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Old 4th Jun 2016, 7:48 pm   #1
dave cox
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Default Tektronix 7633 storage scope repair

I recently picked up a non-working 7633 storage scope locally as I figured it would be useful to have some spares, especially as it was languishing at £20 with no bids. As usual, not really needing something (nor having anywhere to put it), pretty much guarantees bidding success!

The story was that the scope had recently stopped working (without drama) but the fan was operating as normal. It looked in pretty decent nick so a repair was on the cards. A quick inspection revealed not much in the way of voltage coming from the power supply. I thought I got a whiff of burning when it was on power so I quickly pulled the power and had a good sniff all round it taking care not to get my nose too close the HT wiring Finding nothing I applied power a second time, this time I saw the wisp of smoke coming from the H-amp / V-amp area The charred resistor gave it away. Right next door to that resistor were 2 tantalum drop capacitors ...
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Old 4th Jun 2016, 7:53 pm   #2
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Default Re: Tektronix 7633 storage scope repair

A quick in circuit ESR measurement indicated a short so I popped all the connectors, the 3 screws and the 2 wires connecting to the tube ... No, wait a minute it's not so simple. The drive circuit is different and has to be un-soldered
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Old 4th Jun 2016, 7:57 pm   #3
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Default Re: Tektronix 7633 storage scope repair

Anyway, both tantalum caps were shorted but the resistor was still bang on spec so I left it. I've seen the linear power supplies fitted to these scopes give trouble where thermal cycling can crack the solder joints on the power transistors to I re-soldered those as well ...
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Old 4th Jun 2016, 7:59 pm   #4
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Default Re: Tektronix 7633 storage scope repair

So I now have a (another) pretty reasonable scope !
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Old 4th Jun 2016, 8:06 pm   #5
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Default Re: Tektronix 7633 storage scope repair

Quote:
Originally Posted by dave cox View Post
Anyway, both tantalum caps were shorted but the resistor was still bang on spec so I left it. I've seen the linear power supplies fitted to these scopes give trouble where thermal cycling can crack the solder joints on the power transistors to I re-soldered those as well ...
Congrats on getting the 'scope going

....however, I would definitely replace the resistor. It has been operating well outside of its rated tolerance and its long term reliability may well be impaired.
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Old 4th Jun 2016, 8:08 pm   #6
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Default Re: Tektronix 7633 storage scope repair

The storage mode is 'working', AFAIK, but I don't think the tube is in fantastic nick, see the lack of brightness on the left side of the full screen raster although the reduced scan raster looks good

I like to see what these are capable of, with the right plugin. I think the 10uV P-P square wave @ 1Hz is pretty fancy !! Can my rigol storage scope do that ?

dc

PS:
There is also a splash of electrons in the lower right corner of the display in storage mode, you can see it is the 1st attachment. To be investigated ...
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Last edited by dave cox; 4th Jun 2016 at 8:34 pm.
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Old 5th Jun 2016, 4:23 am   #7
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Default Re: Tektronix 7633 storage scope repair

There are some adjustments for the storage mode which might be worth trying.

You must have a 7A22 in there, a very nice plug in and the most sensitive made (along with the 5A22) that I know of. My Rigol certainly won't do this.

I got a 7623A last year and at first switch on the storage mode was not that great but after some running it's fine now.

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Old 5th Jun 2016, 9:45 am   #8
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Default Re: Tektronix 7633 storage scope repair

I agree with Tony. Go through the adjustment procedure for the storage section in the manual. There are quite a few pots to be adjusted.

I don't have the 7633, but I do have a 7623A, two 7603s, one 7603R and two 7704s.

These are fine 'scopes. I've just been given a 7A22, very useful indeed.

I wonder how many of these mainframes have been skipped becuase of failed tantalums. One of the 7704s that I have came with the normal ticking power supply problem, caused by the same thing. The best thing to do is change them all, but as you have discovered, some of the boards are not that easy to get to on the three compartment mainframes.

The only other major problem with these units is as you have discovered, they are just so huge!

Mark.
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Old 5th Jun 2016, 4:22 pm   #9
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Default Re: Tektronix 7633 storage scope repair

Yes, agreement on the tantalum bead replacement. Tek 7000 series 'scopes are indeed works of art, but they DO NOT like to be switched off for long or stored in damp conditions. I make a point of treating them like old valve gear and bringing the up SLOWLY on a variac before plugging them in to the mains. I have a 7603 which worked perfectly when the seller took pics of it before selling it to me. It lay under my bench until I noticed it and ignoring my own rules, plugged it straight in to the bench, as it was working when I bought it. A puff of smoke and a blown fuse heralded the destruction of one of the PSU bridge rectifiers - all because of shorted tants further up the line!
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Old 8th Jun 2016, 11:20 am   #10
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Default Re: Tektronix 7633 storage scope repair

I did actually replace the 2 dipped tantalum caps with new dipped tantalum caps (although I did up the voltage rating from 25V to 35V). One of them had had a small RF choke in series and the other 16R5 to a regulated 15V rail. Quite how they had BOTH failed short circuit I can't even guess !

I did a quick comparison of the ESR of some good quality Al caps (inc rubicon / nichicon etc) against tantalum caps and there was much more than an order of magnitude difference (even at just at a few KHz). I think MLCC would work well for that particular scenario but I would need to work out the effect of bias voltage on capacitance, order the right ones then 'bodge' them into place. So I took the pragmatic route

Thanks for the advice Tony and Mark. I will work through the storage adjustments in the near future

dc
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Old 9th Jun 2016, 7:11 am   #11
tony brady
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Default Re: Tektronix 7633 storage scope repair

good luck with it Dave.

There are quite a few transistors on the Storage System ( diagram 10 in the manual) On more that one occasion I've found the transistors which are in socket pins loose in the 7000 series so it's well worth pushing them firmly home ( before applying power obviously!)

for what it's worth I think your tube is ok, it's better than mine in non store mode so unless it's been dropped I think you will be lucky
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Old 9th Jun 2016, 9:31 am   #12
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Default Re: Tektronix 7633 storage scope repair

My general advice for storage scopes is to not use it unless you really are using the storage. Use an ordinary scope whenever you can. Avoid using a storage scope in 'conventional' mode because you can't see when you're overdoing it and may be damaging the mesh. In storage mode you get a very visible bloom.

Get it really wrong and burn from the beam will eventually cut the mesh if a too bright trace is left running. In 'Conv" there may be no visible clue that you're overdoing it.

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Old 9th Jun 2016, 11:05 am   #13
dave cox
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Default Re: Tektronix 7633 storage scope repair

I pretty much only use the 7633 in a bi-stable mode set up to so that it looks a bit like it has a 5 or 10 second phosphor. This is pretty handy, for ex looking at a small audio signal riding on a drifting DC level when you actually want to see the behavior of the DC level as well. The tek differential amps are good for this but they do drift like crazy as they warm up or you waft air too close to them For the 10uV square wave image posted previously, it took about 25 minutes before you couldn't see any baseline drift

However, I do use a 7934 storage scope almost exclusively in 'normal mode' because I don't have anything else with nearly so much bandwidth. I keep meaning to sell it and get something more suitable to my needs ...

dc
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Old 10th Jun 2016, 9:01 am   #14
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Default Re: Tektronix 7633 storage scope repair

Dave:

I have a 7633 and have also restored a second one plus a couple of 7623A's. I've found that the DC voltages on the storage tubes are consistent, even between types. I can send you measurements if you're interested.

John

PS just moved a bit closer to you in Somerset!
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Old 10th Jun 2016, 5:11 pm   #15
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Default Re: Tektronix 7633 storage scope repair

Yes please John, some actual measurements would be great! The tektronix service manuals are good but the storage setup looks more contorted that typical setup procedures

Wincanton looks really very nice and should be a great place to live. Luckily the nearby A303 bypasses the town, it can be a bit of a nightmare in the school hols !!

dc
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Old 10th Jun 2016, 10:35 pm   #16
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Default Re: Tektronix 7633 storage scope repair

Dave:

Non-store mode, starting at the top (schematic 10).
Front mesh -7V
Fast mesh +126V
Collector mesh +153V
CE3 +68V
CE2 +47V
CE1 +71V

FGA / FGK approx +38V
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Old 14th Jun 2016, 10:16 am   #17
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Default Re: Tektronix 7633 storage scope repair

Thanks for that John. All the voltages check out so I won't make any adjustments.

The weird 3rd attachment got me thinking after I looked at a sketch of how the flood gun(s) operated. The sketch showed 2 flood guns on opposite sides of the tube, way down past where the tube is widening. If the flood guns were only partially working would that give an image like that in attachment 3, i.e illuminated from only the left side gun ? Better take another look at the schematics but I am thinking I have a damaged tube ...

dc
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Old 14th Jun 2016, 2:32 pm   #18
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Default Re: Tektronix 7633 storage scope repair

Dave:

I think you're right: have you read the Tekscope article? (see w140.com/tekwiki/images/f/f9/Tekscope_1972_V4_N4_Jul_1972.pdf )

Is the scope trace OK when used in non-store mode?

John
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