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Author Topic: Plates overheating on a 4-400A Tetrode Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 01-18-2003 03:31 PM                     Paul, Need a High Voltage capacitor???? Call Salt Lake Instrument on 17th south. Ask for Bill. He has thousands of them!! Mark @ CLACO  |  IP: Logged Paul G. Thompson The Weenie Man Posts: 4718 From: Mount Vernon WA USA Registered: Nov 2000 posted 01-18-2003 04:32 PM                     Thanks, Mark. By the way, I'll send a complete schematic (zipped)  |  IP: Logged Jack Ondracek Film God Posts: 2348 From: Port Orchard, WA, USA Registered: Oct 2002 posted 01-18-2003 05:56 PM                        Prism's good stuff... not as common as the others... probably because it's pricy. Your boss made a good move there. The 4-400s in your rf cabinet are probably intermediate drivers for the finals. As such, they'll run at least 2 or 3 times as long as the modulators in your 1kW. That modulator design was really hard on the 4-400 series. The industry should have stayed with the 833 tubes that Gates used. Those were perfect for the job.  |  IP: Logged Kenneth Wuepper Phenomenal Film Handler Posts: 1026 From: Saginaw, MI, USA Registered: Feb 2002 posted 01-18-2003 06:03 PM                     Paul, Got the two diagrams and think you are on the right track. Happy to know that this was an on-going and worsening problem and not a sudden result of the tube change. I used to teach the theory classes for the local ham club. I was servicing 2ways since 1952. Lots of hours on the old commercial ticket. Funny thing though, I never did get a ham ticket, too busy with my film work. KEN  |  IP: Logged Paul G. Thompson The Weenie Man Posts: 4718 From: Mount Vernon WA USA Registered: Nov 2000 posted 01-21-2003 02:43 PM                     Mark, I called those guys. They had one, and it is being shipped and hopefully, I'll have it tomorrow. However, the transmitter died this morning. The plate contactor will not close. So, time to start chasing wires again. I did have a half-assed capacitor checker, and it showed that the output capacitor of the modulation transformer has, basically, gone south. Edit: But the trouble this time was an interlock in the coax switch and a relay failed in a controller. Jeeze, what next? But at least I got the damned thing on the air manually... [ 01-21-2003, 09:35 PM: Message edited by: Paul G. Thompson ]  |  IP: Logged Kenneth Wuepper Phenomenal Film Handler Posts: 1026 From: Saginaw, MI, USA Registered: Feb 2002 posted 01-21-2003 10:35 PM                     Paul, The factory engineer was testing a UHF TV transmitter and when the SWR relay dropped out he simply bypassed it and continued to operate the transmitter. BAD NEWS! The SWR was for real and they had to replace 1,200 ft of Helium filled CoAx. The problem was the expansion fittings holding the center conductor. The fingers were of Nylon and the expansion balls were Teflon. Bad dielectric shorting in these places. It's not nice to defeat failsafes! Please don't get electrocuted, we need you on here! KEN  |  IP: Logged Paul G. Thompson The Weenie Man Posts: 4718 From: Mount Vernon WA USA Registered: Nov 2000 posted 01-22-2003 02:05 AM                     Ken, I know.....but my predecessor did a real hatchet job on the wiring, including using 120 volts on the shields of audio cables which were used for annunciation. But I was doing what I did on a safe bet with myself. The power ladder part of the schematic (left side) shows a connection from TB8-4 to TB8-6 with an attached note. This connection was the final "GO" for plate contactor potential after the coax switch connected the 1kw transmitter to the antenna feed line. There is a microswitch in the coax contactor, but whether it is connected to TB8-4 and TB8-6 directly or a "loop-DE-loop" to a homemade manual change box of which I have no documentation is uncertain. But when I climbed up the ladder with my 120 volts to the plate switch, I knew it was going to run. With the antenna connected, throwing S-6 induces a one second delay before the plate contactor is closed. The screen supply powers up first at a low voltage until K-18 closes which bypasses R-140. (A little rat race there.) As far as the interlocks are concerned, There are only three. They are the airflow (which has been defeated years ago) (which I am going to have to undefeat) and two door interlocks. I just had to jumper TB3-10 to TB3-11 to clear that part of the ladder. (I was running out of time, and only had 45 minutes before that transmitter had to go on the air.) It could be that my predecessor may have used a horses' assed interlock through the Gentner or his little home made box that sits in the middle of a pile of rat shit. As you can determine, I got my hands full! But unlike the Engineer you mentioned, I am very careful about powering up a circuit for test purposes until I know for sure it is not going to fry something else. When working on these transmitters, I take plenty of coffee breaks and study the schematics carefully and well and explore all possibilities of what could happen "if".... Oh, by the way...K-4 pins 2 and 3 are normally closed. My schematic shows they are not. K-4 dumps the power chain in event of various overloads sensed by K-9 through K-11. So far, that was the only error I found in my schematic reproduction from a 40-year old rag that was obliterated and in a million pieces. I'm kinda proud of that reproduction.....  |  IP: Logged Kenneth Wuepper Phenomenal Film Handler Posts: 1026 From: Saginaw, MI, USA Registered: Feb 2002 posted 01-22-2003 07:36 AM                     Sounds like the station is very lucky to have you there. My old high school buddy, Paul, is a real engineer also. We can fix anything! We make a great team on these old theatres as we were both in the high school media (Movie) department. We both had our commercial FCC licenses out of high school and worked for the same 2way outfit. Paul went into communications for the railroad and I was media director for a school system where I operated and maintained the television studio. We even had our own microwave to the local UHF station for a 30min live show once a week. Your reproduction of the schematic is nothing short of incredible. Also the scanning has preserved all of the detail so it can be magnified many times and still is extremely clear. Great Job, Paul. [ 01-22-2003, 05:59 PM: Message edited by: Kenneth Wuepper ]  |  IP: Logged Paul G. Thompson The Weenie Man Posts: 4718 From: Mount Vernon WA USA Registered: Nov 2000 posted 01-22-2003 08:13 PM                     Jeeze....someone must have snuck in on me and took this picture of my equipment racks. This is about what I inherited from my predecessor.  |  IP: Logged Sam Hunter Jedi Master Film Handler Posts: 779 From: West Monroe, LA, USA Registered: Jan 2002 posted 01-22-2003 11:18 PM                     Jeez, you must be freaking kidding? Thats a freaking mess!  |  IP: Logged Paul G. Thompson The Weenie Man Posts: 4718 From: Mount Vernon WA USA Registered: Nov 2000 posted 01-22-2003 11:26 PM                     Sam, I have seen worse. All you have to do is look at mine. True, there may not be as many wires, but mine are tied in a zillion knots. Sorta like 100 each 100-foot microphone cables scrunched and inter-tangled and knotted into one cable 1 foot long with 200 ends.  |  IP: Logged Kenneth Wuepper Phenomenal Film Handler Posts: 1026 From: Saginaw, MI, USA Registered: Feb 2002 posted 01-23-2003 06:28 AM                     Zotts! The person who did that went to the school of "Listen to my great station~" university. Last week he couldn't spell enguneer now he be one! I bet that the control room looks "pretty" and that this whole mess is somehow interfaced to a COMPUTER? And we all know that the computer makes it PURRfect. What brand of headache medication do you use?  |  IP: Logged Jack Ondracek Film God Posts: 2348 From: Port Orchard, WA, USA Registered: Oct 2002 posted 01-23-2003 08:11 PM                        Is that a drop light hanging in the middle of that? You'd need more than that to find anything in there!  |  IP: Logged Paul G. Thompson The Weenie Man Posts: 4718 From: Mount Vernon WA USA Registered: Nov 2000 posted 01-23-2003 09:14 PM                     Ken, My medication is 16 packs of cigarettes and 14 gallons of coffee. Jack, I was amused by the light hanging there, too. Maybe the engineer used it to shine up his butt to see where he came from. Anyway, an update. So far, I got the interlock problem restored, and will test it in the morning. I think it'll run. Also got the autostart of filaments restored, that problem was caused by some wires falling off some terminals in the rat's nest. I ordered the capacitors I needed, but that floozy still has not sent them. But at least, I think the transmitter will power up and power down just fine by itself during tomorrow's test. Then, finally, I'll be able to focus my attention to the nice romantic glow of the 4-400's, high cathode currents, and high screen voltages. According to some data I received on the 4-400A's, the typical operating parameters with the tubes biased class "B" are as follows: Plate Voltage = 3500, Screen Voltage = 500, and Bias = -82 volts. In my application, Plate voltage is low, screen is high, and so is the bias. I might insert a dogbone in line with the modulator screen grids and bring it down a little to see how well it performs. This was also suggested by a guy who went to school with Marconi, and has been rebuilding old vacuum tube transmitters for years. [ 01-24-2003, 03:03 AM: Message edited by: Paul G. Thompson ]  |  IP: Logged William Hooper Phenomenal Film Handler Posts: 1879 From: Mobile, AL USA Registered: Jun 99 posted 01-24-2003 02:27 AM                        "Hey, I'm about to go out to our AM site to look at something. You want to go?" Hell no! I ain't that big anymore. quote: Sam, I have seen worse. Paul is happy because: 1. It all seems to be from the last half-decade or so, as opposed to one of those with intermixed layers from th 1950's to now with ratty brittle cables, going through dirty normals on telco patch panels, then to Rat Shack relays, etc. 2. The environment is not one of those where there's 4" of silt in the bottom of the racks & coating everything as the result of every time it's flooded, & it seems to be not a rat- or wasp-filled nature preserve. The odd thing is that about 1 in 10 installations you'd see like that were the work of blindingly brilliant, intuitive engineers who'd understand complex problems & arrive at brilliant solutions immediately, but the actual work to them was just something to lash up. It wasn't important. They're like the folks with shops & offices that look like earthquake zones, but they know where everything is located in x,y,& z dimensions, plus remember *everything*. They creep me out.  |  IP: Logged

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