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T4X meter shunt resistor

Paul WB0CJB All Messages By This Member #81104 The same T4X I am working on has a 2.7-ohm resistor (R36) across the push button meter switch. The other resistor (R38?) resistor was burnt and cracked. I assumed it was a 330-ohm resistor and put one in. In the OUTPUT power position, the meter pegged. I have a second T4 transmitter that was in a house fire and has been partially cannibalized. The underside was unaffected, so I removed the board that had the R38 (another 2.7-ohm resistor) in it. I soldered the 2.7-ohm resistor in. Now the meter does not peg but I am not sure if the 2.7-ohm resistor is the right value.? ? Before I started to work on the T4X I checked out the AC4 power supply. The bias supply works fine. From what I have been reading the R36 resistor "reads" the plate current. R38 should show the output of the transmitter. Right now, the transmitter does not have any output. I am in the process of tracing the signal chain to locate where the signal is lost.? ? When I first started on the rig the B+ was shorted to ground due to a shorted trimmer 1.8 MHz trimmer. I replaced it and reconnected the B+ to the RFC6 (the shield feedthrough was missing). I went through the PA section and replaced the screen and cathode resistors (wrong value). I went through the PA and C48 was burnt and literally was about to fall apart. So, I replaced it as well.? ? Right now, I am checking the frequencies. The 5.645 MHz signal is being transmitted (I am using a 7300 receiver). The PTO is working fine. The heterodyne oscillator is working on most of the bands. The 1.8-3.0 MHz band I can't hear the signal on the 7300, and the 28.5 MHz band I don't hear the signal. The 7.0, 14, and 21 MHz bands are fine. The 28.5 MHz position on the rotary switch is intermittent as viewed on my scope. Thus, my previous post about the excess lubricant on the switches.? ? Paul WB0CJB ? ?
Steve Wedge, W1ES/4 All Messages By This Member #81106 Hi, Paul, Does this tx have a 12.6 MHz crystal in one of the AUX XTAL sockets?? Forgive me if I'm asking the obvious.? The 2.7 sounds like the right value, though the schematic shows a 3.3 there (later units went to 2.7).? Can't remember the value of R38 (it's a S.A.T.) but 330 sounds familiar. If you're getting all of your oscillators working and you have a bias current showing up of 70 mA, then your PA is biased correctly.? Check the plate resistor of the driver (12BY7).? I have seen these crack.? If you have a scope, probe that resistor on the driver side of the coupling cap. You should have over 50V when you turn up the gain. It is a possibility that the finals are completely knackered but getting some idle current suggests that if the signal path is okay, you should get SOMETHING out... 73, Steve Wedge, W1ES Time flies like an arrow.? Fruit flies like a banana. Sent with secure email. On Saturday, December 14th, 2024 at 10:22 PM, Paul WB0CJB via groups.io <wb0cjb@...> wrote: toggle quoted message Show quoted text The same T4X I am working on has a 2.7-ohm resistor (R36) across the push button meter switch. The other resistor (R38?) resistor was burnt and cracked. I assumed it was a 330-ohm resistor and put one in. In the OUTPUT power position, the meter pegged. I have a second T4 transmitter that was in a house fire and has been partially cannibalized. The underside was unaffected, so I removed the board that had the R38 (another 2.7-ohm resistor) in it. I soldered the 2.7-ohm resistor in. Now the meter does not peg but I am not sure if the 2.7-ohm resistor is the right value.? ? Before I started to work on the T4X I checked out the AC4 power supply. The bias supply works fine. From what I have been reading the R36 resistor "reads" the plate current. R38 should show the output of the transmitter. Right now, the transmitter does not have any output. I am in the process of tracing the signal chain to locate where the signal is lost.? ? When I first started on the rig the B+ was shorted to ground due to a shorted trimmer 1.8 MHz trimmer. I replaced it and reconnected the B+ to the RFC6 (the shield feedthrough was missing). I went through the PA section and replaced the screen and cathode resistors (wrong value). I went through the PA and C48 was burnt and literally was about to fall apart. So, I replaced it as well.? ? Right now, I am checking the frequencies. The 5.645 MHz signal is being transmitted (I am using a 7300 receiver). The PTO is working fine. The heterodyne oscillator is working on most of the bands. The 1.8-3.0 MHz band I can't hear the signal on the 7300, and the 28.5 MHz band I don't hear the signal. The 7.0, 14, and 21 MHz bands are fine. The 28.5 MHz position on the rotary switch is intermittent as viewed on my scope. Thus, my previous post about the excess lubricant on the switches.? ? Paul WB0CJB ? ?
Frank KG9H Member Profile All Messages By This Member #81107 开云体育 Steve, good point mentioning??70 mA. Most folks see 0.1A and stop reading the manual. Have had many many Drakes, re-reading the manuals again, LOL ? ? ?Frank Krozel Life Member, ARRL ? ? (630) 924-1600 kg9hfrank@...
Steve Wedge, W1ES/4 All Messages By This Member #81109 Good-O.? I have one T-4XB that has caused me to pull hair out!? Everything works except for the carrier oscillator.? I can't find the culprit and I have rebuilt the oscillator after checking all component values.? The damned thing just won't oscillate! So I am sure that my tubes are good (I checked them, anyway) but can't get more than idle current (because the CO is toast).? There's a heavy coating of some sort of greasy goo all around the switch that changes the function.? The CO only comes on in a normal tx when it is in transmit, so my guess is that something isn't connecting but haven't found the cause yet. When all the oscillators are working, lack of output is somewhat easier to find.? I'd check the driver at the low-Z driver side.? I suspect a bad part more than a bad tube.? I may have seen one bad 12BY7 in Drake equipment over the years -- the opposite of what the Kenwood Hybrid folks see... 73, Steve Wedge, W1ES Time flies like an arrow.? Fruit flies like a banana. Sent with secure email. On Sunday, December 15th, 2024 at 11:08 AM, kg9hfrank@... via groups.io <kg9hfrank@...> wrote: toggle quoted message Show quoted text Steve, good point mentioning??70 mA. Most folks see 0.1A and stop reading the manual. Have had many many Drakes, re-reading the manuals again, LOL ? ? ?Frank Krozel Life Member, ARRL ? ? (630) 924-1600 kg9hfrank@...

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