The above image shows my boat anchor receiver, a Drake R4A of 1965 vintage hooked up to my 2001 vintage K2. The R4A is being used as an outboard receiver with a buffer amplifier fitted internally within the K2, together with an external mute unit. Here is a link to the full article in Adobe Acrobat format (400 KB file size).

Having the access to some quite sophisticated test equipment, I thought I would share some measurements I've made on my K2 transceiver. The serial number is 1892. The K2 uses an "A" series motherboard partly modified to the "B" version.
Later versions of the K2, i.e. those you could buy today from Elecraft, include the PLL and "A to B" modifications.

My K2 is fitted with internal ATU, DSP audio filter, RS232, 160m, SSB and internal battery pack. I have also fitted the "non Elecraft" K6XX CW tuning indicator, which uses the last segment of the bargraph LED to indicate a correct beat note. The SSB and CW filter crystals have been replaced with the newer tighter specification crystals. The SSB bandwidth is now 2.6 KHz using the wider bandwidth capacitor kit.

The following table shows the image figures obtained after modifying the band pass filters (as part of the A to B mod kit), while they are not as good as an up conversion transceiver, they are adequate for most users. Beware the 2nd receiver input option (K160RX) gives much worse image figures on some bands as the TX low pass filter isn't used via that input. Measurements done with Marconi 2018.

The "A to B" modifications made no difference to the 21 MHz figures.

Another part of the "A to B" modifications involve changes to the PLL and BFO in an effort to improve frequency drift, while the modification does improve it, the figures aren't impressive. There is a later modification that replaces the PLL crystal and adds a thermistor board in place of RP3. Details of my measurements of drift and frequency error are shown below.

Frequency error varies from band to band on a K2 as it doesn't use a single master oscillator. The errors measured after 30 minutes warm up with my K2 plus thermistor board are as follows:

Power output levels are quite near the indicated figures.
Power level in Watts has been calculated from dBm levels and rounded to nearest tenth of a Watt.

Sensitivity measurements , made with Marconi 2018 and DVM. These were measured after the A to B mods were done on the K2. The reason for poor sensitivity on 15m and 10m has been found to be the band pass filters are now narrower than original (due to the A to B mods to improve image rejection), this seems to have resulted in the filters drifting off frequency when warm. I will re-align and post the new figures when I have a spare hour. The "new" sensitivity on 10m is 0.16uV without pre-amp and 0.1uV with. I suspect the poor performance of the pre-amp in the figures below has to do with amplifying more noise than signal due to the filters needing re-alignment. There was an issue with early trimmers fitted to the first K1 four band boards, these were found to be temperature sensitive and were replaced with blue coloured trimmers. The same issue may have now arisen with the narrower filters in the K2.

The other significant modification (official Elecraft 2nd filter modification) flattens the response of the 2nd filter and improves the SSB receive audio significantly, the following plots show the "before" and "after" results:

I used to run a Microwave Modules MMT144/28 transverter with my K2. If you would like details of the interface, attenuator and interconnections, download this Adobe Acrobat file (92 KB).