Author Topic: Tektronix 465 horizontal issue  (Read 12238 times)

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Offline bob808

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Tektronix 465 horizontal issue
« on: November 16, 2013, 02:23:23 pm »
Hello.
I got a Tek 465 scope from ebay and I was surprised to see that it worked with no problems. Perfect cosmetic condition, came with original service manual, pouch and a blue screen (I don't know what that's used for, there's already a clear one installed, and a metal mesh in front of that).

After a few tests it was clear that I need to recalibrate it.
Calibration signal was about 270mV with 960hz. I could get it to 300mV but there's nothing on frequency adjustment in the manual. If anyone knows how that's done...
Voltages are very close to spec:
5v - 35mV ripple
55.74V - 47mv ripple
15V - 29mv ripple
-8V - 10mv ripple
112V - 30mv ripple.

This is my first scope (somehow, as I just got a digital one as well so it's a first for me anyway).
I followed the calibration procedure about 4 times until I understood what was happening, and I got everything right. I don't have a function generator so I used the 1khz signal from the digital scope, I had another 4mhz tcxo and 16mhz from arduino crystal :D I only used the 16mhz sine wave for trigger calibration as that's not that reliable. 1khz and 4mhz generators are spot on with digital scope and dmm.
The problems that I have are with the horizontal part of the oscilloscope.
First the horizontal line goes beyond the visible area of the CRT. I can adjust the horizontal knob and get more of signal both ends. The trace also fades away in a fuzzy way instead of a sharp dead stop (only for sine waves, for rectangular it ends nice). If I increase the brightness then I get even more trace at both ends. At maximum crt intensity I get really fuzzy endings on the trace.
On faster time sweeps (faster than 5us) I get a dimmed portion of the display that moves with the horizontal knob, it's signal dependent, doesn't seem to be from the crt itself. The "faded" part of the signal moves with changing the knob, with signal and with gnd couple on either channel.
Also when I gnd couple both channels (on slower sweep speeds) the line is nice and sharp but it has a "belly" in the center part, oriented upwards, and is about a micro division on horizontal. If I move the line to the upper part the shape inverts, with the "belly" downwards. On the left side the trace tends to distort more on the extremes. I couldn't test the HV as I don't have proper probes but I did adjust the CRT bias as per instructions.
Any ideas?
 

Offline bob808

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Re: Tektronix 465 horizontal issue
« Reply #1 on: November 17, 2013, 09:58:46 am »
I made some more tests and as I cleaned my face plate I must have offset the horizontal knob a bit, but let's say the trace starts from the first division, my trace extends to the right for about one more division.
Also I tested the horizontal amplifier and all transistors seem ok, also I got the same shapes as per manual in the test points indicated.
Also I did a measurement for the signals going to the CRT plates and they are identical.
I did another test in XY mode with 5Vpp sine wave on CH1 and I get a 45 degrees line with the same vertical divisions as the signal so the horizontal amplifier seems ok.
I was thinking maybe the sweep generator but there are two for A and B modes and the dimming problem presents itself in both modes, although in B mode the trace moves a micro division to the right and the dimming moves on the trace.
Also I put a 1khz signal on ch1 and 16mhz signal on ch2 and in ALT mode I get the dimming but as I switch to either channel the dimming becomes brighter, although is still there.
 

Offline sync

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Re: Tektronix 465 horizontal issue
« Reply #2 on: November 17, 2013, 12:34:04 pm »
I made some more tests and as I cleaned my face plate I must have offset the horizontal knob a bit, but let's say the trace starts from the first division, my trace extends to the right for about one more division.
That's ok. The trace on an analog scope should be longer than the screen. Usually 11 divisions.
 

Offline bob808

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Re: Tektronix 465 horizontal issue
« Reply #3 on: November 17, 2013, 01:11:16 pm »
I so than that's very good but I'd like to solve the dimming issue.
I hope I get some time tomorrow to check the sweeping circuits.
 

Offline sync

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Re: Tektronix 465 horizontal issue
« Reply #4 on: November 17, 2013, 01:41:04 pm »
Calibration signal was about 270mV with 960hz. I could get it to 300mV but there's nothing on frequency adjustment in the manual. If anyone knows how that's done...
It's ok. The calibrator is only spec as "approximate 1kHz".

Quote
The trace also fades away in a fuzzy way instead of a sharp dead stop (only for sine waves, for rectangular it ends nice). If I increase the brightness then I get even more trace at both ends. At maximum crt intensity I get really fuzzy endings on the trace.
Fading in and out is normal for faster sweeps. The intensity modulation has only a limited bandwidth. At what sweep speeds do you get this?

Quote
On faster time sweeps (faster than 5us) I get a dimmed portion of the display that moves with the horizontal knob, it's signal dependent, doesn't seem to be from the crt itself. The "faded" part of the signal moves with changing the knob, with signal and with gnd couple on either channel.
This is not normal. Maybe caused the z-axis (intensity) modulation circuit. Nasty area. Parts of the circuit are at -2500V!

btw: Only do adjustments when you know what you are doing and have the necessary equipment. Make the performance tests from the service manual first.
 

Offline bob808

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Re: Tektronix 465 horizontal issue
« Reply #5 on: November 18, 2013, 12:12:20 am »
Calibration signal was about 270mV with 960hz. I could get it to 300mV but there's nothing on frequency adjustment in the manual. If anyone knows how that's done...
It's ok. The calibrator is only spec as "approximate 1kHz".

Quote
The trace also fades away in a fuzzy way instead of a sharp dead stop (only for sine waves, for rectangular it ends nice). If I increase the brightness then I get even more trace at both ends. At maximum crt intensity I get really fuzzy endings on the trace.
Fading in and out is normal for faster sweeps. The intensity modulation has only a limited bandwidth. At what sweep speeds do you get this?

Quote
On faster time sweeps (faster than 5us) I get a dimmed portion of the display that moves with the horizontal knob, it's signal dependent, doesn't seem to be from the crt itself. The "faded" part of the signal moves with changing the knob, with signal and with gnd couple on either channel.
This is not normal. Maybe caused the z-axis (intensity) modulation circuit. Nasty area. Parts of the circuit are at -2500V!

btw: Only do adjustments when you know what you are doing and have the necessary equipment. Make the performance tests from the service manual first.

On a tektronix scope the test signal is "approximately" 1khz? Didn't know that but ok, not that important anyway.
I get the dimming on some portions of the signal at faster than 5us sweep speeds. Also as I said if I move the horizontal knob the trace moves with the dimmed part together, so it's like it's "glued" to the signal. It's there even without any signal.
I attached pictures with GND coupled CH1 at all sweep speeds where the first problem appears in the order: 5us, 2us, 1us, 0.5us, 0.2us, 0.1us and 0.05us.

*later edit
Also even if the extension beyond the visible area of the trace is normal, can I adjust it somehow to stop at the last graticule?
« Last Edit: November 18, 2013, 12:22:10 am by bob808 »
 

Offline sync

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Re: Tektronix 465 horizontal issue
« Reply #6 on: November 18, 2013, 02:05:46 am »
I get the dimming on some portions of the signal at faster than 5us sweep speeds. Also as I said if I move the horizontal knob the trace moves with the dimmed part together, so it's like it's "glued" to the signal. It's there even without any signal.
I attached pictures with GND coupled CH1 at all sweep speeds where the first problem appears in the order: 5us, 2us, 1us, 0.5us, 0.2us, 0.1us and 0.05us.
On faster sweep speeds raise the intensity until you see the beginning of trace. I think you will see the trace in dimmed portion then. Reduce the intensity on slower sweep speeds again.

Quote
Also even if the extension beyond the visible area of the trace is normal, can I adjust it somehow to stop at the last graticule?
Why? You would loose the availability to do measurements over the full 10 divisions.

You should learn how to use an analog scope first and get experience. On all of your pictures the scope is working. There is this small dimming issue. But that's only cosmetic. No need to try a repair and risk damaging the scope or hurting yourself.
 

Offline bob808

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Re: Tektronix 465 horizontal issue
« Reply #7 on: November 18, 2013, 02:35:39 am »
Well I'd have a full signal to measure. Maybe I didn't said it right, I mean I'd like the trace to stop where the last vertical line is noted on the CRT. So I have signal over the whole 10 divisions, but not extend to the outside of the display.
I do like to work on tube amplifiers so I learnt my lessons a few years ago regarding HV  O0
Also there would be no problem if I end up breaking the whole thing. It's not like it will ruin me financially and I am also learning in the process of repairing. So if something is not working right I pretty much would like to try and repair it. I have my digital scope for other kinds of work/repairs.
 

Online tautech

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Re: Tektronix 465 horizontal issue
« Reply #8 on: November 18, 2013, 05:37:44 am »
Good on you! IMO there is nothing worse than 2 pieces of your kit giving you 2 different measurements. There will be a way to adjust the trace in manual, as I have done a few, with similar problems and it may be linked to HV if my memory serves me. Check the HV is correct first then the plate amps. Just remember the voltages you are working with!
Avid Rabid Hobbyist
 

Offline bob808

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Re: Tektronix 465 horizontal issue
« Reply #9 on: November 18, 2013, 06:12:49 am »
I was just looking for enough 1Mohm resistors in the box :)
 

Offline bob808

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Re: Tektronix 465 horizontal issue
« Reply #10 on: November 18, 2013, 11:08:01 am »
I adjusted the HV supply to -2450V, but I couldn't find any other problem. All test waveforms are as the manual shows. I think the problem is that the manual suggests you do the tests with 1ms sweep times. The problem presents itself at the fastest speeds so if anyone is bored with their 465 and want to make a measurement I'd appreciate a photo/screenshot of the waveform for test points 64, 87, 88 and 89 at 0.05uS sweep ;D
 

Offline bob808

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Re: Tektronix 465 horizontal issue
« Reply #11 on: November 19, 2013, 10:36:50 am »
Well I fixed the horizontal extension problem, now it fits perfect on screen :) I totally screwed up the previous calibration and it seems that X1 gain adjusts this.

Now all that's left is get to the bottom of the faded signal. It's really annoying over 1mHz and I need to crank up the "brightness" of the CRT and it hurts my eyes. Since all the waveforms check out ok at the settings in the manual I'm really in the dark right now. I notice that those waveforms do start to look very ugly once I am past 5us and go to worse when going faster to 0.05us. And I saw that in many test points. All the transistors that I've checked are working, nothing is shorted out. Power supply voltages are spot on. I do have some 20ish mV ripple on some of them but I have lots of noise around me, lights, UPS, computer etc.
 

Offline sync

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Re: Tektronix 465 horizontal issue
« Reply #12 on: November 19, 2013, 11:18:37 am »
Well I fixed the horizontal extension problem, now it fits perfect on screen :) I totally screwed up the previous calibration and it seems that X1 gain adjusts this.
*double facepalm*
The X1 gain is for adjusting the time/div value. Read the service manual. Now your timebase is not calibrated anymore. Measuring times and frequencies give wrong results now. |O
 

Offline bob808

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Re: Tektronix 465 horizontal issue
« Reply #13 on: November 19, 2013, 12:00:31 pm »
I was suspecting that it maybe alters the initial calibration but that was just something I did while searching for the fault. Ofcourse that I will calibrate it again and see if I can somehow manage that issue. Initially I did that with gnd coupled so the line got shorter and I had a brief moment of joy until I remembered that I used that pot in calibration so I wasn't 100% sure.
I am not even discussing calibration at the moment :) i need to find the problem and unless someone who knows this issue is found, it is going to take a while :) It's nice at the moment as it keeps me busy but I hope I won't become frustrated in the near future.
Again, I screwed the calibration completely while searching for the issue and I'm not planning on using this scope in the near future unless I fix it.
Any ideas at least on what kind of problem I'm facing? Is it an interference or a blanking problem or something recognizable? On B sweep the problem is less obvious and starts at about 2us but still pretty ugly at 0.05us.
 

Offline bob808

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Re: Tektronix 465 horizontal issue
« Reply #14 on: November 20, 2013, 10:15:25 am »
This issue has been addressed in the manual, but I couldn't fix it in the first calibration I did so that's why I resorted to looking somewhere else.
I added the part of the manual that describes the procedure, I also made some photos from the 465 screen and screenshot from the signal viewed on digital scope.
Basically there's this variable capacitor that you adjust for best square wave from a test point, and also has to fill the screen with the signal, especially the first two divisions. This test is made at 0.05us. I even cut one of the resistors leads in series to test individually and they test in spec. I just re-soldered back the leads. My dmm cannot read 0.25-1.5pF so I don't know if that variable cap is in spec but seems well built and there's some change in the shape between fully screwed in and almost all out. It's just not enough, the change is minimal.
Is there something obvious that I'm missing?
 

Offline PaulAm

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Re: Tektronix 465 horizontal issue
« Reply #15 on: November 22, 2013, 03:19:22 am »
That's a weird enough problem that it might be worthwhile to send it off to the yahoo tekscopes group.  It sort of looks like something is wrong with the blanking or Z modulation.  You horizontal display switch is in "A Lock Knobs", right?
 

Offline bob808

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Re: Tektronix 465 horizontal issue
« Reply #16 on: November 22, 2013, 04:29:26 am »
Yes, it's in that position.
I looked in the manual and Q1466 is part number 151-0223-00 and I found it to be MPS2369A so I bought a couple of 2N2369 today. They are a faster type of transistor with turn-on/turn-off times of 12/18ns. I thought maybe the bc549c couldn't make it that fast but...nothing changed.

Unblanking comes on the Q1466 emitter. I measured the emitter of the Q1466 and the unblanking signal looks perfect at 0.05us, but on the collector the thing is a mess. The signal on the collector starts to get ugly when I get past the 10us setting and faster, until it's really bad at 0.05us. I added some photos with side by side comparison of emitter/collector for Q1466. Also the description of the circuit.
The emitter is the red signal.

p.s. I joined that group a few days ago but I still haven't been accepted.
 

Offline bob808

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Re: Tektronix 465 horizontal issue
« Reply #17 on: November 22, 2013, 04:46:15 am »
Well, I just remembered that I was looking at waveforms with averaging on. In sample mode I can see something else that's happening there. And I can see it at slower sweeps. It looks like there's another ~50khz riding on the collector signal, and at faster sweeps it just messes up the signal. Maybe this could be the cause. I need to understand what's happening at Q1466 :)
My knowledge is minor so I have to do some research.
I added some screenshots in sample mode. Duh, I forgot to change Acqu Mode to Sample sometime ago...  :palm:
As before this is emitter vs collector on Q1466. Red line is the emitter.
 

Offline PaulAm

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Re: Tektronix 465 horizontal issue
« Reply #18 on: November 22, 2013, 05:12:12 am »
That looks seriously bad.

Do you see anything like that at tp 1486?  Looks like it might be a little bleed through from the hv supply oscillator.
 

Offline bob808

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Re: Tektronix 465 horizontal issue
« Reply #19 on: November 22, 2013, 05:20:34 am »
Yes, seems like it.
 

Offline PaulAm

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Re: Tektronix 465 horizontal issue
« Reply #20 on: November 22, 2013, 06:48:37 am »
On the bright side, there aren't too many parts in that section.

Just to be on the safe side, check the +55V supply R1480 connects to.  I got caught once because there are a couple of +55V sources on some of these scopes.

Otherwise, C1488, C1487 and C1420 are possibilities as well as a leaky diode.
 

Offline bob808

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Re: Tektronix 465 horizontal issue
« Reply #21 on: November 22, 2013, 07:06:12 am »
Seems like the HV oscillator is at around 50khz and that's consistent with what I get.
Another thing to notice is that the previous owner replaced C1418/C1419 with a 47uf capacitor. I don't know if there was supposed to be two 22uF in parallel or a double capacitor but the actual capacitor looks new, I mean new production. And I can see that one pin in the + hole is not properly covered in solder, but it has good contact.
On the + pin of the capacitor I get about 23.4V unregulated and on the fuse about 22V. On the fuse I don't get the 50khz extra but on capacitor it's there as you can see in the attached photos. On test point 90 there's 316V at 52.45khz :)
+15V supply is unregulated and at a fast look in the manual I couldn't find it in the power supply schematic. Maybe derived from +55 unregulated.
 

Offline bob808

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Re: Tektronix 465 horizontal issue
« Reply #22 on: November 22, 2013, 07:27:27 am »
C1488 shows about 2000pF, C1487 about 5000pF and C1420 I've tried with LC meter, ESR meter and DMM on capacitance but I get high values of 700-1000pF and it's oscillanting. I guess 47pF is way too small and in circuit to be measured. But it's small and sturdy :) And is not shorted.
 

Online tautech

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Re: Tektronix 465 horizontal issue
« Reply #23 on: November 22, 2013, 07:38:35 am »
"On the fuse I don't get the 50khz extra but on capacitor it's there..."

So the inductor is doing its job.  :)
Suggest you revisit previous cap replacement and research OM spec for best replacements. I feel 2 caps were put there for good reason. Not sure that low ESR is required.
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Offline bob808

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Re: Tektronix 465 horizontal issue
« Reply #24 on: November 22, 2013, 07:58:59 am »
I looked in the parts list and saw that until an earlier model than mine (28xxxx series) they used only C1419 with a value of 47uF/25V then moved to two 22uF/35V caps. Mine is now 47uF/40V. Maybe they decided to up the voltage rating on the capacitor since I get there about 23V. Don't know about the two, a higher voltage spec cap usually has lower ESR than same capacity and lower voltage spec cap. I will first try to get a low ESR 22uF/50V caps and try it that way. The thing is that I have to remove the trigger cicuit board to reach on the other side. Also as I saw in the manual one of the caps may be on the other side. I may leave it alone for the moment and see if I can find something else.
 


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