Ok. New update.
First a confession... The unit I'm working on is actually a 2308, which is the updated version of the 2306:
https://forum.tek.com/viewtopic.php?t=94760#p153847But... The bottom board that is giving me trouble is shared with the 2306, just with a different firmware build (it actually has the 2306 check mark on the silk screen for the board model). I haven't seen anyone here with a 2308 and I didn't want to scare everyone off since the board that's having issues appears to be the same.
Update:
I hooked a logic analyzer up to the serial lines on the display connector and captured the messages on Pin4 and Pin5: [UART decoding at 9600 8n1]. Pin4 is coming from the main processor because it gets traffic even when the cable to the display is unplugged.
During successful power up without the top board installed:
Pin4 has the following messages:\x0C\x01\x06\0 2308 ADDR 6 A01 00000 60HZ
\x05\xFF\x06\0 start response model INVALID
\x05\xFF\x06\0 start response rev INVALID
\x05\xFF\x06\0 start response key INVALID
\05\0\06\0 0.000 V #1 OFFOVERFLOW
\05\0\06\0 0.000 V #1 OFFOVERFLOW
...[repeating]
Pin5 has the following messages:2304 B02 \x80
![](https://www.eevblog.com/forum/testgear/keithley-2306-dual-battery-simulator-repair-(maybe)/?action=dlattach;attach=1543453;image)
The overflow message is because the top board isn't installed. That doesn't happen when the top board is installed.
When the unit doesn't power up correctly I get these messages, then nothing...
Pin4 has the following messages:\x07
\t\x17
\x07
\x0C\x01\x06\0\x20
Pin5 has the following messages:2304 B02 \x80
2304 B02 \x80
![](https://www.eevblog.com/forum/testgear/keithley-2306-dual-battery-simulator-repair-(maybe)/?action=dlattach;attach=1543459;image)
With the remote display plugged in, I get the following message on Pin4:
\x0C\x01\x03 REMOTE PANEL ENABLED
Does any of that set off any alarms for anyone?
Any more ideas on ways to troubleshoot this? I haven't see any other diagnostic interfaces.