Tek 468 triggering issue

[Aldobrandi]

Hello everyone.

I just got this wonderful 468 that is in great physical shape, in and out (the boards are gorgeous!), but sadly I believe that it has a triggering issue. I am rather new to electronics and this is my first scope so bear with me.

Simply put, I can't seem to trigger on any signal and adjusting the trigger level does not seem to do anything. Let's pretend for a minute that I want to display a stable cal. signal (300mV, 1kHz square wave). Here are my settings :

Non store mode.

Vertical
- Volts/div: .1V
- Vert mode: Ch1
- Coupling: AC

Horizontal
- Horiz Display: A
- Delay time position: fully counterclockwise
- Time/div: .1ms
-X10 mag: OFF

Trigger
Trig mode: auto
A trigger source: norm
Trigger slope: positive
A trigger holdoff: norm

No mater how I adjust the trigger level, what I get on the screen are two solid traces, shaking ever so slightly, which allows me to guess that there's a waveform somewhere in there. The trigger LED is off.


The really bizarre thing happens when I switch to "norm" trigger mode and fidget with the slope switch: every time I change the trigger slope from + to - and vice versa, the scope triggers for a very brief instant, the trigger LED goes on and immediately turns off, and the ghost of the square wave appears on the screen. There is one way that I can somewhat stabilize the signal: by adjusting the trigger holdoff level until the trace drifts slowly from one side of the screen to the other (it sometimes goes back and forth a few times a minute), at which point I'm able to clearly see a square wave (which by the way seems way out of spec: I estimated at about 1.7kHz).


As far as troubleshooting is concerned, the only thing I was able to do with my minimal test equipment is check the DC voltages, especially the +55V rail, and they all appear to be within spec. I haven't checked the digital supply voltages but I assume this wouldn't have an effect on the analogue end.

I am sorry for this long wall of dry text, with no pictures but I am currently away from the scope.

Thanks in advance for your help!

[ModemHead]

... The really bizarre thing happens when I switch to "norm" trigger mode and fidget with the slope switch: every time I change the trigger slope from + to - and vice versa, the scope triggers for a very brief instant, the trigger LED goes on and immediately turns off, and the ghost of the square wave appears on the screen. ...

This is a very good thing. It means the trigger section may be working, it just isn't getting an input signal.  It may also trigger when moving the level control back and forth, somewhere near mid-rotation.

Flip the A trigger source to 'Line'.  Does the trigger light come on continuously?  If so, more evidence that the trigger section is functional.

There may be a problem with the trigger input selection and coupling, possibly even just dirty or malfunctioning switch contacts.  Exercise the trigger source and coupling switches a bit and look for improvement.  Switch to Channel 2 and see what happens. Set the trigger source to 'Ext' and use a coax tee connector to put a copy of your input signal into the external trigger input and see what happens.

As a side note: for best results when viewing the calibrate signal (especially in the presence of HF noise), engage the 20MHz bandwidth limiter, set the trigger coupling to 'HF reject' and the trigger source to the channel you're using.

[Aldobrandi]

Thanks, ModemHead, for your very encouraging reply!

I have already tried a few things that you mention and here's what I observed (I'll post pictures in a few hours when I get home from work):

Flip the A trigger source to 'Line'.  Does the trigger light come on continuously?  If so, more evidence that the trigger section is functional.

The trigger light doesn't come on but a straight trace comes one at the center of the screen. Here again adjusting the level doesn't do anything.

Exercise the trigger source and coupling switches a bit and look for improvement.

I'll do some more of this tonight. Would you recommend using Deoxit if I do not see any improvement? Also is Deoxit 100 alright (it's the kind formulated without solvent)?

Switch to Channel 2 and see what happens.

Here again I'm pretty sure channel 2 exhibits a similar behavior (I am writing this from memory, but I'll make sure to check again tonight).

Set the trigger source to 'Ext' and use a coax tee connector to put a copy of your input signal into the external trigger input and see what happens.

Someone on the TekScopes group recommended this as well. I used my second oscilloscope probe to couple the cal. signal into the ext. input. but alas to no avail: I still witness my sad untriggered sig. One detail that might give you some more information is that in this situation, if I push the "beam find" button a straight solid trace appears below my signal, hinting at least at the presence of something, just not on the screen. Sadly here again moving the trigger pot doesn't do anything.

Thanks so much for your help. Although it's probably going to give me some troubles I'm already in love with this scope. Looking for test points is a true pleasure: man did the tek engineers did a beautiful work at laying out this puppy properly for servicing!

Aldo.

[ModemHead]

If you're already on TekScopes, you're getting the best Tek trouble-shooting help available on the net (no offense intended to anyone at EEVBlog.)  And so I'm guessing you already have a good copy of the service manual for this scope, essential!

The trigger light doesn't come on but a straight trace comes one at the center of the screen. Here again adjusting the level doesn't do anything.

Wouldn't hurt to check the level control pot.  Might be open/short?  I think the B-sweep trigger level is the same pot and can be swapped.  In fact, the B-sweep section has a lot of duplicated components that can be used for trouble-shooting the A-sweep.

Would you recommend using Deoxit if I do not see any improvement? Also is Deoxit 100 alright (it's the kind formulated without solvent)?

Oh my, that question can trigger a religious debate. All I can say is that I have seen situations where I get better results using Deoxit than just plain IPA.  I use the diluted spray version, but I do not spray it willy-nilly everywhere.  I use cotton buds, paper strips and so forth to limit application to just where it's needed.  (I suppose I should be using the non-solvent version.)

From what you've reported, another working scope may be necessary for trouble-shooting.  Set the bad scope to Line triggering, find the place where the mains sample signal comes in to the trigger section and follow it forward.  With line triggering, your signals will be nice and slow and will not depend on the vertical inputs.  The schematic usually has some DC voltages noted at various key points, check them. Be aware that some of those voltage readings can depend on front-panel settings.

Although it's probably going to give me some troubles I'm already in love with this scope. Looking for test points is a true pleasure: man did the tek engineers did a beautiful work at laying out this puppy properly for servicing!

I know what you mean. I have 3 Tek scopes and the oldest model, a 465, is actually my favorite.  I have poked around in it quite a bit during it's recovery from the scrap heap.

Stick with it, I think you will find the source of this problem!

[Aldobrandi]

Wouldn't hurt to check the level control pot.  Might be open/short?

That'll be the first thing I do tonight. I'll clean it with IPA and use a very small amount of Deoxit for good measure. Do you recommend I take the pot apart if first cleaning is not effective?

From what you've reported, another working scope may be necessary for trouble-shooting.

It may be a couple months before I save enough money to afford something, but my next scope will be a new one. I will then proceed to checking voltages across the boards. I indeed have the service manual and have perused it (although I haven't gone religiously through it, cover to cover, quite yet!) and there's almost too much information for someone with my level of knowledge. I am currently focusing on the theory of operation chapters as well as the troubleshooting charts, which are glorious!

I have poked around in it quite a bit during it's recovery from the scrap heap.

Wow thanks for reminding me of this fantastic journey of yours, this is going to be a gold mine of troubleshooting and repairing tips for me. I'm going to go over it in details and learn from it. Thanks again for making this available!

I have already learned so much in just a couple of days, and feel like by the time I'll be done I'll have a solid understanding of some aspects of analogue scopes. Buying a broken scope is one of the best decisions I've made in my journey to learn electronics... now if only I could find some money to buy a functional one

Cheers!
Aldo.

[ModemHead]

Do you recommend I take the pot apart if first cleaning is not effective?

I would never recommend taking a pot apart unless an ohm-meter check indicated the pot was completely non-functional, in which case you've got nothing to lose.

I am currently focusing on the theory of operation chapters as well as the troubleshooting charts, which are glorious!

Good. It has been my experience if that if you can use all available evidence to narrow a problem down to a particular functional block, you can begin to focus on trying to understand the circuitry one small section at a time. You can often draw imaginary borders around fairly small groups of components that are meant to do a specific task.  The trick is to find the place where everything coming 'in' is good, and what comes 'out' is wrong.  And that process is usually a very good learning experience.

[PA4TIM]

Nice sope, but before you start to mess things up:

Put the sourche knob, right from the screen on 1. ( it s hard to read on my mall sscreen but it looks like it is on the delay position. There probably is line and there must be channel 1 and 2. Your probe is channel 1 so the trigger source must be channel 1 too. I think it is labeled  "on 1"

Trigger is a circuit that you can see as a sort of level switch. If a signal goes over the level you choose with the level knob and slope knob the "switch" goes on.( the voltage triggers it only if it goes up in positive slope or on the negative slope position, when goes down )

When the trigger  "fires" it starts the sawtooth that makes your horizontal trace. If you do not have autotrigger you will not have a trace. In auto it fires allways and for repetive signals this most times works. But if you tell the scope to measure the triggerlevel on channel 2 but the probe is on channel 1 it never sees the level and does not fire. Flipping the switch like you did causes a new sweep  so you see the trace or a moment.

There is a delay function, this is to complicated to tell in a few words, see manual.
Then channel 1 or 2
Line, here it does nt look at the signal but at the mains line supply. If you look at signals in sync with powerlines this is helpfull, but I never used it, think it used to be populair in tv repair
External, very handy. A separate trigger channel. Many generators have a trigger out but you can also use it to for instance to hook a probe to a gate and channel one to a drain or source so the signal s synced to the gate, or hook it to a clock signal so the digital signals you are looking at are synced with the clock.