2465 TekTronix repair attempt with (horizontal?)  issues, help needed - Page 1
Forum section: electronics / repair

author: Satbeginner
author_rank: Regular Contributor
timestamp: April 30, 2019, 06:56:33 am
content: Hi all,I got (yet another for my collection) 2465, it was stated it had problems before I got it, so no problem there.What have I done so far:Recapped the Power Supply, cleaned the Siemens fan and powered the scope up.Power supply is 100% OK now, all voltages are well within range in terms of value and ripple, I set the reference voltage for 10.00V.Control DAC and reference are checked and set for a 2.5V range.After Power-On I have a straight trace, not really focussed perfectly yet, but more interesting: I got no proper readout, I have two blurry lines just outside the top and bottom of the screen.When I apply a calibration signal from the scope itself when on Auto Lvl, the triggered light goes on, but the image looks like if it is not triggered, because I see just two horizontal lines.They seem to be the high and low level of the calibration signal.When I switch on the "Voltage cursors" (horizontal lines) they seem OK, a little vague, but their position responds to the delta-settings knobs, but when I switch to the vertical (Time Cursors) I see some vague light in the tube as if the beam is hitting the sides?The Ch1 trace stays where it is.Now the weird thing: as I am looking for causes, I tried the following:When I put the scope on X-Y mode using Ch2 I get a horizontal line instead of a dot??When I check the outputs of the U800 (second scope) I have clean static DC voltages (with cursors and readout switched off) approx. between 20 and 60V, depending on the Ch1 position, so that should show a dot, but it is a horizontal line......?Vertical position of this line (that should be a dot) is nicely controlled by the Ch2 position.When not in X-Y mode, the horizontal trace appears to be way to wide as well, so it looks like something other than the U800 is causing a horizontal deflection?Any ideas where this line / horizontal deflection is coming from?It's a line, so it should be some changing signal, but now I could use some tips and tricks.Thanks and un saludo,LeoP.S.In my search for the cause I compared signals with my other 2465 and found this so far:On the A9 High Voltage PCB, in my working 2465 I see a sinus signal on the output of opamp U1890B (Pin7), while on the not working scope I see a negative voltage?

author: tautech
author_rank: Super Contributor
timestamp: April 30, 2019, 11:22:01 am
content: I'd recommend you post a link of this thread here so to get the many 2465 owners attention and perhaps get the help you need:https://www.eevblog.com/forum/testgear/tektronix-2465b-oscilloscope-teardown/

author: Satbeginner
author_rank: Regular Contributor
timestamp: April 30, 2019, 01:02:52 pm
content: Hi, I will because that's the thread to go when you have a 2465x scope, been there many times :-)But the good news is: I solved it!!My assumption was that the horizontal shaped line must have been some kind of deflection??In a way it was, but not some hum or ripple, it turned out to be the control of the last set of convergence grids.On the A9 High Voltage board there is the Edge Focus potmeter (R1864), that sets the voltage on the vertical convergence grid.Amplifier U1890B and Q1890 are supposed to invert that voltage to be used on the horizontal convergence grid as an inverting voltage follower.After some searching I found that the voltage on the Quad Pole #3 was always -285V, regardless of the setting of the R1864 potmeter that did change the voltage on quad pole #4.It was the €0,03 1MOhm feedback resistor R1891 that went all open :-), so no feedback so the output went as negative as negative can be.So, instead of extra focussing the beam it did exactly the opposite, it spread the beam out completely, hence the line and the way too wide traces!After replacing that resistor I have about the sharpest trace of all my scopes, so I am a happy bunny!Thanks for reading and pointing me to the other thread.Now I will put it all together again and start the calibration.Un saludo,Leo