Old Tek Oscilloscope With Bad Trigger a Boat Anchor?

[Justa]

I have the opportunity to pick up an old Tektronix vacuum tube scope in with a malfunctioning trigger if I drive 80 miles to pick for free.  It has been years since I used a scope and I know very little about them.  I believe this scope triggers at different points on the waveform so it is impossible to freeze the waveform.  Right now I really can’t afford even a used scope but would love to have one to work on tube amps and suspect this would be better than no scope at all.  Is this worth the drive or is it just a boat anchor?

[sluckey]

Which model? There's a good chance that the trigger problem is just a tube. You may not be able to find tubes for it though.

[Justa]

The owner didn't know the model number offhand and doesn't like to be bugged so I am apprehensive to bother him about it since it sounds like he is just going to give it to me.  He is a sharp Electrical Engineer that grew up building tube equipment so I am guessing I will not find an easy or cheap fix.  If the trigger problem is not worth fixing on this scope will it be better than not having a scope at all?

[tubesornothing]

I am a huge fan of scopes, I use them in a traditional sense, with a sign gen, but also in some not so common ways.

About 30% of my scope use for amps, is just connecting up the scope to various locations and watching it while I play guitar through it.  Not only is it mesmerizing, it helps me see little oscillations and spikes.  This I find useful to help fix real small problems.  It does not require a trigger circuit.

However...  a scope with a working trigger and a sig gen make it pretty easy for other tasks of checking gain, fix crossover distortion, fix a reproducible osc or spike.

I say go and pick up the scope, but also do a few other errands on the way so the trip was not a waste.  There are lots of guys on the net who like to fix those things, so you might get lucky.  If not, it looks cool on the bench and you can still use it a bit with your guitar - and learn some neat stuff.

Ton

[sluckey]

If the trigger problem can't be fixed, the scope is almost useless. There are several different trigger problems. Basically, the horizontal sweep (trace) on the scope is generated from a free running sawtooth generator. If the sweep is allowed to free run, then any waveform you try to put into the vertical input will appear non synchronous. It may slip slowly across the screen or it may fly across the screen too fast to even recognize it.

The trigger circuit allows you to trigger (synchronize) the horizontal sweep such that the waveform you want to see appears stable and in the same place on screen each time the horizontal sweep is fired. Most of the old tek scopes had several choices of trigger input signals. With source set to INTERNAL, a sample of the vertical input waveform is used to generate the trigger for the horizontal sweep, so the display will be stable. EXTERNAL trigger source is useful whenever you want a stable display that doesn't rely on the vertical waveform. And LINE trigger source uses a sample of the AC line voltage as a trigger, useful when looking at power supply ripple.

What's my point? Oh yeah. The point is there are several ways the trigger circuit can fail. Maybe internal works but external doesn't. Maybe it's the other way around. Maybe nothing can trigger the sweep and the sweep is always free running. Maybe there's not even a visible sweep. Shotgun troubleshooting is not recommended. You'll need a schematic (probably available), and an understanding of basic scope circuits. And remember, suspect tubes first. Hopefully you can find them.

The only way to figure out what the symptoms are is to get it. If the guy said the scope is a single channel scope with a single timebase, I might just decline. If I had to figure my time was worth even $25.00/hour, I'd definitely decline. If the scope was an old classic 545, I'd be on the way already.   :)

[jjasilli]

My 2 cents:  Unless you want a whole new hobby in scope maintenance and constant re-calibration, there is no point to acquiring a vacuum tube oscilloscope.  Also, unless you enjoy repairing or refurbishing test equipment, there is no point in acquiring malfunctioning test equipment of any type. 

[Justa]

Thanks guys,
Sounds like it might indeed may be a boat anchor.  But then again I could get lucky but reality has always kicked my butt.  It would definitely look cool around those glowing tubes though.

[sluckey]

Oh go on. Make the drive.   ;)

[Justa]

Actually the coolness factor and just getting my hands on it really turns me on.  Now I will need to find an excuse, ehh he does go to a Church up there.  Maybe I will make the drive Sunday and be saved from my uncontrollable cravings and desires without having to use again, the checkbook of course!

[bnchwrmr]

You might JUST get lucky with the tube hunting, too.
Antique Electronic Supply claims to have a very
huge selection of NOS tubes...check 'em out. They
might have what you need.

And don't forget about possible faulty or dead
capacitors for the passing or storage of the trigger
signal. You need a scope to fix a scope sometimes,
so you might ask around......!
Tektronix scopes have a rep for being rugged,
dependable, and highly sought after, and you might
be able to sell it on eBay.....if all else fails.
Ask for the manual(s), if available. Copies might be had
fron a few sites for free or for $$ to $$$$.

All the Best
bnchwrmr?

[tubesornothing]

If looking for strange tubes, also check out John at Pacific TV:  http://members.shaw.ca/pacifictv/

[Justa]

Thanks, I appreciate the tips on locating tubes and the potential cap issues.  Something I always wanted to ask you guys about is using a DVM to look for bad caps with DC leakage.  I have a Fluke 189 that will display both DC and AC signal levels similtaneously and when I rebuilt my TR I had already replaced all the caps but I noticed that I could see the DC leakage levels on signal caps in circuit.  I just never saw a post of anyone using a DVM to do this but is this a legit method to find leaky caps easily without having to remove them from the circuit?  I could buzz through a lot of caps quickly this way and I bet there are a tons of caps on a scope.

[sluckey]

I noticed that I could see the DC leakage levels on signal caps in circuit.  I just never saw a post of anyone using a DVM to do this but is this a legit method to find leaky caps easily without having to remove them from the circuit?

Well you saw a dc level at some point in a circuit, but how can you say it was due to a leaky cap? You first use the dvm to check voltages in a circuit and when you find a suspicious voltage, you have to investigate to find the cause. If you suspect a leaking cap, you have to disconnect the side of the cap that connects to the point in the circuit that has the suspicious voltage. Did the suspicious voltage go away? If so, the cap is suspect. Now connect your dvm lead to the flying end of the cap you disconnected. If you have voltage, the cap is leaky.

I'm not aware of a surefire, foolproof method to determine a cap is leaky without disconnecting one end. By disconnecting one end, you eliminate all the other variables in the circuit.

[Justa]

Weird, I must have had one end of the caps disconnected somewhere as I remember seeing a high VDC on one side of a cap and just a few tenths VDC of DC sitting on the other end of the cap and thinking that I saw a normal level of DC leakage on a cap.  After looking at a schematic as I do not see where it this would have been possible to have had it in circuit.
Thanks Sluckey

[tubesornothing]

... I remember seeing a high VDC on one side of a cap and just a few tenths VDC of DC sitting on the other end of the cap ...

That sounds normal to me for a coupling cap - thats their job.