Curtis EK-39

A Rare Keyer

Acquired from AntiqueRadios.com classifieds as 'For Parts or Repair', for $20. The enclosure alone is worth the price.

As advertised, the keyer didn't work on arrival - no problem there! Since there was no fuse in the holder, and the holder is broken that points to possible power issues. Replacing the fuse and powering up resulted in the neon lamp glowing briefly and then flashing out. Diagnosis: the transformer is shorted. Applying 5VDC directly to the PC Board resulted in a working keyer, so this confirms the source of the problem is the transformer.

The original transformer is huge - in fact, it's slanted on it's chassis mounting ears in order to fit. The circuit is based up on DTL Logic, a predecessor to TTL. Power requirements are low, so it's unknown why a larger transformer was used. In any case, primary winding was shorted. A Radio Shack replacement (273-1365), 12V 450ma appeared to fit very well and works. The secondary is passed through a full-wave bridge, then regulated to 4.7 V by a Zener. The connectors for the paddle and rig are Amphenol coaxial connectors. An additional straight-key input is a simple 1/4" mono jack.

IC's are Signetics DTL Series, and although the Parts List has them as LU- prefix, the actual ICs on the PC board are SP- prefix, indicating 0° to 75° operating temperature range. According to the 1969 Signetics UtilLogic II Handbook:

  • 3 - Signetics LU380A, QUAD 2-Input NOR
  • 2 - Signetics LU370A, TRIPLE 3-Input NOR
  • 2 - Signetics LU387A, QUAD 2-Input NAND
  • 1 - Signetics LU322B, DUAL J-K Binary (Flip-Flop)

EK-39 could be ordered in one of three color schemes to match the prevailing rigs of the day: grey panel and cabinet to match Collins, grey panel and black cabinet to match SWAN, Drake and Galaxy radios, and according to brochures, there was a third combination of light blue and dark green to match Heathkit SB-series, but I've not seen that color combination.

Replaced or Updated

  • Transformer - R.S. 273-1365, 12 VAC, 450ma
  • Fuse Holder - specifically, the Littelfuse 342014A is the exact part, to fit the existing 1/2" hole - Talon
  • Fuse - 3AG, 500ma, as indicated
  • Terminated the supplied cables for use with 1/4" or 3.5mm stereo keyer and 1/4" mono or RCA Rig connector
  • Updated the two .01uf ceramic disk capacitors on the Mains entry jack with proper X2 capacitors, valued .0047uf

Documents

Owner's Manual is courtesy of Brad Mitchell, N8YG, who has written an article about the Keyers from John Curtis (K6KU), called A History of Curtis Keyers, which is available at the ARRL Website.

Current Status

Operational, with plans to repaint the outer cabinet shell with Krylon Semi Flat Black #1613, per WB4HFN Drake Tech Info. For future consideration, although the keyer or it's PCB has obviously survived a power surge significant enough to blow the transformer primary, it may be possible to repair or re-engineer the keyer such that the DTL ICs might be replaced with regular TTL, Schottky-TTL, or CMOS equivalents. To that end, it may be necessary to reverse-engineer the schematic from the PC Board.

Parts Sources

Keyer End of Paddle connector is - AMPHENOL 91-MC3M 3-Pin Mic Connector (Male)
Paddle End is an In-Line, 1/4" Stereo Jack, which also has a 3.5mm stereo adapter, to handle two sizes of keyer plug.

Keyer to Rig connector is - AMPHENOL 1-Pin (Male) connector, for which I have not found the Part Number.
Rig end is a male RCA Connector, which can then connect via an RCA-to-1/4" Mono Adapter so the rig could be used with both Heathkit HW-101 (RCA) and Drake (1/4" Mono).

Fuse Holder - Littelfuse, 342 Series, 342014A (3AG Fuse,Panel Mount, Fluted Knob) which has a keyed, 1/2" hole. This is an oddball part, since current series of both Buss and Littelfuse holders are larger and there just isn't any room on the back panel to enlarge the hole.

  • Ralphs has the Knurled Knob - www.ralphselectronics.com, search: 342012A
  • Talon Electronics has the fluted - www.talonix.com, search: 342014A
  • TTI Inc - www.ttiinc.com, search 342014