Bill when the filament switch is turned on with the Valiant, HV for the
audio section is applied as well, so you have voltage on the plate and
the audio section is hot.
These voltages are supplied by T2. My Valiant 2 book lists that
resistor as 820 ohms. For that resistor to go bad, you must have a
short somewhere or the socket has been miswired by someone. Double
check that. My resistance chart says you should see about 1K ohms to
ground with no power applied (everything turned off).
73
Jim/W5JO
In my eternal quest to bring some life back into my (once total basket
case) 1957 kit-version Valiant transmitter, I now have a "gremlin" of
sorts living in the 12AU7 audio driver stage (V15).
The 1k, 1/2w cathode bypass resistor R45 (in parallel with a 10uf, 35v
electrolytic cap C99) connected from pin 3 & 8 of the 12AU7 driver
tube to ground "lets all of its smoke out" after the power is on for
about 8-10 minutes.
No high voltage is applied, just sitting there on the bench idling and
I can watch the resistor burn in the middle. It's weird (to me anyway)
that it only happens after the rig is powered up for about 8-10
minutes.
On the off chance the resistor was bad, I replaced it and the
electrolytic cap but it burned out again anyway. Checked the chassis
grounds and solder joints, all OK. ed.
I took some voltage measurements and discovered that there is ZERO
voltage at the 12AU7 cathode after a brief warmup. The manual says
there should be 14 volts at pin 3 & 8. When I turn on the filament
switch (with the rig cold) there is a slight momentary + voltage on
the cathode as the heater warms up but after a few seconds, ZERO
voltage on the cathode (???). All other voltages at the other tube
pins are in spec. I'm using 117vac line voltage from a Variac.
I've gone thru the wiring to look for any obvious problems, swapped
known good 12AU7's but for the life of me don't know why the cathode
bypass resistor keeps overheating (until it is actually smoking) and
why there is no cathode voltage.
I'm hoping some of you more experienced folks can lend a suggestion or
two to point me in the right direction. I'd like to avoid encouraging
anymore grey hair on my head if I can, hi. Thanks, Bill, w0ng
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