Hey Jim, Thanks for the advice. When I said that there was no HV I meant that no plate voltage was applied to the finals. I must have had a temporary brain-fart. When the filament switch is ON, I do indeed have audio plate voltage.
I'll check ALL the wiring again and let you know what I find. Appreciate the quick response. 73 Bill w0ng -------------- Original message -------------- From: "Jim Wilhite" <[EMAIL PROTECTED]> > 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 > > ______________________________________________________________ > > Our Main Website: http://www.amfone.net > > AMRadio mailing list > > List Rules (must read!): http://w5ami.net/amradiofaq.html > > List Home: http://mailman.qth.net/mailman/listinfo/amradio > > Help: http://mailman.qth.net/mmfaq.html > > Post: mailto:[email protected] > > To unsubscribe, send an email to [EMAIL PROTECTED] with > > the word unsubscribe in the message body. > > > > ______________________________________________________________ > Our Main Website: http://www.amfone.net > AMRadio mailing list > List Rules (must read!): http://w5ami.net/amradiofaq.html > List Home: http://mailman.qth.net/mailman/listinfo/amradio > Help: http://mailman.qth.net/mmfaq.html > Post: mailto:[email protected] > To unsubscribe, send an email to [EMAIL PROTECTED] with > the word unsubscribe in the message body. ______________________________________________________________ Our Main Website: http://www.amfone.net AMRadio mailing list List Rules (must read!): http://w5ami.net/amradiofaq.html List Home: http://mailman.qth.net/mailman/listinfo/amradio Help: http://mailman.qth.net/mmfaq.html Post: mailto:[email protected] To unsubscribe, send an email to [EMAIL PROTECTED] with the word unsubscribe in the message body.
