tech stuff – Page 22 – Engineering Radio

Graphical Network Simulator

I have been working with GNS3 (Graphical Network Simulator) in some of my classes. It is a fine tool with which one can build simulated computer networks using various routers and switches. The software program itself is free, however, the Cisco IOS images are not included and must be found elsewhere due to copyright issues. This detail is a bit of a pain, but not too bad. Once the program is set up and the appropriate IOS images are loaded, the console functions exactly like whatever router is being simulated. This includes running whichever terminal program is preferred, e.g. hyperthermia, putty, or if using the Linux version, x-term, etc.

GNS3 screen shot, topology and router console — GNS3 screenshot, topology, and router console

The advantages to this over something like Cisco’s Packet Tracer program are many. In Packet Tracer, certain functions are locked out and generally there is only one acceptable way to complete any given task. With GNS3, the IOS is fully functional, which means that experimentation and failure are available to play with. Failure is a great way to learn things in any hands-on environment. The advantage of virtual failure is that only you know about it.

For real-world applications, this means that router and switch configurations can be created, tested, and tuned ahead of time and then loaded into working devices, saving downtime and potentially handfuls of hair.

A few things about using GNS3, the PC idle tuning is required. Each instance of IOS assumes that the entire processor is available to use, thus starting several routers can work a PC’s processor to 100% and windows will never fully recover. Secondly, when starting each router, wait 10 to 20 seconds before starting another one. Again, this has to do with the way IOS uses processors. Also, to save time, store the IOS image as a decompressed file. This saves quite a bit of time on startup. Finally, do not forget to copy the running config to startup-config. Even though GNS3 says it is saving the router configs, it does not save the running config unless you issue the copy run start command, just like a real router.

The Raytheon RL10 Limiting Amplifier

Update: Apparently this is quite interesting to a number of people. I have rescanned the manual, properly compressed it and which you may find it here.

Found this manual at one of the older transmitter sites:

Raytheon RL10 limiting amplifier manual cover

The entire manual is available for your reading pleasure here: Raytheon RL10 limiting amplifier

As this is an older design than either the Gates Sta level or the Collins 26U, it may not be as useful to tube audio enthusiasts.

The main issue with the Gates and Collins unit is the GE 6386 remote cutoff triode used, which were great tubes, but very difficult to come by these days. This design calls for a 1612 or 6L7, which is a pentagrid amplifier. Feedback is provided by the screen of the following stage, a 6SJ7GT. Anyway, perhaps it will give somebody some idea of how to make a good tube compressor limiter.

It is kind of tragic, in a way

Working on another AM directional station (WGDJ) which was damaged by lightning recently. In this case, the antenna array controller ceased working and one of the towers in the daytime pattern was out tolerance. Before we stared working, I told the owner to have all the vegetation cut down around the towers. This is what we ended up with:

I can’t really fault them for this, but it does make work more difficult. That strip of tall green grass; that is the catwalk. The grass itself is called Phragmites, which is tall, tough, reedy stuff that can scratch and cut person unaware. The array is in a low swampy area next to the Hudson River in East Greenbush, NY. Stepping off of the catwalk, one can sometimes find solid ground, or find ones feet six inches under water.

WGDJ tower one ATU cleanout — WGDJ tower one ATU clean out

This is Mike cleaning out the mice and bees nests out of the tower #1 ATU. Notice the can of bee spray in his back pocket. This was after he was stung in the forehead.

Mouse nest, WGDJ daytime ATU coil — Mouse nest, WGDJ tower #5 daytime ATU coil

This mouse nest, at the attendant dead body in it, was responsible for a -10 degree phase shift in the daytime pattern for that tower. I hate cleaning this stuff out, it is a dirty, nasty job but necessary nonetheless. While doing this work, I wore gloves and a dust mask. The entrance hole where the AC power and control cables come into the bottom of the ATU was plugged up with some steel wool. There is still a bad capacitor in this ATU for the daytime array, that damage was likely caused by lightning.

At the end of the day, we repaired the antenna array/phasor controller; bad AC transformer and rectifier bridge and several bad logic steering diodes for tower 4 and 5, cleaned out all the vermin nests and isolated the remaining problem with the daytime antenna system. Parts should be in next week to finalize repairs.

All in all, not a bad day’s worth of work.

Modifying the old Broadcast Electronics Transmitter

In the previous post, the issue with the WVOS-FM transmitter was detailed: The PA feed through/bypass capacitor had arced to the PA cavity causing lots of unwanted off-air time. When I went to order the replacement parts, of course, they were not available. It seems that Broadcast Electronics changed the design of its transmitters in the late 1980s to use a different feed-through arrangement.

They were nice enough to send us a nifty retrofit kit; BE part number 959-0272 which replaces BE part number 959-0115. If interested, the six pages of installation instructions are available here, for your reading pleasure.

The retrofitting itself was quite the job; drilling six mounting holes and one one-inch feed through hole in the PA cavity, mounting the new feed through housing, rewiring the high voltage connection to the tube and back to the HV bleeder assembly, etc. What with all of the drilling, sawing, filing, deburring, and whatnot, I began to wonder if the transmitter would ever run again. This is the transmitter before the modification:

This is the old high voltage feed through hole, arc mark clearly evident.

Broadcast Electronics FM3.5A old high voltage feed though

This is the modified feed through/bypass configuration.

Broadcast Electronics FM3.5A new PA feed through capacitor — Broadcast Electronics FM3.5A new PA feed through/bypass capacitor

While doing this work, I removed the tube and put a plastic sheet in the bottom of the PA cavity and around the HV parts at the bottom of the transmitter. Somehow, getting aluminum filings in the tube socket seemed like a bad idea. I also thoroughly vacuumed out the entire transmitter once all of the metal work was done.

I removed the Kapton capacitor plates from the old feed-through arrangement and reinstalled the Teflon insulating plates to keep the air flow out of the tube cavity going in the correct direction. The new capacitor looks very beefy, perhaps it will never fail again.

Once the installation work was done, I brought up the transmitter first with no screen and no connection to the tube anode. Then with the tube connected, and finally with the screen supply turned on. The tuning needed a brief touch up but all in all, the transmitter came up and ran well with the new feed-through arrangement.