PK-232 Checkout & Alignment

I acquired an old AEA PK-232MBX controller for amateur radio digital communication. I already have a more modern Raspberry Pi running Direwolf, but the prospect of creating a 1980s/1990s vintage station with an Apple II is irrationally appealing. The opportunity to combine two hobbies, amateur radio and retro computing, is interesting.

I purchased my PK-232 untested from eBay. While it appears in good physical condition, I started with the loopback test from section 2.5 of the manual. After passing that, I moved on to the alignment procedure in Chapter 6 of the service manual. Although written for a slightly older PK-232 revision, the alignment procedure remains applicable. The alignment procedure is mostly straightforward, but I found two points confusing.

First, the procedure references the R160/C59 junction test point. The silk screen label for R160 is underneath the resistor and impossible to read. The scanned service manual does contain the board layout identifying components, but that diagram is out-of-order in the scan. The diagram is actually stuck in “Appendix D – Waveforms”. Of course, I found that after locating the test point through trial and error.

PK-232MBX circuit board, showing oscilloscope probe connected to R160/C59 junction

Second, the “AFSK null adjustment” (Step 12 in Section 6.2.1) didn’t make sense. No matter the adjustment, I observed no change in the signal. The procedure made a bit more sense after reading the Heathkit HK-232 user manual. The HK-232 is essentially a kit version of the PK-232, so design and operation is identical. To complete this adjustment, exit “CAL” mode with the “Q” command. With the oscilloscope connected to the R160/C59 junction, adjust the oscilloscope vertical scale to show the noise. Then adjust R157 to minimize the amplitude of the noise. The range of adjustment is very small and changes in the noise amplitude are barely noticeable. R157 sets the “tone generator off” output level.

Receiving packets!

In the end, the PK-232 works, and I’m able to transmit and receive packets. Unfortunately, there isn’t much packet activity nearby, and I’m unsure what’s next for the unit. I’m not done with packet yet, and I look forward to trying a better VHF/UHF antenna and HF operation.