Dear Group,
I am trying to repair my SB-1000 which died 3 years ago when I was
transmitting
on 7mhz while the antenna broke down.
(my earlier post asking for a spare part for the bandswitch is for the same
reason)
The amp is equipped with the QSK board option. One transistor on this board had
died: during transmit it switches the zener diode to ground which is
connected to
the center tap of the filament transformer. This zener diode sets the
operating bias and
carries the idle plate current (around 80mA)
The transistor had become a short, so as soon as I switched the amp from
"standby"
to "operate", the idle current started to flow.
I put in a new transistor and that problem is solved. I carefully tested
the amp and
the output power easily reached at least 500W.
But there still seems to be a problem:
When I transmit, the plate current starts to rise slowly after a few
seconds. A few seconds
later the rising becomes faster. I stopped transmitting because I dont want
to kill
any more parts in the amp... The process is repeatable after a few seconds.
I have even noticed that the current starts to rise when I only let the
plate idle current
run for a while. The effect is just much more pronounced and faster when
transmitting.
Can anybody help me, what would be the reason for this problem?
Many thanks,
73 Con DF4SA
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