Dan sheds light on various Hammarlund SP-600 models

Many thanks to SWLing Post contributor, Daniel Hawkins, who leaves the following comment in reply to our previous post about the Northern Radio SP-600 and discussion about diversity operation. Dan writes:

Diversity operation: two or more receivers and antennas used to copy CW or RTTY from one or more transmitters.

Most of the Hammarlund SP-600s were models built for diversity use including the well-known JX-17, the most common SP-600. Diversity models can be used as single receivers. In this eBay example Northern Radio has modified a SP-600 J-11 for diversity use.

SP-600 nomenclature: J means joint army/navy (JAN) mil-spec components. L means low frequency. X means crystal frequency control in addition to VFO. My SP-600 is a JX-21, which is not a diversity model. Higher model numbers do not necessarily mean later production dates. All SP-600s use the same serial number sequence regardless of model. Somewhere between serial numbers 15,000 and 17,000 (mid 1950’s) Hammarlund stopped using molded black beauty capacitors and switched to installing ceramic capacitors.

The two-digit model numbers indicate model types. JX-1, 7, 10 and 21 were similar non-diversity receivers. SP-600s built for military contracts will have an additional tag showing the military model number(s).

Here is a great page showing Northern Radio modified SP-600s in action with accompanying Northern Radio RTTY gear.


Thanks for the primer, Dan! I believe I have one of the X models with crystal control, but I’ll need to verify once back home. Any other SP-600 owners out there in the Post readership?

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6 thoughts on “Dan sheds light on various Hammarlund SP-600 models

  1. Stymie

    Just got a SP 600 JX 21 Ser. Number 19841 I do not see a JX21 listed in the SW Receivers book only a SX 21A. Any info on the model would be greatly appreciated. Year built etc. Thanks

  2. Bob LaRose - W6ACU

    My first job after I got my EE was with Harris/RF Communications in Rochester, NY. I remember that Harris/RF bought Northern Radio about 1969. If I remember correctly they let it run in NYC for awhile but then moved what was left . We repackaged some of their stuff but I think the product line died very quickly after that. Northern was big in FDM multiplexed FSK modems for HF so it makes a lot of sense that they also sold specially modified receivers (including diversity) to optimize the reception of the FDM signals. These are the signals you used to hear a lot on HF that sounded like a buzz saw. The signal actually consisted of a whole bunch of individual FSK signals spaced apart. There were different versions based upon how many FSK channels and the related related FSK shift.

    1. RICK A.

      Thanks Bob for the information on Northern history. Northern made some receivers with copper chassis! I believe the 805 was one of them. It had a gray heavy duty cabinet and the face was green Vintage was 1946 as they had mostly loctal tubes.

      Thanks for sharing.

      Rick K0HE

  3. DanH

    Thanks, Thomas. Let us know which SP-600 model you have when you can. Hammarlund included an ID boiler plate on top of the SP-600 tuning capacitor shield including model and serial numbers. It is visible from the top of the radio without top cover and case. Unfortunately, these tags often go missing over the years. If yours is an X model it may still be possible to find crystals for the XTAL frequency unit. I noticed that JAN Crystals in Florida still has a rudimentary web page. I placed an order for crystals with them before but that was many years ago. They did excellent work. I have considered buying rocks for my JX-21 to provide WWV frequencies on XTAL control.

  4. RICK A.

    Thanks for the information. I ran this ad by a friend wwho is a Hammarlund expert and he agrees with me. I recommend passing on this one – it is overpriced.
    It will immediately need a full recapping. In fact, it should not be powered up before recapping – there are a couple of the BBODs that if they decide to short, will take out one or both of the big chokes with them. That is a big job (42 capacitors)
    The front panel is nice, but behind the panel, it is not in great condition.



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