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1930s Rack Transmitter

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The original PP 814 transmitter used for Field Day this year has been rebuilt into a 6ft wooden rack with new power supplies and some changes to the RF sections.  The amplifier’s HV supply now uses a recently acquired UTC S-48 transformer (refinished in gloss black with a wrinkled top plate) with taps for 1kV, 1.25kV, and 1.5kV.  These taps can be selected via banana jacks/plugs on a strip of bakelite between the transformer and the 866 mercury vapor rectifiers.

There are additional supplies for the filaments (6.3VAC and 10 VAC), the oscillator and doubler (75V and 225V regulated), buffer (600V using 816 mercury vapor rectifiers), amplifier screen (300V using 816s), and amplifier bias (-90V regulated with a 874), all of which, including the HV supply, populate the lower four decks.

The exciter was modified from it’s FD configuration, now using an 89 ECO oscillator choke coupled to a 42 Class A buffer (plate tuned to second harmonic of ECO grid) followed by an 802 buffer/driver.  Operation is much more stable than with the original two-stage circuit.

The amplifier now uses a pair of 814s in push-pull, providing more output power than the original PP 804s.

The antenna tuner is also being physically reconfigured, putting the RF current meter in the center of the panel and the tuning capacitors on either side, similar to many period tuners.  The circuit itself is unchanged.

The plan is to operate it Field Day style from the back porch in the PA QSO Party (SOLP CW only – Crawford Co.) and well as on 630M Sunday night (0000Z Monday), which is the first day of Amateur operations on the band for most stations.

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Model 28 KSR Teletype

Picked up this nice Model 28 KSR Teletype machine last week from a ham in EPA.  Should be lots of fun operating in future RTTY contests.

Video of the M28 in action (believe it is copying the W1AW RTTY bulletin on 40M):

1930s Transmitter Update

dscf7164-croppedThis year’s Field Day transmitter employed a 59-59 (ECO/Buffer) exciter and Push-Pull 804 amplifier.  While it worked relatively well for the circuit in use, I wanted to get better performance out of the exciter.  Tuning was very critical at each stage due to insufficient isolation (incomplete screening in the 59s), causing chirp and in some cases a raspy note when optimal settings were not used.  A particular “good” setting of the tank tuning at each stage was also only good for 10-15 kc before re-tuning was needed to clean up the signal again.

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