hi gents,
Recently I was asked to repair a Heathkit SB-1000 amp
from a local friend. At first the Eimac 3-500Z installed was
arcing pink like stabilo instead of orange.
For many hours I have baked the tube with Ep of 1600V
with 2k-ohm 100 W resistor in series of the plate as to
W8JI's page and now it sustains up to 300 mA of Ip without
the ark. Also, I had to exchange a faulty bias zener diode
to a small zener diode + NPN boost transistor to suppress
extra zero-signal Ip - it worked nice.
On the other hand I have a Chinese graphite plate 3-500Z
and it works nice on 80 m-10 m band with Ip <= 420 mA,
Ig<150 mA, 60 W drive and nearly 850-900 W out.
The problem is the power-down of its 160m band.
Seemingly it has 800 W out but within seconds the power
goes down to 400 W or less when key-down.
No burning or heating, or degradation of input SWR can be seen.
I have replaced three adding capacitors of its output
tank circuit - two 170 pF and one 500 pF but seemingly
no improvement obtained.
Or, are the capacitance of bypass- or coupling capacitors
insufficient on 1.8 MHz band?
Anyone please tell me the possible cause and suitable
solution of this trouble?
de Han JE1BMJ
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