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Old 16th Mar 2020, 3:03 pm   #1
IlResca
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Default Tektronix 465 not sweeping

First of all, let me say I have no analog electronics background because I used for many years oscilloscopes but only to troubleshoot mainframes, in the 70's, so only "digital" skill! Presently I would like to see some signals in my Arduino/ESP projects.

My Tek 465 is not showing any trace. A dot is present at the leftmost point of the grid and it becomes a vertical line when applying a signal. Setting the scope in X-Y mode, I can successfully notice the horizontal amplifier works fine while, applying a couple of signals I can successfully display Lissajous figures.

So I focused on trigger (wrong?) and I can say that the sweep never starts, even in "Auto" mode. Luckily I have a second scope to try to diagnose this one and I followed the trigger signal when channel 1 is connected to the internal signal generator. Focusing on diagram 5, adjusting the trigger level control, I can see a quite good signal at point 39 and 40 (pin 9 and pin 8 of U640) but I see no signal and 0V at points 41 and 42. No signal at pin 1 of U870.

Should I remove the two tunnel diodes CR650 and CR652 I see a huge signal at points 41 and 42 and also at pin 1 of U870. But, anyway, I have no trace and no pin of U870 shows any sign of life.

That said, at page 3-13, description of U870, I read that pin 3, even if no trigger signal is present on pin 1, after 100mS: "pin 3 steps LO to turn Q864 on which initiates a sweep". Pin 3 IS low: 0.2V. It is totally unclear to me if pin 3 has to turn low "every" 100mS to initiate the sweeps or, once it is low, the sweeps should continue forever.

By the way, I also noticed a ripple of 10mV on all the voltage rails. I already replaced some electrolytics due to 50Hz ripple. Now the ripple is at 100Mhz and looks almost triangular: something I didn't expect to see. May be this is the cause of U870 to fail, if it even is failing? How to investigate where this ripple come from?

I think the 465 Service Manual is of public domain (I downloaded it here: https://elektrotanya.com/tektronix_4...wnload.html#dl) but I attach the pages I am referring to.

I want to thank in advance who will be so kind to help!
Attached Files
File Type: pdf MaxRipple.pdf (194.8 KB, 12 views)
File Type: pdf 5-TriggerGen.pdf (706.0 KB, 9 views)
File Type: pdf 8-Sweep.pdf (684.8 KB, 12 views)
File Type: pdf U870.pdf (1.11 MB, 10 views)
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Old 16th Mar 2020, 3:40 pm   #2
ajgriff
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Default Re: Tektronix 465 not sweeping

The horizontal section can be a bit of a challenge when it comes to fault finding. You may already have a copy but the attached document is worth studying.

Alan
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File Type: pdf Tek Troubleshooting Scopes.pdf (3.57 MB, 23 views)
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Old 16th Mar 2020, 4:14 pm   #3
IlResca
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Default Re: Tektronix 465 not sweeping

Thank you Alan: start reading now!
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Old 16th Mar 2020, 8:13 pm   #4
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Default Re: Tektronix 465 not sweeping

Spot at the left edge of the screen is just what the timebase sweep generator should be doing while it waits for a trigger.

'Norm' trigger will leave it waiting forever if no trigger event comes along. With auto trigger selected, there is a timer that initiates a trigger and a sweep after some time has gone by since the last sweep. There is a switch and capacitor group on the timebase A (main timebase) switch to set a delay suitable for the timebase speed.

Select 'norm' and have a play with the trigger controls, turning the trigger level knob up and down to see if you can trigger a sweep. If you can't get sweeps this way, and can't get auto sweeps, then you need to find where the two paths converge and follow it into the sweep generator itself. Something has to act as a flip-flop to hold the I am sweeping/I am not sweeping state.

David
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Old 17th Mar 2020, 9:26 am   #5
IlResca
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Default Re: Tektronix 465 not sweeping

Quote:
Originally Posted by Radio Wrangler View Post
'Norm' trigger will leave it waiting forever if no trigger event comes along. With auto trigger selected, there is a timer that initiates a trigger and a sweep after some time has gone by since the last sweep.
Are you meaning the timer "inside" U870? The book says that, in AUTO mode, it turns low "after" 100mS with no trigger signal. What I asked in my initial question is if this signal has to toggle every 100mS in order to have a continuous sweep or if it is enough it stays low forever to enable continuous sweeping.
Quote:
There is a switch and capacitor group on the timebase A (main timebase) switch to set a delay suitable for the timebase speed.
Can you clarify? This sounds totally obscure to me.
Quote:
Select 'norm' and have a play with the trigger controls, turning the trigger level knob up and down to see if you can trigger a sweep.
Of course, this is what I have done since the beginning: no way to trigger, in NORM mode. That said, playing with the trigger level knob, I see the trigger signal at points 39 and 40 appear, increase to a maximum, decrease and disappear depending on the position of the knob. This looks the correct behaviour.
Quote:
If you can't get sweeps this way, and can't get auto sweeps, then you need to find where the two paths converge and follow it into the sweep generator itself. Something has to act as a flip-flop to hold the I am sweeping/I am not sweeping state.
Yes: this is the reason I am asking here for some help. I have quite good experience in mainframe repairs with a scope but I am lost in this kind of electronics.
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Old 17th Mar 2020, 3:10 pm   #6
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Default Re: Tektronix 465 not sweeping

I don't have the service info handy at the moment and I've only been in my 465Bs a couple of times (I used to design instruments for 'the other brand' including bits of one scope) So I'm not exactly familiar with the circuitry.

Try to identify the latch that holds the sweep running/sweep stopped information. See if it is stuck in stopped (it likely is) then look to see if it gets trigger impulses from the trigger channel or from the auto-run timer.

David
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Old 17th Mar 2020, 6:23 pm   #7
RogerEvans
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Default Re: Tektronix 465 not sweeping

I would like to try and help on this but I am struggling with the online scanned manuals which seem a bit garbled in the schematics of the sweep circuitry. There is a high quality scan available online from Artek manuals in the US (artekmanuals.com), make sure you order the 465 or 465B as appropriate.

You could try asking on TekScopes (groups.io/g/TekScopes), there are several people there with 465s and many old threads on 465 repairs. I have two 454s and a 475, all of which have socketed transistors in the sweep circuitry. All three have at some time required attention to sweep and triggering problems and all three were solved simply by pulling transistors and testing them on one of the cheap Chinese component testers. I also keep a small supply of 2N3904 and 2N3906 so that I can swap in a PNP or NPN as appropriate and this is nearly always good enough to prove that the circuitry then works at moderate sweep speeds.

The bad news is that I had a 7000 series timebase that would not sweep and the fault there was a failed U870 (part no 155-0049-01). They are available but are not cheap.

Best of luck,

Roger
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Old 17th Mar 2020, 8:10 pm   #8
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Arrow Re: Tektronix 465 not sweeping

It's been my experience with repairing Tek. 'scopes in the range 455 → 475 that a lot of patience, perseverance, persistence (the 'Three Ps') and much time is required: there is lots of complicated circuitry to analyse. Yes, you can have the service manual in front of you and spend ages going back and forth though its pages and wandering around the pcbs trying to interpret the cct. descriptions and wandering around said defective 'scope with another 'scope, a DMM and a component tester. I'm sure that there are a few people who are absolute whizzos with the manual and a few instruments & get it fixed relatively quickly. Me? I'm not and never have been that clever. I've always used the "this item looks like it could be the trouble: I'll test it".

It's failure of a transistor (or more) that is often the culprit - with regard to your described fault.
My fault-finding approach would be as follows.

The first place I would look at is the A & B sweep generator / A9 interface board. It features transistors Q1012, 1014, 1002, 1024, 1030, 1036, 1038, and various diodes. I would remove each one in turn and check it in the traditional manner (some of those are FETs). If any are found to be defective, replace with suitable alternatives.

Next, the sweep & Z axis logic / A8 trig. gen. & sweep control. U870 is featured here, along with many transistors & diodes. Although you could remove each one in turn and test it, I would look at Q862 / 864, 854, 1052 / 1054 / 1042 / 1044 first.

Of course, this approach is the 'blindly test (almost everything)' method , but for reasons stated at the start of this post, overall, it has usually caused me less headaches than the alternative & has (usually) restored normal operation.

Best of luck with it: there's a failed component(s) in there somewhere - just a matter of finding it! The 'Three Ps' are your best tools with this generation of Tek. 'scopes.

Al.
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