Tektronix 468 fails power on test - Page 1
Forum section: electronics / repair

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: September 16, 2020, 05:54:44 pm
content: I have recently acquired a Tektronix 468 and am not sure whether it can be rescued.When powered up (with digital mode turned off) it will occasionally display a good trace but usually there is no horizontal sweep, only vertical motion of the trace. The timebase seems to working however channel display logic is a bit mixed up. CH1 shows OK but CH2 shows CH2 input on both channels. All power rails are operating fine and spot on voltage. Having checked through all the signals on the Horizontal Amp, Sweep Logic and Trigger circuits, most signals are present, but one or two are missing. Indications are that there may be a problem with the sweep logic, possibly the 74LS74 IC on the main board, but I suspect this may not be the full story.The digital section does not work at all. On power-up, the 468 is meant to start with a power-on self-test. The first test is the ROM checksum followed by the Lamp Test and then the RAM Verification Test. The manual indicates that if the ROM checksum fails then it will "neither respond to the front-panel switches nor will it operate". It also says that "Front Panel indication of a ROM failure is that the Lamp test is never started".Well none of the digital mode switches work regardless of whether of whether storage mode is turned on or off, however the main oscilloscope function switches do seem to be working. There is nothing on the 7-seg LED display except 3 dots. The Lamp Test where all the segments of the display light up does not happen which would seem to indicate that the ROM test has failed.My question is whether the oscilloscope should still operate in non-storage mode? There is a lot of complex connectivity between the analogue and storage parts of the scope so I was wondering whether the failure of the storage mode is what is causing the scope to behave in a strange way?I think my next task may be to remove, clean contacts and re-seat all the ICs in the storage computer section at the back, but I have also read that a ROM can succumb to something called "ROM rot". I don't know much about what that is but it sounds fatal. Can the ROM be re-programmed or replaced?

author: SoundTech-LG
author_rank: Frequent Contributor
timestamp: September 17, 2020, 06:49:52 pm
content: Hi,I have one here that works, at least the last time I powered it on it did. If you need to know some normal indications during power up, let me know.

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: September 18, 2020, 03:35:22 pm
content: Soundtech, that is very kind and I will bear that in mind. In the meantime I also found a "TekScopes" group on groups.io and hoping to get a more detailed response posted my dilema there as well. The ongoing discussion is here:https://groups.io/g/TekScopes/topic/76894521#171610As it happens, someone did a quick run-through of the 8085 processor signals for me today and a comparison does show a number of differences. It is quite possible that at least one of the two ROMs is faulty and the processor is halting because it has no instructions to follow although this has not been absolutely confirmed. I have replacements on the way. What has been confirmed is that the trace is not running because the digital side is down. When I followed the procedure in the service manual to "isolate the nonstorage circuitry from the digital storage circuitry to allow checkout of conventional oscilloscope functions" I did get a good trace although only on CH2, so there are some problems to be resolved even after the digital side is fixed. Still, it it was sufficient to prove that the analogue side works.I have no real expertise in diagnosing the computer side of things so this has been an interesting problem to work on!