Headlines - Transmitter 298 Lives!
- K6KPH is Everyone's Sweetheart
- Operations Report
Transmitter 298 Lives!
Faithful readers of these pages will doubtless recall our earlier reports on the astounding work being done on "H"set transmitter S/N 298 at Bolinas. As those reports described a team of dedicated and talented current and former HP engineers took up the challenge to restore this transmitter to operational status more than a year ago.
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The RF section of No. 298 in an early stage of restoration
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While there are several "H"sets at Bolinas none had been restored. That's because of the dual intimidating facts that the are a> the most complex transmitters on site, having independent sideband capability, and b> all examples were seriously degraded by rust and corrosion, the result of sitting unused in an essentially marine environment for more than a decade.
See Newsletter No. 10 for the initial report on this transmitter including hard to believe before and after photos showing the level of workmanship being applied to this restoration. Then read the updates in Newsletter No. 11 and Newsletter No. 14.
We are happy to report that on Saturday 9 February transmitter 298 put 5kW of power into a H over 2 antenna on the KSM 22Mc frequency. There is still work to do before No. 298 enters full service but we thought you's want to hear this news.
Now comes Mr. Bob Dildine of the H set crew with his narration of Saturday's events:
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Bob, Larry, Greg, and Kurk went out to Bolinas February 9. Kurk brought out the spare screen supply he restored and Bob brought out the spare linear amp he checked out.
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| The open wire feeders are followed through the H frames out to the antenna by a member of the H set crew |
We walked the transmission line for our antenna all the way out to the antenna and it looked good except for one spot where a jumper around a cross-arm has been mashed down on the cross-arm, probably by a heavy bird.
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Note the unhappy result of heavy bird sitting
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We secured all the exciter units into the H-set's rack, but found that some of the quarter-turn fasteners that secure the front panels to the shields are broken. Larry will look into what can be done to repair them, but all front panels are secured to the rack frame now. Unfortunately, that didn't seem to stabilize the fluctuations in the screen current and that remains a mystery.
We connected the H-set to its antenna and drove it with a synthesizer set for KSM's 22Mc frequency. The tuning was somewhat different than for the dummy load and we got significantly more antenna current which suggests the antenna impedance as transformed by the transmission line is lower than 600 ohms.
When we keyed the transmitter, it has a raspy note that sounded like 120 Hz modulation. Earlier tests several months ago showed the H-set to have a nice clean tone. After several experiments, we found the raspy note is the same whether it's driven by the synthesizer or its internal TCXO. We also found that the tone is pure coming out of the exciter and also coming out of the 4CX250 drivers. But when the finals are activated, the note becomes raspy.
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| Video: Steve contacts the receive site for a signal report |
Line-related modulation clearly shows up on an oscilloscope connected to the transmitter's output. We couldn't track it down but more experiments will be done next time. The screen supply or final bias supply are the suspects at this time. Steve H found a dual RF ammeter assembly that we will install on the H-set antenna output next time out.
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As you can imagine, it was a very exciting day. The moment when we push the "money button" on No. 298 to put it into regular service can't be far away. As always, True Believers are welcome to visit with us at the transmit and receive sites. If you have an interest in being there on the day when No. 298 is officially returned to service please let us know by writing to [email protected]. We'll do our best to give you fair warning of the date and time.
If you've been following these reports of our activities you know that we're all volunteers. And that includes the H set crew who not only lend their skills and pay for parts but also make a 100 mile round trip from their home base every time they come to Bolinas.
Of course they do it happily, as we all do. In truth we consider ourselves to be the luckiest radio squirrels there are. But you know... a little help from our fellow True Believers would be a great help.
