Tek 7603 with strange focus behaviour

[david77]

I've had this 7603 sitting here for a while but only last week got some plugins for it. I did some work in the PSU upon receiving it, the original caps weren't too good so I replaced them all.
Now I've got the 7B53 and a 7A18 I dicovered this strange focusing problem.
It looks like something in the scope is oscillating wildly, the traces seem to throw a sort of shadow. It's not a static shadow, it rather looks like a high frequency oscillation that changes position depending on the vertical position of the trace.
I've attached three pics to make it more clear what I mean.

The trace it self is only in focus around the middle of the screen, if you move it up or down it becomes a bit fuzzy. Once the scope has warmed up a bit it's less severe, so I suspect a temperature related problem somewhere. Haven't been able to locate it with freezer spray, though.
That "shadow" is there all the time.

Does anybody have a clue what could cause this?

[bktemp]

Looks like a high frequency superimposed on the sine wave or more precisely on the deflection voltage.
If the scope uses a SMPS, check the caps. If not maybe some amplifier is oscillating.

[Grapsus]

7603 is a modular scope right ? You should have at least two vertical amplifier plugins, do they both show the defect ? If yes, the problem must be in the chassis, otherwise swapping the two vertical plugins can help to isolate the problem further.

**Edit:** Sorry just saw that you have only one vertical plugin. Does the problem happen on both channels ? Try to move the plugin between the two possible slots in the chassis.

[tautech]

Forgive me for being dismissive, but you give no details of the sine wave source. Just FG is not enough.
How do you have the signal source connected to the scope? Properly terminated BNC we hope.
And
You state you replaced some caps in the PSU, why? Leaking, Looked bad or high PSU ripple?
Is the PSU entirely within Service manual spec?

You see there are many things that might affect displayed waveform, including a fault within the scope.

[david77]

The problem appears on both plugin slots. I've got another vertical plugin on the way.

I had to replace the PSU caps because they showed low capacitance and high ESR values. Replaced them with brand new caps.
The FG is not the source of this oscillation, it shows a perfectly good sinewave on another scope, and yes it's terminated into 50 ohms. In fact the anomaly shows up with no signal source attached as well. It's also present with just the horizontal trace visible.

And yes, the PSU is in spec. I checked all that.

[tautech]

Does the Cal squarewave show a similar display?
Can you see this oscillation on the vertical amp outputs?

Do you have the Tek troubleshooting guide?

[david77]

Yes, it's always there. Just checked with the built in square wave.
There's some very weird AC voltage happening on the vertical amp outputs, yes.
Looks like a square wave fed through a cap, spikes going up slowly falling off and then spikes going down and slowly rising to the centre line. There's also this oscillation superimposed on that.
Sorry, can't post a  picture right now, camera's dead.

Yes, I do have that booklet.

edit: That square wavey stuff was a red herring. The oscillation is clearly there, though

[david77]

Oh well. It seems I have to change my previous statements a bit.

The error only exists in the right vertical plugin bay. If I switch the vertical amp into the left bay it works perfectly.

[Grapsus]

That's good news, it means that the problem is in the chassis and since you have one slot functioning, you can compare signals in both slots by following the signal path until you find the stage where the noise is injected. It would be easier with two identical vertical plugins plugged at the same time, but you can get away with only one by placing it in one slot, measuring, then moving the plugin to the other slot, measuring again etc. That's how I'd do it.

[tautech]

That's good news, it means that the problem is in the chassis and since you have one slot functioning, you can compare signals in both slots by following the signal path until you find the stage where the noise is injected. It would be easier with two identical vertical plugins plugged at the same time, but you can get away with only one by placing it in one slot, measuring, then moving the plugin to the other slot, measuring again etc. That's how I'd do it.

+1 

You might like to check any e caps that are specific to that slot also

[david77]

There's nothing specific to the slots. The output of both goes directly into a switching IC and from there to the vertical amp.

The logic control in this scope seems to  be pretty much toast. The line that should control wich vertical plugin is selected doesn't do what it's supposed to do. Also there's a clock generator that generates nothing, of course it's made with a special Tek IC 155-0011-00. I think I will have to go and find one of those.
I'm not quite sure how or if it is related to my present problem but it sure looks like it.

[tautech]

Hmmm, dirty slot socket contacts ?
Never overlook the simple stuff.

[david77]

Nope. Cleaned the contacts already with a piece of cardboard soaked in contact cleaner, doesn't change a thing.

I think I'm really onto something with that digital board. Need to re-read the manual and understand what this digital stuff is supposed to do.

[Grapsus]

It's confusing, you say that the switching logic isn't working, but both of the slots display your sine wave on the screen right ? even if one of them has the noise added. Have you tried comparing all the lines at the connector for each slot with the vertical module plugged in ? In addition to the signals, the connectors must have power lines ?

[david77]

It is confusing, yeah.
I pulled the suspect IC out now and guess what - both vertical bays work fine now.

Before I pulled the IC I did some measurements. It turned out the broken chip prevented the control line that tells the scope which vertikal plug in to select to reach the right logic state. It was kept somewhere between HI and LO.
I suspect that prevented the vertical switch IC to fully route through the right vertical bay, resulting in crap on the display.

The main thing that broken IC does is control the chop function and vertikal screen blanking. So now a chopped display isn't possible and maybe there are also problems in ALT mode, haven't checked that out yet.

Now I need to find a new Tek IC with the PN 155-0011-00 to replace that broken one. Maybe I'm lucky and it works fully then.

[tautech]

Seems like Clock, chop blanking 155 0011 00 is a Tek only IC.
Sphere have them SH. http://www.sphere.bc.ca/test/tek-parts/tekparts3.html

While searching for them I came across this cross reference pdf.
http://bee.mif.pg.gda.pl/ciasteczkowypotwor/Tek/publikacje/Common_Design_Parts_Catalog_Semiconductors.pdf

[miguelvp]

http://www.markhannahsurplus.com/tekparts.htm

has them too

TEK  #               Description                                   Qty     Price each
155-0011-00     CLOCK AND CHOP BLANKING      15      $5.0000

I never bought anything from them but I had that link for a while in case I need some parts for my Tek equipment.

[david77]

I already ordered one from the US that I found on ebay.

[MadTux]

I've also a 7603 parts unit here, if you still need some chips (CRT HV board and some readout chips are already gone, CRT is broken)

[david77]

The new IC arrived today, the chop mode on the scope is now kind of working.
The clock signal is there and the chop mode works but it's only producing badly distorted traces.

I think I've already found out why the traces appear distorted on the screen but not what causes this to happen. It's not a focus issue as it first appeared to be.
It is a bit hard to describe what I'm seeing here. I guess I will have to make a video to explain it, maybe on Sunday.

[dom0]

7603 parts unit ... CRT is broken

That seems to happen very often to 7603?

[david77]

The problem seems to be how they mounted the CRT. It's only fixed to the chassis with a clamp round the neck.