Friday, August 25, 2017

Tektronix 2465B Repair - Part II

After a couple months I finally found some time to revisit my sad Tektronix 2465B auction find. At this point the power supply was rebuilt, and I had isolated U950 being a culprit by grabbing good hybrid ICs out of my working 2445B to test. Once the replacement U950 arrived it was placed in the 2465B which solved the numerous display issues and returned a horizontal sweep.

 Tek 2465B with horizontal trace restored

To make matters worse, while troubleshooting I did not have one of the hybrids (U700) lined up exactly correctly after swapping it out from my 2445B. As a result of this when torquing it back down very lightly I heard a terrible crack. The edge of the ceramic board inside the hybrid had caught on the connector mounted on the board and cracked the hybrid substrate. So this would also have to be replaced because of my own mistake. I still had a good U700 from my 2445B for testing until the new hybrid arrived. Luckily these are cheap.

Tektronix 2465B Hybrids

A new issue was apparent at this time involving the horizontal sweep and possibly trigger systems. At low sweep speeds the scope would behave correctly, a nice sweep would occur with a waveform being properly triggered. The issue I was seeing involved at higher sweeps above 1ms/div. At these speeds the horizontal sweep would intermittently show up, sometimes not being visible altogether. If a signal was present I could get a waveform to display for a single sweep when changing either the sweep speed or trigger options.

Researching into the issue I could not locate anyone who had this exact issue on any forums, most issues with horizontal sweep resulted in people just recommending to replace various hybrids. I knew this was not the case for me, looking back to the service manual(s), there was no exact description of this issue. At this point I began following the troubleshooting flowchart for the horizontal section item by item verifying voltages and waveforms. With everything looking fine, I was beginning to reach desperation as there was not much more to check. Towards the end of the flowchart there was a test involving removing the plug to J102 which is a ribbon cable that runs to an option board. This specific option board was for TV sync options which I really have no need for. Testing this I removed this connector and powering on the unit resulted in a fully functional unit, sweep and trigger was now working at all time divisions. All issues were now gone.

I really don't care to look into repairing this TV option board, pulling it out didn't show any obvious signs of problems. I don't plan on needing to troubleshoot NTSC waveforms so I am just going to leave it disconnected for now.

Tektronix 2465B Working

Some last items that I need to look at are trivial. I need to replace the scale illumination light bulbs, with 2400+ hours on the scope, all three are burnt out. I need a replacement knob for the channel 2 vertical position. This has actually been harder to find than I expected. Being patient I'm sure one will show up on eBay soon. The last issue will require some more investigation though. The traces themselves are slightly fuzzy, not a crisp trace like I see on my 2246 or 2445B. The rest of the display is perfectly focused, on screen menus look nice and sharp. I adjusted the focus, astig, and the gain + focus knobs on the high voltage board, none of them helped the traces. So not quite sure about this one. It's not a super big issue but still bothers me somewhat.

As a final note I will need to get some proper Tektronix probes for this scope, my P6105s are not designed for this 400MHz bandwidth.