Hello a local radio club has given me a very ill Yaesu FL2100Z to repair.It
contains a pair of Svetlana 572B's .These are the symptoms
1.At switch on after about 10 secs the tubes conduct and glow white drawing
about 400 mA.One tube appears to be hotter than the other.
I assumed that this was because there was no bias on the tubes to cut them
off so I took a look at the bias board designated PB-1903.Sure enough there
was a 47k resistor (R303) which had been vapourised.I replaced it and
checked the diodes on the bias board and fired it up again.Same symptoms.I
then took out the tubes and checked the bias conditions.The voltage present
is -152v in standby -2v with the amp keyed.The other thing I noticed was
that the input circuit had had the 2 33 ohm carbon 3 watt resistors changed
to wait for it..... 2 x 33 ohm 10 watt wirewounds!This surely is a recipe
for disaster.
The other thing I noticed was that moving the tune and load control around
reduces the anode current suggesting oscillation.The power meter indicates
about 10 watts max output in this condition.The amplifier will show an
output peak of about the same as the input drive suggesting that this is
merely feedthrough from the input circuit.Driving the amplifier does not
increase the anode current.
I have never had any experience with "zero" bias triodes before but would I
be correct in assuming that the amp has been oscillating thus taking out the
bias circuitry.I think that the tubes are probably ok although may have
suffered some damage.Are there any improvements that can be made to the bias
board or indeed can I get a spare board from anywhere as the existing one is
a bit of a mess.
Any help would be greatly appreciated.
73 de Conrad G0RUZ
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