[Elecraft] K2 Un-Modules
from [Mark J. Dulcy]
I wrote an article about some little things I built recently... I'm thinking this could become a Tech Note at Elecraft, but I'd like comments first.
The KSB2, KNB2, and K160RX options add desirable features to the K2. However, once they’ve been installed there’s no easy way to temporarily remove them. Prior to their installation, one or more bypass jumper wires must be removed from the RF board. In addition, for some options a few other components must also be removed or replaced. Because of these changes, you can’t just unplug these options because the radio won’t operate without them.
Until now, the commonly accepted method of removing an option has been to undo the changes that were made to the RF board when the option was first installed. That might be an acceptable solution if you are permanently removing it, but what if you only want to take one out temporarily for any of the following reasons?
- To modify it
- For high-level fault isolation (in case the radio stops working altogether)
- For signal tracing parts of the RF board that are otherwise hidden
Enter the Un-Modules! By using these simple and easy-to-build devices, you can quickly remove their associated options without modifying your K2! You’ll note that un-modules aren’t exactly the same as removing an option by the traditional K2 PCB rework method, since the resulting signal paths will be a little longer. Thus, signal leakage and stray inductance and/or capacitance may be slightly higher. Even so, with proper construction practices they still ought to be better than with the actual options installed, so the impact of un-modules is probably negligible. More importantly, by using un-modules you’ll be sparing your RF board from repeated PCB rework and the very real possibility of serious damage as a result! In summary, the benefits of un-modules … 1) they make it really easy to remove the options and 2) they eliminate the chance of damaging your K2 … should outweigh any concerns about their possible negligible impact on performance.
What you’ll need. Foremost, you need the proper Single Inline Package (SIP) terminal strip connectors. They must have gold-plated mating contacts, whereas tin plating is actually preferable for the solder-tails. The mating contacts (posts) should be at least 230 mils (.230") in length and have a 25 mil (.025") square cross section. Do not use terminal strips that have round pins! The exact length of the solder-tails isn’t critical; somewhere around 120 mils (.120") should work fine. You can either buy individual connectors with the required number of pins, or just get one long strip – like I did – and cut it up to create the lengths you need. You want connectors with a single row of pins spaced 100 mils (.100") apart. Slightly “smaller” metric headers also exist with pins that are 2 mm apart; you don’t want those! Also, don’t get connectors that are designed to be easily broken apart because they are far too flimsy and the longer headers would almost certainly break apart under the forces of insertion and removal!
The other parts you’ll need are a 560 ohm ¼ W resistor and replacements for most of the parts you removed from the RF board when you first installed the options. Don’t try to reuse the original K2 parts because their leads won’t be long enough! You will also need the following tools and supplies: a soldering iron and solder, some insulated or bare hookup wire, diagonal cutters, needle-nose pliers, a small vise, and some brightly-colored quick-drying paint. Don’t use spray paint because it might contaminate the connector contacts! I used orange nail polish leftover from Halloween.
- Solder all components and jumper wires to the connector tails, never to the posts! If you are unable to determine the difference … the posts must be gold-plated while the tails should be tin-plated and noticeably shorter than the posts.
- Never insert or remove an option or an un-module while the radio is powered on, or even while power is connected to the radio!
- I haven’t bothered to create an UN-KAF2 because I don’t have a KAF2. Such an un-module has to span two K2 mating connectors, so creating one would require a circuit board to properly space the connectors. (Unfortunately the separation between the KAF2’s mating connectors isn’t a multiple of 100 mils, so standard 0.1" perfboard can’t be used.)
The SSB Adaptor option can be replaced by the trio of simple UN-KSB2 headers shown above. When properly installed, they duplicate W2, W3, and C167 which you removed from the RF board before installing the KSB2.
- 2 3-pin .100" SIP terminal strip connector
- 1 12-pin .100" SIP terminal strip connector
- 1 .001 µF ceramic capacitor
- Using quick drying paint, mark pin 1 on one end of the 12-pin connector.
- While the paint is drying, solder a jumper wire between pins 1 and 3 of both 3-pin connectors. (Since they are symmetrical there’s no reason to mark pin 1.)
- After the paint has dried, solder the capacitor between pins 7 and 12 on the 12-pin connector.
Remove the SSB Adaptor and plug in the 12-pin UN-KSB2 header at J11. As shown in the silkscreen on the RF board, pin 1 goes toward the left side of the radio when it’s viewed from the front. Also plug in the two 3-pin UN-KSB2 headers at J9 and J10. They are near the right edge of where the KSB2 mounts on the RF board. ALWAYS INSTALL ALL THREE UN-KSB2 HEADERS (AS A SET) WHENEVER THE ACTUAL KSB2 OPTION IS REMOVED FROM YOUR K2!
The Noise Blanker option can be replaced by the clever UN-KNB2 header shown above. In conjunction with the installation of an option-supplied alternate value for RF-R88, its circuitry duplicates the attenuator that was originally formed by the three resistors you removed before installing the KNB2.
- 1 8-pin .100" SIP terminal strip connector
- 1 100 Ω ¼ W resistor
- 1 470 Ω ¼ W resistor
- 1 560 Ω ¼ W resistor
- Using quick drying paint, mark pin 1 on one end of the connector and wait for it to dry.
- When installing the resistors in the next step, position them either “above” or “behind” the connector relative to the image above. This is important because there’s very little clearance between J12 on the RF board and the Elecraft® KSB2 option (when it’s installed). If the resistors extend even slightly beyond the “front” edge of the connector, then their leads might touch component leads on the KSB2!
- In this step, install the three resistors by forming their leads around the tails of the connector pins but don’t solder them yet:
- The 470 ohm resistor goes between pins 1 and 2.
- The 560 ohm resistor goes between pins 5 and 6.
- The 100 ohm resistor goes between pins 1 and 6.
- Now solder the resistor leads.
Remove the Noise Blanker and plug in the UN-KNB2 header at J12. As shown in the silkscreen on the RF board, pin 1 goes toward the left side of the radio when it’s viewed from the front.
There are no issues regarding the use of this un-module.
The 160 Meters & Receive-Only Antenna option can be replaced by the simple UN-K160RX header shown above. It duplicates jumper wire W1 (RX Antenna Bypass) which you removed from the RF board before installing the K160RX. I haven’t actually tested this un-module but I’m confident it will work.
- 1 16-pin .100" SIP terminal strip connector
- Using quick drying paint, mark pin 1 on one end of the 16-pin connector and wait for it to dry.
- After the paint has dried, solder a jumper wire between pins 1 and 7. WARNING! IF YOU LOOK AT THE SILKSCREEN ON THE RF BOARD YOU MIGHT BE MISLED TO THINK THAT THE JUMPER SHOULD GO BETWEEN PINS 1 AND 6, BUT IT SHOULD NOT! YOU CAN CONFIRM THAT IT GOES BETWEEN PINS 1 AND 7 BY LOCATING JUMPER WIRE W1 ON THE K2 SCHEMATICS ➚ AND/OR BY FOLLOWING THE TRACES ON THE UNDERSIDE OF THE RF PCB.
Installing the UN-K160RX is more involved than the other un-modules because the K160RX is hard-wired to the rear-panel RX Antenna jack, making it difficult to remove. Also, note that there’s no pin-1 indicator in the silkscreen artwork for J14. Pin 1 goes toward the left side of the radio when it’s viewed from the front.