While surfing the net killing time I stumbled onto a "repair" for the AL-811H
amp. It is on mods.dk.
The fellow posting the repair stated that when keying this amp there should be
no current showing on the grid meter (no power from exciter just using radio to
key amp). He said that in many cases D16 a rectifier diode will fail and cause
you to see current flow on the grid meter. His he said was 30 ma keyed no
power. Well I tried it with mine and sure enough I noticed 35 ma when radio
keyed the amp. I happened to also notice that the plate current was only about
75 to 80 ma (book says it will be around 125 plate current at idle.
Well I cut out the D16 diode replaced with another 1n4007. 1000v 1 amp. rect
Sure enough grid current remains at zero now when keyed and to my surprise the
lost 35 ms current seems now to of corrected the plate current which is just
about exactly 120 ma very close now to the 125 stated current.
Here is his entire paragraph on mods.dk:
This fix is for the ameritron AL811 amplifier.
Problem is high grid current reading (mine was approx 30mA) with the amp keyed
and no exciter drive check D16 on the schematic which is a general purpose
rectifier diode 1N4007 1 amp max current 1000v reverse voltage needs replacing.
The 1N4007 diode acts as a fuse for temporary anode to grid shorts and is
located on the top board(right hand side)same board as the tank coils. It is
nearly ALWAYS this diode that causes the problem not the tubes.
My question.. I know the repair DID infact correct a problem I didn't not even
know I had. What improvements or better said what affect does repairing this
diode have in the function of the amp.
I did check the cut out diode it was confirmed defective..
Anyway, no big deal but I know this group will know what the diode really does
and how the affect may or may not have on the amp.
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