Garey Barrell <[EMAIL PROTECTED]> made an utterance to the drakelist gang
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Frank -
IF you are seeing 500 mA of plate current, you'll probably find that the
meter shunt resistor has gone up in value. It's the 3.3 ohm, 1/2 W
resistor on the relay board under the chassis. The problem is that this
same meter is used to set the 100 mA idling plate current of the finals,
and if the meter is reading incorrectly, the bias is also set
incorrectly, affecting the linearity of the transmitter. While you're
in there, it's also a good idea to check the 15 ohm, 1W cathode
resistors for the finals. They also change value with age and should be
matched within 5% of each other for best linearity.
The TUNE signal is developed by converting the Mic input stage to an
oscillator, so there isn't much different between the TUNE and AF signal
path. What microphone are you using? It _must_ be a Hi-Z mic, such
as a crystal or ceramic. A dynamic mic will usually require a step-up
transformer, sometimes internal and switchable on the mic stand. A
steady whistle into the mic should produce the same output power as the
TUNE mode. If not, I'd suspect the mic (correct Z?) or wiring. With
normal speech, about one half the max plate current _as read on the
meter_ is about right. A scope (or peak-reading wattmeter) would show
that the true peaks are at the max power level as TUNE.
73, Garey - K4OAH
Atlanta
Drake 2-B, 4-B & C-Line Service Data
<http://www.k4oah.com>
Frank Woolfe wrote:
Gang
Have recently replaced output tubes in my T4XC, I can obtain a tune
carrier of over 150/180watts with max tune gain around
500ma Ic however can only obtain around 50watts on voice peaks on
20/40m around 250ma Ic, I would expect to see the same amount of
output power on voice as carrier, is there something I have missed or
should be replacing also?
Frank VK5MFW
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