tektronix D755 CRT problem?

Telequipment was taken over by Tektronix about that time, maybe this repair tip is useful Telequipment D63 scope.

3 faults 1 High voltage discharge crackling noise. Even in the dark it was not possible to see where the discharge was so probably in the step-up transformer or the quintupler. The display was too bright and post deflection accelerator voltage was too high so much reduced trace size in X and Y sense.The CRT 2cnd electrode should have been

-1400V but was -2200V so EHT probably 19KV rather than 12KV. The EHT oscillator transistor had been replaced by a BD131. Changing the biasing pot to the preliminary drive transistor made no effect. The primary of the oscillator o/p transformer was 56V pk-pk not the correct 38V. Although the DC voltage varied from 60V to

100V altering this pot. One of the DC divider chain of 3x 5.1M was o/c so no -ve feedback component to the oscillator. 2 Traces in X sense squashed up to left of screen. One of the SPS5286 final X plate drivers was o/c replaced with BF338. 3 The third trace (Right hand PI) was static bright line with no Y displacement. A trace to 330 ohm,1.5W dropper from 105V to one of the Y2 driver transistors was loose and touching another trace. With this 330 removed and no 105V then no 3rd trace, only appearing on beam finding activation.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

That's because you're posting through Google, where the spam comes from. Get a real NNTP news serve and you will see very little.

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You can\'t have a sense of humor, if you have no sense!

On the 'no signal' trace distortion, check the position of the trace vertical position pot for centre of rotation and see if the trace is in approximately the centre of the screen [vertically]. It is possible that the DC balance of the vertical amplifier is way off centre due to misadjustment or a component failure in the vertical amplifier or failure of one of the V+ or V- supply lines into the amp. I have seen what I think you are describing if the trace is shifted off screen due to a large DC component on the measured signal and trying to bring it on screen with the vertical position control. The Telequipment 'scopes of that period [I have a D83] have very poor resistors. I eventually changed out all resistors in the power supply and several in the trigger circuits. The high value ones [10meg] in the feedback divider mostly went open circuit causing the EHT to skyrocket, and the metal film ones in the trigger circuits became very temperature sensitive and would drift like crazy. Otherwise a nice

60MHz 'scope. Still going strong.

Neil S.

ok, I did it and you're right (fortunately the old "tin" newsreader still exists). However is a bit a p.i.t.a. to remember (or find out) what news server to use whenever I change ISP (I travel lot).

Best regards and sorry for the OT.

Frank IZ8DWF

Well, yes, it at least gives me an idea on the SPS5286 substitution. I'm still very curious to understand if the fault I'm observing can be caused by an electronic problem or if it's definitely a bad CRT, that changes very much the time I'm going to spend fixing this scope. Thanks and best regards.

Frank IZ8DWF

the vertical position control works as expected. The track is only distorted in the vicinity of the right side and when it is positioned in the upper half of the screen.

Yes, this had 4 open resistors in the EHT board, one 10M, two 4M7 and one 3M9. So this scope probably suffered for a long time from too high EHT, I don't know wether this can damage the CRT and result in the very localized track distortion I'm observing. After replacing the EHT resistors and reconnecting everything it developed a short in the 105V supply caused by two transistors in the Z grid controller going short circuit. This fact still puzzles me, but I measured the voltages on the two dead transistor's sockets and they look very normal. I checked already all resistors in the power supply board, in the EHT board and in the Z driver board. I'm in travel until the end of the week, so I won't be able to replace the two blown transistors before next saturday and see what happens. Thanks for all suggestions.

Frank IZ8DWF

No problem. The more people we can get off of Google Groups the better. Then you can use Nfilter to drop anything posted from Google, and not see enough spam to worry about. :)

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You can\'t have a sense of humor, if you have no sense!

-snip-

The effect is independent of the sweep rate? I would expect a hard mechanical hit has loosen or bended a deflection plate, that often shows such effects and they are then sweeptime independent. If it depends on sweep speed there is a very little chance for ripple on the supply to show some weird screens but that's normaly not position sensitive.

Jorgen dj0ud

Jorgen Lund-Nielsen wrote in news:h8aj2v$fvr$ snipped-for-privacy@it-news01.desy.de:

could be a geometry problem with the CRT; input a square wave signal,one cycle per division,turn up intensity to see the (fast)rising edges,and check for distortion. If your tube is bad,this will show it. If so,the internal mesh lens(final gun element) is probably boogered. you may even see or hear tiny bits of glass rattling inside,from a cracked E-gun rod.Those usually show up on the phosphor as dark or bright spots,though.

One very rare possibility;look for tiny magnets(beam centering magnets) that have come loose and migrated elsewhere on the tube or MU- shield.they're about 2mm in diameter and length.

CRTs typically have some trace bowing,should be less than a minor division (0.2div) at top and bottom of graticule.

This assumes all power supplies are at proper V and within ripple specs.

--
Jim Yanik
jyanik
at
kua.net

yes, totally independent of sweep rate. Track bending is always at the same position and of the same entity.

That's also my guess, but as I said, I'm not at all an expert of scopes and CRT. Maybe not a deflection plate as for more than half screen the track looks absolutely perfect? Could it be a problem caused by too high EHT for a long time?

Thanks Frank IZ8DWF

[snip]

I'm going to do this test on saturday when back home.

There's a totally dark spot in the lower half of the CRT, not big, probably it's the size of the electron beam when focused in a single dot. Infacts I thought it's due to a burning caused for just leaving a bright dot for a too long time on the CRT (just guessing of course). I haven't found any other spot with noticeable brightness difference.

I had thought about magnets too, but couldn't find any around. I'm not sure about what a MU-shield is, but the whole neck of the tube is inside a metallic tube that goes from the socket to the point where the tube starts to enlarge. Of course it has openings for the X and Y plates connections.

power supplies are at proper V, I didn't check for ripples (and I will do anyway) but I'd assume that deflection artifacts due to supply ripple wouldn't appear only on a spot less than a quarter of the whole CRT area.

Thanks Frank IZ8DWF

frank wrote in news:4aa90d39$0$697$ snipped-for-privacy@news.tiscali.it:

Not trace geometry problems. too much beam current can burn the phosphor and create a dark spot or line. and raising the EHT voltage shrinks the display size and then your gain adjusts don't have enough range.

Deflection plates are pretty hard to bend.It's more likely that the glass rods that hold the plates in position are broken or cracked,or a mesh expansion lens has had it's screen distorted or torn.I don't know if your CRT has the mesh lens,though.(they result in a less focused or more fuzzy beam spot.)

--
Jim Yanik
jyanik
at
kua.net

I've replaced the two dead transistors in the Z driver board, now the intensity control works. I replaced the unknown FRB749/SPS5286 with a BF393 (probably overkill, but I hadn't anything else with the same E-B-C configuration). The focus control is however dependent on what's displayed on the CRT, I have to tweak it when passing from a straight line to a 4 or more divisions sine wave. I have made two screenshots of the CRT distortion, one 20 MHz sinewave and one 50 MHz sinewave both occupying most of the CRT area. Any way to make them public easy? If someone is curious and wants to have a look at them, send me an email to iz8dwf at amsat dot org.

Best regards

Frank IZ8DWF

the vertical edges are straigth on the "good" part of the CRT area, they just aren't focused, look a bit broader than the horizontal lines. Distortion is visible on the "bad" part of the CRT, but that's with any kind of signal.

Regards. Frank IZ8DWF

Sounds like your EHT is dodgy. I'm sorry I can't be any more help than that.

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