Tektronix 465B time/div problem - Page 1
Forum section: electronics / repair

author: john_boca
author_rank: Newbie
timestamp: November 29, 2024, 06:50:19 pm
content: This is a new-to-me scope. It has not worked correctly since I got it. When turning the time/div control, I can only get a trace on certain values - 20ms, 10ms, 0.2ms, and .1ms. On 50ms I can get a dot, but that is it. On most of the other time/div settings, I can get a dot when pressing the beam finder. The dot can be centered vertically but is always way off to the left horizontally, either at edge of screen or just off screen. It is same results for both channels 1 and 2, A and B traces.I do have the service manual as well as the "troubleshooting your oscilloscope" guide but am getting confused where to start (other than checking power supply, which I did. Seems fine.)Following the horizontal section troubleshooting, I got as far as the "armed" and "fired" events. In single sweep mode, it says to rotate trigger level fully clockwise, press the reset button, then rotate the level control counter-clockwise to its stop. When I start the CCW rotation, I get to about 2:00 when the trigger light flashes, a trace flashes across screen, and ready light goes out. The TDs are then low. Can't remember exact sequence, but I can sometimes get both the ready and trig lights to light at same time. (So "fired", I guess.) However, no trace visible. Can still get beam finder dot. Both TDs are high.The troubleshooting guide doesn't really say what to check for the "start sweep gate" event.From here, I am at a loss. Am I even in the right ballpark (circuit)? Any suggestions for what to test?Disclaimer: I am very new to all this, so assume statements that might seem obvious to experts are not obvious to me. Bought the scope to learn how to test/repair things, not to learn how to repair it. I just figured if I can't diagnose/fix equipment with probably the best documented SM I will ever hope to see and biggest community of experts, then I have little hope of fixing other things.Last comment: I do have another (working) scope to test with.

author: tggzzz
author_rank: Super Contributor
timestamp: November 29, 2024, 08:49:35 pm
content: Welcome.to the forum. There is a lot of info on these scopes here, which can be found using the forum search or gurgle.You don't say what the input signal is. The cal out should be sufficient.II wouldn't worry about single sweep until the normal sweep with autotrigger gives a trace at all sweep speeds.I presume you have tested all PSU lines for both voltage and ripple.If the trigger works OK at some sweep speeds, then I would expect it works OK at all sweep speeds. That rules out gross tunnel diode faults.IIn old scopes oxides can sometimes interfere with contacts. Wiggle all internal comnectors, and see if that makes a difference. Ditto transistors and ICs.A common problem with 4x5 scopes is that the gold finger switches become contaminated with dirt or fluff, and need cleaning with IPA on shiny paper.Look at the manual schematic to see what (if anything) is common between the sweep speeds that do and don't work. That might give a hint as to where a fault lies.

author: TimFox
author_rank: Super Contributor
timestamp: November 29, 2024, 08:52:27 pm
content: Specifically, each sweep speed requires a resistor and a capacitor.The switching on the big rotary switch shares resistors and capacitors between different ranges, so the bad result with "5" in the range is a clue.Similarly, bad results over a range of values is another clue.

author: john_boca
author_rank: Newbie
timestamp: November 29, 2024, 10:21:35 pm
content: Thanks for super-fast replies.Just edited subject to correct that this is a 465B, not a 465. Not that this makes a difference for your advice so far. Same advice applies to both scopes.Two different "input signals" - one is just GRD (no input at all), the other is the calibrator.Yes, tested the PSU lines for voltage and ripple.I have removed, cleaned, re-attached all connectors to the A7 Trigger Generator and Sweep Logic board. Actually removed that board to get to the Time/Div cam.Thanks for confirming/agreeing with the logic of if it works at some speeds, then it should work at all speeds. In the back of my mind I was thinking that maybe certain speeds produced more or less voltages, which might not be enough to trigger the sweep. Not saying that makes sense, but when you run out of things that make sense, you look to the ones that don't make sense.Cleaning those gold finger switches was (and maybe is) the plan. I did visual inspection and they seemed clean (famous last words).Reason I haven't done this already: I just envision putting alcohol soaked paper where needed and paper falls apart, making things worse.So before doing that, just wanted to make sure I was looking in the right direction. If someone said "Oh, that is clearly indicating a fault in X, Y, or Z component", then I wouldn't have to risk the cleaning procedure.Pretty sure I was looking at wrong place to clean, though.The Time/Div cam has little raised parts which push down on a contact lead under it (on the board). Those contacts don't look like the finger switches I have seen in pictures.Maybe the finger switches are on opposite side of board, and I need to remove the entire board?

author: tggzzz
author_rank: Super Contributor
timestamp: November 29, 2024, 10:42:53 pm
content: IIRC there are finger switches on both sides of the board.On a 465B I couldn't be bothered to disassemble one of the vertical attenuators, so I simply forcefully squirted IPA across the fingers using a small plastic disposable pipette (switch open, squirt, close/open multiple times) To my surprise, it worked.Use shiny paper (i.e. not rough/abrasive), cut so strip is just wider than the "hand", soak in IPA (pull paper to verify it doesn't disintegrate), open switch, insert paper and close switch, pull paper ensuring fingers don't become bent.