SP-600-JX-28 s/n 15,017
acquired in Fall 2011
above "as found," before initial washdown
above after first wash
things to note -
meter, only 1 scale, larger
no meter switch
no "Hammarlund" name on front panel
has disc ceramics, ser. no. is 15,xxx
terminal board construction
rectangular side panels
additional stiffening under pwr xfmr & chokes
Was said to have been in operation about 6 mo.s before I got it,, I doubt it. Ckd filter cap ESR, was low, brought up on variac fairly quickly, monitoring line current and HV. HV rectifier filaments were open, after replacement installed, still no signals, tho' audio stages do work. I put it aside then.
Now, over a yr later, Mar., 2012, I'm getting back to it, and have found some information:
SP-600-JX-28 Signal Corps. R-620/FRR, order no. 25693-Phila-53-61, contract DA-36-039-SC-49453, Oct. 5, 1953, 100 units
built, s/n 15001-15100. Std. frequency range, 54O khz-54mhz. Several ckt modifications.
Much of the following appeared in an ebay ad, and has been edited. (e.g. that seller thought MFP was "gold anodizing")
There were only 100 of these made, and only a handful that have ever surfaced. Looking at it, most will make the mistake and
think someone had made homebrew modifications.
Of all the SP-600 receivers, this easily is the most unique, specialized, and refined. According to the manual, this rare R-620
variation had a nomenclature tag that had serial numbers 1-100. The JX-28 version with serial numbers 15000 - 15099 were this
These are some of the features the military had incorporated into this special receiver:
1. This receiver utilizes 21 tubes, not 20 as in most SP-600's. An additional voltage regulator tube V21 0B2 is added.
2. The frame and chassis is MFP'd and the sides have a similar design to the side panels of an R-390, versus the V cuts seen on
a standard SP-600, this increases mechanical rigidity.
3. This utilized all ceramic disc capacitors.
4. The wafers of the selectivity switch are ceramic, not phenolic!
5. Uses a 6BA6, rather than a 6C4 in a stable electron-coupled oscillator. also, there is no BFO buffer amplifier.
6. The output from the BFO V13 has no affect on the AVC bias developed. V12 is the avc amplifier.
7. Has a neon bulb in the antenna circuit to dissipate any electrical overload to antenna.
8. There are another 16 circuits variations for added improvement to this receiver, that are highlighted in the manual.
I do have it operating now, late March, 2012, but still am checking things out. The signal for developing AVC voltage is taken from ahead of the "IF Driver", and the BFO is injected after it, so BFO does not load the AVC, and the RF gain control can be run wide open, with AVC in operation when copying CW or SSB. At this time, I think the AVC is not reducing early stage gain enough, and the signal getting to the detector is too high. The BFO signal is much lower, so correct "demodulation" of an SSB signal still isn't correct on strong signals. I'm still looking into this problem, I think it is unique to this particular receiver, not to all -JX-28's. The SP-600-VLF uses much the same system in this area.