Gates BC-250L BC Transmitter

Western Historic

RADIO MUSEUM

GATES RADIO COMPANY

BC-250L AM Broadcast Transmitter

KOWL 1490KC - Lake Tahoe

~ Moving the Gates BC-250L ~

~ Restoration ~

by: Henry Rogers W7YBS

The 1964 photo to the left was taken inside the transmitter shack of KOWL and shows the Gates BC-250L and the monitoring equipment when it was in actual use. KOWL went on the air in 1956 with studios located in Harrah's Club at Stateline, Nevada. The transmitter and antenna were located about two miles south in the city of South Lake Tahoe, California, on Blackwood Street. Most of the big name entertainers that played Harrah's-Tahoe ended up being interviewed at the KOWL studio, their voices going out over the airwaves via this transmitter. The BC-250L was in use 24 hours per day, seven days a week - continuous operation - from 1956 until the late 1960s when it was placed in the "back-up" transmitter status. The last time the Gates was powered up at KOWL was 1993.

I got a telephone call from KOWL chief engineer, Bill Kingman, in early August 2007. He asked if I would be interested in a Gates Modulation Monitor and Gates Frequency Monitor as a donation. I assured him that I would be more than happy to have the monitors. During the conversation Bill brought up the BC-250L that the station was reluctantly thinking about scrapping and wondered if I would be interested in it also. After discussing weight and size, I told him that I would be glad to have the Gates transmitter. Here are the photos of how we moved it and its restoration back to its original glory.

Moving the Gates

The moving crew consisted of fellow hams Chuck Cusick, K0DWC and his wife, Karla, W4DVQ, my wife Sharon, KK7EI and myself, W7HTR. Sharon is standing next to the Gates BC-250L with the modulator deck removed. This is just before we started moving the transmitter out of the KOWL Transmitter Shack. Behind Sharon is the current KOWL transmitter. A few minutes later we are going out the shack door with the Gates. As can be seen, it barely fit through the door.

After Chuck and I got the transmitter out of the shack we are deciding how we are going to get it into the van. This photo is taken looking west down Blackwood St. in South Lake Tahoe.

Chuck and I have the Gates at the "Gate" and we are ready to lower it into the van. We ended up having to move the van in order to get the angle correct so the transmitter would side into the van. Far right is the modulator deck. Chuck carried that in his pick-up truck along with the transmitter's rear door and some of the spares.

Left, KOWL Chief Engineer Bill Kingman and I pose in front of the van with the Gates finally loaded and ready for transport to Virginia City. Bill took all of the photos of the Gates removal. Along with the transmitter we also were given all of the spare tubes, manuals, letters regarding purchase (in 1956) with another letter about repairing the modulator deck, the modulation monitor and frequency monitor and some spare components. To the right is a photo of the "moving crew" in front of the van with the Gates loaded. Left to right, Karla W4DVQ, Sharon KK7EI, Henry W7HTR and Chuck K0DWC.

When we got to the museum we moved the transmitter in on its side using a heavy-duty dolly. We then flipped the Gates on its back to have minimum height so when lifted vertical it would clear the low ceiling in the museum (6'8".) I already knew that we would have to cut a relief hole in the sheet rock in order to have enough clearance. The hole measured 15" x 46". All calculations were correct and the transmitter was raised into a vertical position with no problem. It only remained to put the sheet rock piece back into position and then patch up with plaster. Photo right shows Chuck inspecting the hole before we patched it up.

Cleaning up

After setting in the corner of the KOWL transmitter shack since 1993, the Gates BC-250L had accumulated a lot of dirt, dust, crud and spider webs along with evidence that past problems had really not been cleaned up. First we wiped down the outside of the cabinet then proceeded to remove the RF deck for cleaning and repairing of anything that looked suspicious.

Obvious was the missing PA plate blocking capacitor - fortunately we were given all of the spare parts and among them was another NOS PA plate blocking cap. I removed all of the large inductors for cleaning and lubrication. They were almost frozen in place but operated well after a lube job. The wiring had a few "emergency" repairs that needed to be cleaned up along with the repair of the crystal switch. The crystal switch selects one or the other of two duplicate 1490kc crystal-oven assemblies. An "emergency" repair had disconnected the switch and hard-wired only one crystal into the circuit. Most of the problems with the RF deck were just due to age and non-use. These were easy to repair. However, another problem was the lack of hardware. Over the years various repairs had left many of the components only held in place with one or two screws or bolts. In fact, two transformers, one on the Modulation deck and one in the bottom of the cabinet, were only setting in place after repairs had been made years ago. The RF deck also needed a rebuild on the turns counter for the PA Loading requiring a new shaft and new crank-handle be made. The photo to the left shows the RF deck chassis cleaned on the right side but the left side remaining to be cleaned.

The photo upper right shows the Modulator deck where T7 "blew its top" and spewed black tar everywhere in the transmitter. Cleaning this mess up took a lot of time. We ended up removing T7, T10, the tube socket chassis and two chokes to have access to the "tar flow." Fortunately, WD-40 works wonders on the tar - helped along with a razor blade and holder for removal of the thicker deposits. In one area of the Modulator deck the tar was .25" thick covering the choke mounting screw heads entirely. Although this was easily removed, I soon found out that if even a small fleck of the tar that got on the floor was soon smeared and tracked around creating another mess. I tried to keep this from spreading by cleaning the floor every day. When all of the tar was entirely cleaned from the RF deck, Mod deck and cabinet, I was then able to go back and clean the floor up. Fortunately, the floor is linoleum and cleaned easily.

The photo to the left shows the underside of the Modulation deck after thorough cleaning and sorting out of the wiring. The repairs done to the chassis over the years had left the wiring in a mess. Since most repairs done on the transmitter were "emergency" type, neatness was not even considered - speed in getting the transmitter back on the air was of primary importance. The replacement of T7 had utilized a make shift dual transformer set-up at first. This was wired in with a harness. When the new replacement T7 was installed the harness was just "tack soldered" to the terminals and the transformer set in place without the nuts being installed.

To the left is the Gates BC-250L after the cleaning along side the modulation monitor and frequency monitor mounted in a six foot tall open rack. There was a switch on the front lower panel of the Gates that was an addition to replace the plate voltage rear mounted safety inter-lock switch on the rear door. Originally there were two switches, one for filaments and one for plate voltage. The filament switch had been removed and bypassed while the plate voltage switch was replaced with the front panel switch. I removed the switch and temporarily bypassed the plate voltage interlock. I used the small "Danger High Voltage" tag to cover the hole in the perf metal panel. After testing was completed we installed the correct type interlock switches so the transmitter would be inoperable if the back door was off.

The exterior of the cabinet had some over-spray of green paint on the left side which was removed with "Glass Plus" and 0000 steel wool. The dymo labels were left on the transmitter and monitoring gear since they were very nicely installed (straight) and the information on them made sense. In fact, the labels are color coded so green labels indicated current "off air" (no plate voltage) and red labels are the current while "on air." Orange labels are set points or general information.

Note in the photo to the left, the Gates is "ON" and feeding into a 50 ohm dummy load. The meters are showing reading, the tube filaments are on as are the two pilot lamps.

Inside the transmitter now appears as it did when it was first delivered and set-up at KOWL in 1956. I removed the remote boards and motor drive that allowed adjustment of the Plate Voltage remotely. The motor was frozen anyway. Additionally, the Filament Voltage adjustment pot was in poor condition. It needed disassembly, cleaning and reassembly before it would actually control the filament voltage again. The fan assembly mounted in the top of the cabinet was not connected since the fan motor had "fried" years ago. Inspection showed that the motor was beyond repair so a replacement was found and the fan assembly reinstalled.

Testing & Operation

After going through the BC-250L and correcting everything that looked suspicious, we were ready to find out what else was going to need repair. We decided before anything else that we needed to see how the transmitter worked on 1490kc into a 50 ohm dummy load. The Gates manual guides the engineer step-by-step in the initial set-up at 1400kc. In the back of the manual is a frequency chart that gives all of the approximate settings for operation on any frequency from 540 up to 1600kc. There are a few capacitors that are not connected into the circuit when operating above about 850kc. They remain on the chassis but are not hooked up. Running the 230vac to the transmitter was not too difficult. We installed a three conductor, 10ga. cable that was 25 feet long with a 20 amp connector on the end. This allowed us to access the 230vac outlet in the ham shack. Since the power requirements for the Gates was 230vac at 2000 watts at 100% modulation, the current draw would be about 8.7 amps and the voltage drop across 25 feet of 10 gauge cable would be negligible.

Initial power up revealed that the power on lamps were either missing or burnt out but the tube filaments came on. We installed some indicator lamps and applied power again - about 10 seconds later the low voltage B+ and Bias supplies came on, running at 600vdc and 400vdc while the bias for the audio was at -21vdc. The bias for the 810 modulator tubes can run as high as -100vdc but I set it for -65vdc for the present. After a quick check of the voltages the transmitter was shut down and checked over for any physical problems. With nothing found, power was again applied and left running for 45 minutes to see if anything developed. While the transmitter was running in we went ahead and tuned the driver stages up with the final grid drive to the 810s running at 145ma with no plate voltage and the plate current on the 6146 drivers running at 215ma. The transmitter ran fine during this period with no problems. We tested the neutralization by running the PA tuning inductor through resonance to see if the 810 grid current varied - it didn't, which signified that neutralization was fine for 1490kc. We connected a 50 ohm dummy load to the output and attempted to apply plate voltage. Immediately the 20A fuses in the 230vac box blew signifying that something was connecting to ground when the plate relay closed.

The first thing I checked was the relay board since there were several connections that were changed when the remote control circuits were removed. I found a factory wiring error that had been corrected by installing a wire to a different terminal. I went ahead and corrected the relay board so the wiring would match the schematic. Nothing else was found on the relay board so we proceeded to remove the load from the plate transformer and see if the PLATE control circuits would power up the transformer and hold the relays. No problem with no load. Next we connected the rectifiers and again no problem but we were using solid-state substitutes for the 8008 tubes. Next we connected T-12, the filament transformer for the 8008s but left the solid-state rectifiers in. This combination blew the fuse. Pulling out T-12 showed that arcing had happened where it was mounted to the cabinet floor. When checked for direct shorts to case nothing was found, however there was probably enough leakage current to cause the fuse to blow. I noticed that the two chokes were mounted on ceramic standoffs but I had bolted T-12 directly to the cabinet floor.

When we picked-up the Gates, T-12 had been setting on a piece of cardboard but I didn't attach any importance to that but was more concerned that T-12 was bouncing around since it wasn't bolted to the cabinet floor. Obviously, we had discovered why the cardboard piece was important. I wasn't going to use the cardboard "trick" - instead I mounted T-12 using ceramic standoffs similar to how the chokes were mounted. A quick test allowed us to power up the entire Plate supply with 8008s installed though the RF deck and Modulator deck were not connected. No problems were encountered and the 8008s had a slight blue glow inside the plate structure. We then went for it and connected the entire transmitter together but disconnected one of the plates on the 8008s. This would give us about half voltage for tuning purposes. Power on gave us about 600vdc and about 150ma at resonance. With both plates connected we had 1250vdc plate voltage and about 250ma of plate current, or about 315 watts input power. We adjusted the modulator bias for 50ma idling current and checked all of the other readings finding them to be pretty close to the expected values. Next was the modulation test.

Using an HP Function Generator (50 ohm output Z) fed into the audio input (transformer 600 ohm Z) set at 800 Hz. We wrapped a few turns around the output lead from the Antenna Inductor and attached that to the Tek 453 oscilloscope input. When the transmitter was powered up we had ample coupling to the 'scope. While watching the 'scope, we increased the function generator level until we began to see an indication of modulation on the transmitter waveform. Checking the modulator current, I found it was just slightly above idling current. I increased the audio level until about 50% modulation was indicated. The modulator current was at 100 ma. so I further increased the audio until 125 ma of modulator current was showing. This was about half of the plate current of 275 ma. and the 'scope showed about 60% modulation. Since the Gates uses a shunt connection for the modulation transformer the audio power required for 100% modulation is the same as the PA plate input power.

Problem Develops

I should have known that the rectifier filament transformer T-12's leakage would eventually develop into a full-blown problem - and in a spectacular manner. After several hours of running with the MV rectifiers with no issues, a demonstration of the Gates to fellow hams resulted in arcing at T-12 and a blown fuse. The top of T-12 was scarred with several "tracks" showing that the path had been from HV to the AC line. Fortunately, among the spare parts were the SSRs that KOWL had been using in the transmitter. I removed T-12 and isolated the primary AC feed and then connected the SSR output to the swing choke. Testing revealed no problems and a slightly higher plate voltage now.

Final Testing

Next was to try voice modulation. Bill Kingman also is the engineer at KTHO 590, also located at South Lake Tahoe. Bill donated a load of old equipment from KTHO - mostly tape carts but among the gear was a Broadcast Engineers (BE) Audio Console. This would allow various audio inputs to be amplified for input to the Gates. I found that other than a few broken wires, nothing was really wrong with the BE console. It was all op-amp technology and dated from the 1980s but it worked great and gave ample audio signal control.

I decided to try the Shure 55 CV microphone since it worked and was not being used on other equipment. Using the Shure 55 through the BE console worked great. The audio sounded very nice listening on a wide audio bandwidth receiver - the RCA SC-88 on "broad."

Museum Display

The next step was to set-up the Gates so it could be on display in the museum and operated into a dummy load at 1490kc for demonstrations. I used a military dummy load that has a direct reading watt meter so it is obvious that the transmitter is putting out 250 watts. I also brought out the audio feedline so the Gates can also be modulated if necessary.

The Gates BC-250L is just about the first thing you see when entering the museum. It is pretty much as it was in 1956, when delivered to KOWL. Demonstrations of the transmitter's operation are sometimes included in a museum visit - depending on the visitor's interest. It is impressive to "fire up" the Gates and see the 810 filaments light, to watch the plate current meter jump to 275ma and see the dummy load watt meter rise to 250 watts output power.

Sometime in the future we may retune the BC-250L for 1925 kc operation. 160 meter operation would allow us to actually radiate a signal with this marvelous transmitter. There are several minor problems to overcome first before 160 meter operation becomes a reality but the desire to have the Gates actually putting out a signal into the airwaves is ample motivation to solve those problems.

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P.O. Box 73 - Virginia City, Nevada 89440

Owner/Operators:

Henry Rogers W7HTR
Sharon Rogers KK7EI