at the KWA shack.  So we decided to operate in the May contest for a few hours, or until the Linear failed, on the Sunday.

Elisabeth and Mike

Daniel gets the cold job

We made several contacts over a three hour period before the screen regulator showed signs of over-heating and voltage stability was lost.   In all other respects the unit performed very well, the other problems previously noted and thought corrected seemed resolved as well.

Tests with the contest group at Castle Camps (with G4NBS on the mike at the time) gave us very good audio reports with a tight signal not detectable +/- 10 Khz either side.  This was with some 600 watts PEP at the output, close to legal limit at my aerial.

A glum Daniel

Daniel looking miserable as the tests came to an early close.

Since this test the screen regulator output devices have been bolted to the case, the over-heating zener diodes have been replaced by a cleaver little circuit suggested by John Bonner G0GKP, this increases the dissipation with the use of a power transistor.  Works a treat, later I found the circuit in the latest ARRL handbook as well.  A hugh heatsink has been bolted to the back of the case too ,overkill.  Tests so far with Martin Cranage G8OFA in Salisbury show all to be working well and leaving the linear on for extended periods only just warms the heatsink.  Nearly all set for the September Trophy contest, just a couple of things to tidy up.


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