[Amps] Dentron MLA2500 SWR Problem

[Adrian Rees]

Hi all
 
Just an update to my previous emails.
 
Last night I completed the modifications suggested by the late Chris Lund. (See http://support2.overkalix.se/~sm2yer/shack/amp_mla2500/dentron_mla-2500b_cooling.pdf <http://support2.overkalix.se/~sm2yer/shack/amp_mla2500/dentron_mla-2500b_cooling.pdf> ) 
 
These modifications are to improve the high input SWR problem with this amp. After very carefully checking all the joints and components (except the RFC, more later) I powered the amp up and left it switched on, idling for 20 or so minutes.
 
I then systematically went through each band 10 - 160. The amp was generating lots of RF and was stable on all bands, including 80m (if you recall 80m was giving me lots of trouble).
 
I then had a QSO on 80m with M0HEM, and received RST Reports as expected. However, after around 5 or 6 overs, a crackling noise came from the amp, the panel lamps dimmed, and the power falied to the amp.
 
It turns out that the internal power fuse ( 10A Anti surge, or Slo Blow in the US) had failed. I removed the covers and could not see any obvious signs of arcing. The RFC that was replaced is RFC 4 on the ALC board. In the manual I have (not the original) its down as a 15MH Choke. Obviously not a 15 Mega Henry Choke.In other versions of the manual, its down as 1.5mH  (Milli Henry) and others as 15uH (15 Micro Henry).
 
I suspect this has caused the problem, together with the very close proximity of the moved components to the outer (earthed) case. I would appreciate comments on the above, as I know think I'm getting somewhere with the amp......as long as I havn't damaged the 8875's !!!!!!
 
Thanks all
 
Adrian MW1LCR