LF: Re: Ferrite cores from TVs

["James Moritz" <james.moritz@btopenworld.com>]

Dear John, LF Group,

The line output transformer cores are made of a grade of ferrite similar to 
that used for switch-mode PSUs, such as 3C85, etc. It performs well at 
136kHz although, like all ferrites, the loss increases with frequency. I 
built a version of the G0MRF TX design that used one of these cores with 
about 400W output.
It can be quite hard to extract the cores from the transformer without 
breaking them, since they are usually glued in - I have had reasonable 
success by cutting away the windings using a hacksaw. The other problem is 
that for the LOPT application an air gap in the core is required, but not in 
a PA output transformer. If the air gap has been obtained by inserting 
plastic spacers between the core halves, removing these are no problem, but 
if the core legs have been machined to unequal lengths to obtain the gap, 
tedious grinding will be needed. SMPSU cores bought from suppliers like RS 
and Farnell in the UK are not particularly expensive, so you may consider it 
not worth the trouble.
Cheers, Jim Moritz
73 de M0BMU