Broken Tektronix TDS 460

A colleague asked me if I wanted his old Tektronix scope that he was throwing out. It worked sometimes, but only for ten minutes. After that there was a sine wave added to the measured signal. Probably something with the powersupply then... I took the scope and started started it when I got home. It actually didn't get past the boot up test and it failed on the Fp Fp/CPU test, which means that there is something wrong with the front panel and or CPU, or something on the way in between.

Failed self diagnostics on start up

I followed the troubleshooting guide in the excellent service manual. It was a bit tricky to follow until I understood how the manual worked. I finally got to the verdict that I should check the cabling between the front panel and the CPU board. I took the scope apart and checked the cabling, nothing wrong. The guide now said that I should replace the CPU board. Bummer...
 Now, before I was done I wanted to check the CPU board to see if I could find anything fixable. I took the board out, but there was nothing visible. The board did, however, have two edge contacts. According to the manual, one of them was for connecting the scope to a computer for diagnostics. The manual did only say that you should use an adapter to connect the edge connector to a DB9 contact, and not which pins where which on the connector. Looking at the board, I could see that both connectors had signals routed to a MC145406DW chip, which is an RS232 driver. Looking at the data sheet I could see which pins on the connector where connected to which pins on the chip. Turns out that edge connector pin 9 is ground, 4 is TX and 6 is RX. I connected a serial connector to the pins and fired up Putty. Unfortunately none of the connectors started talking during start up. One of the connector RX pins showed and idle pattern when the scope was in standby, but that was it.

RS232 connection to CPU board

 So, I'm out of ideas. A replacement board costs from 35€ and up, and maybe it would be worth getting one, since a working scope goes between 20 and 30€. It would be a bummer if I got a replacement board just to see the power problem ruining the functionality. I need to think a bit about this. It would be nice with a digital scope, but new ones are quite cheap, and very much lighter.