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When we last left transmitter 298 (a RCA T3 20kW independent sideband "H Set" transmitter designed for point to point work - see Newsletter No. 10) we saw the wonderful restoration work that had been done by the "H Set Team", a group of dedicated volunteers, current and former HP engineers. But some problems were mentioned as well, problems that might be expected when bringing a powerful transmitter back on line that had been dormant for decades.
Now comes Mr. Bob Dildine of the H Set Team to report on the progress that was made on Saturday 11 August.
WARNING: Lots of transmitter geek information included below.
Saturday as a productive day at Bolinas.
Steve Pazar and I went out to continue troubleshooting the H-set. The objective was to check for parasitic oscillations and track down the cause of the excess plate current we've been seeing.
Here's what we found.
When we first turned on the transmitter to check for parasitics, we noticed that the total plate idling current drifted up over the course of 5 minutes or so for no apparent reason. After some experiments that included swapping tubes and monitoring the individual cathode currents we found that the 4CX5000 that was rebuilt by Econco (s/n 4GC73) for this project was the culprit.
Its cathode current started out at 0.7 amps and increased to 0.95 amps in less than 10 minutes of operation. The other tube is a new one from Eimac (s/n JEG197) and its cathode current remained constant at 0.55 amps. This was with 4,500 volts on the plate, 1,300 volts on the screen, and -325 volts on the control grid and with no RF drive. We checked for parasitic oscillations using an HP 8554 spectrum analyzer connected through an attenuator to the sampling loop on the transmitter's output. This is before the low pass filter. No indication of oscillations were found up to about 1500 MHz.
There were no other indications such as erratic tuning either. Then we installed a pair of well used 4CX5000's that were pulled from KPFA's transmitter about 35 years ago. Cathode current on these were 0.55 amp and 0.6 amp and these currents were steady and unchanging with time. We tuned up at 22 MHz and were able to see about 4,700 watts output for an input power of 6,900 watts, an efficiency of about 68%. Output power was measured with an RF ammeter in series with the 600 ohm dummy load but we're not sure of the ammeter's calibration.
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| Bob Dildine (L) and Steve Pazar (R) watch the meters intently as they tune up transmitter 298 |
In the meantime, Steve Hawes connected the H-set to the station's keying network and we had it sending the KSM traffic into the dummy load. We looked at the output waveform wave shape and it looked pretty good, with reasonable rise and fall times. Next time out we'll do a more careful measurement.
So it was a pretty productive day.
Regards,
Bob Dildine
