I'm out of ideas for trying to fix a Tek 2755 (2754/494AP) spectrum analyzer so I thought I would as the group for some ideas...
The symptoms are the all of the front panel LEDs are on and there's some random characters displayed on the CRT. It didn't always turn on so I recapped the AC power supply which fixed the turn on issue. I also found that the backup battery on the address card leaked all over the card making a real mess. I have cleaned and resoldered it a few times and it seems to work now, but I'm not 100% sure.
I have the service documentation for the 2754 and 494AP which describes how to initiate a self hardware test which is what I've been using to try and fix this thing. The self test mode sequence is:
1. Turn on DS1047 on the GPIB card
2. Verify the checksum for the ROM on the address card and turn off DS1047 if the sum passes.
3. Verify the instrument bus, writing "PROCESSOR SYSTEM TEST" on the CRT if it's ok or illuminating DS1050 if the test fails.
4. Continue remaining tests...
My 2755 is getting to step 3 and nothing visible happens, ie no words on the CRT and DS1050 is off. The fact that the ROM is passing the checksum suggests to me that my fixed address card is likely working. The processor is also able to control the LEDs because it's able to control DS1047 (they're on the same memory mapped latch) so it's unlikely DS1050 is faulty.
I'm assuming that something is wrong on the instrument bus so I tried pulling the cards one at a time to see if the fault went away but it didn't. Then I tried the instrument bus test mode while probing the bus but the waveforms do not match the service manual.
Any ideas on what to try next? Thanks,