the 'best' tektronics 7000 series plugin scope

[sigmatechnica]

Hi all,
I have decided i require a 4 bay tektronix 7000 series scope.  There is much information on  the 7k series, both 3 and 4 bay on the web, but not much in the way of actual joined up comparisons, on either the mainframes or plugins.  The situation is not helped by the apparent high pace of development at tektronix, with some early models lasting little more than a year and some many years. 
   to compound matters these things are all getting on a bit now and finding interesting ways to fail, with some models being more troublesome to repair than others

So which one is ‘best’?   
Some criteria:
*CRT crispness.  Tek had by this stage started using scan expansion meshes which resulted in a fatter blurrier trace.  stroage tubes had additional meshes, worsening the situation further.  CRTs developed too though and some are worse than others over the range

*Reliability.  PSUs seem to be the worst culprit here, particularly the early switchmodes.  Some models, particularly the higher speed ones use many custom hybrids what may just not be replacable

*Physical robustness.  designs varied considerably, any winners?

*Performance.  Bandwidth is an obvious factor here though not so important  for me. are there any other distinguishing features?  the 7844 was true dual beam (difficult to calibrate apparently) and the 7854 had some digital crazyness,  even in the same model there deems to be some variations (there are 7704As with and without a selector switch for calibration to the right of the screen.

So, there must be a few happy 7k owners on here, what are your opinions?

[David Hess]

There is lots of information available on this subject in the tekscopes@groups.io archives and you may want to ask over there.

*CRT crispness.  Tek had by this stage started using scan expansion meshes which resulted in a fatter blurrier trace.  stroage tubes had additional meshes, worsening the situation further.  CRTs developed too though and some are worse than others over the range

The mainframes with a 24 kilovolt acceleration voltage are all very crisp and bright.  This includes all of the non-storage 4-bay mainframes starting with the 7704A so 7704A, 7844, 7854, 7904, and 7904A.

The 7844 dual beam and 7854 digitizing are more complex.  The 7904A replaced the 7704A and 7904 and uses the same advanced construction as the 7934 storage and 7104 1GHz mainframes.

The 3-bay 7603 is worth considering because of its larger screen, popularity, and easier maintenance with a linear power supply.

I think the most "general" would be one of the 3-bay 7623 or 7633 storage mainframes because variable persistence is very useful with some plug-ins but of course these sacrifice the large screen of the 7603 and sharp and bright display of the 24 kilovolt mainframes mentioned above.

*Reliability.  PSUs seem to be the worst culprit here, particularly the early switchmodes.  Some models, particularly the higher speed ones use many custom hybrids what may just not be replaceable

The hybrids and custom ICs are among the more reliable parts.  Problems are usually mechanical, worn out aluminum electrolytic capacitors, or shorted epoxy dipped solid tantalum capacitors.

The switching power supplies in the 4-bay 7000 mainframes started with the 7704 and are all basically identical but with different transformers for different output requirements.  The 7834 and 7934 storage mainframes seem to have more problems with transformer failures.

*Physical robustness.  designs varied considerably, any winners?

I am not aware of any.  The 7904 is all one piece but later instruments separate into halves which might make maintenance easier.

*Performance.  Bandwidth is an obvious factor here though not so important  for me. are there any other distinguishing features?  the 7844 was true dual beam (difficult to calibrate apparently) and the 7854 had some digital crazyness,  even in the same model there deems to be some variations (there are 7704As with and without a selector switch for calibration to the right of the screen.

The fastest common high impedance amplifier was 200MHz so unless you accept 50 ohm inputs, the 400 and 500 MHz mainframes are not necessary as they do not have a CRT any better than the 7704A.

[MadTux]

7904A, because it's fastest universal 7000 mainframe without microchannel plate CRT.
7854 with digital storage for slow things, perfect for work with 7A22.
7603, largest CRT for 7CT1N curve tracer or 7D20 DSO plugin (unfortunately a bit outdated nowadays).

7844 is also nice, but more complicated than normal 7000 scopes.
I don't have any 7000 scopes with analog storage tube or microchannel plate CRT, since these CRTs wear out faster than normal ones and are more complicated to service. If you mostly do digital stuff, better buy a nice DSO, like TDS7404. But for analog work, 7000 series are the best.

[David Hess]

7904A, because it's fastest universal 7000 mainframe without microchannel plate CRT.

The 7904A will be the youngest of the 4-bay non-storage mainframes so potentially more reliable but the 7704A and 7904 sold for a much longer period of time so parts are more readily available.

The 7904A is not specified to be any faster than the 7904 but my guess is that it probably is but not in any way that matters.

[Tom45]

4 bay frames gives more flexibility.

I have a 7844 and a 7854 but my 7904 is the one that lives on my bench while the others mostly sit unused in a corner.

[particleman]

I have a 7633. They are great scopes. I use mine mostly for the plugins 7CT1N, 7A11 ect. Just remember they are old and will most likely need some type of work. The plugins are old also and may need work.  It took me several months to get my scope right. Between trouble shooting the scope and plugins. Then fixing the minor issues that were not immediately visible until the scope started running properly. I do a lot of repair of older transistor packed audio stuff so the curve tracer is my friend I also have a pretty big collection of Tek stuff and you know those are packed with transistors. My plan was to buy the 7633 to use not really as a scope and thats how it worked out.

[PaulAm]

7904 - high bandwidth, common.  I picked up my last 7904 mainframe for $25 at a swap when my original developed the ps ticking disease.  It's a versatile scope and can be used as a high bw scope or evan as a curve tracer with a 7ct1n.  If you were going to get just one (haha), I'd get a 7904. In fact, a 7904 was my first 7000 mainframe.

It's not the coolest 7000 scope, but it is common and cheap.

[David Hess]

4 bay frames gives more flexibility.

They do but I use my 7904 in place of my 7603 because the CRT is so much sharper and brighter.

[Tom45]

4 bay frames gives more flexibility.

They do but I use my 7904 in place of my 7603 because the CRT is so much sharper and brighter.

Interesting.  Must be that the 7904's 24 kvolts makes that much of a difference to the trace vs. the 7603's 15 kvolts.

[David Hess]

4 bay frames gives more flexibility.

They do but I use my 7904 in place of my 7603 because the CRT is so much sharper and brighter.

Interesting.  Must be that the 7904's 24 kvolts makes that much of a difference to the trace vs. the 7603's 15 kvolts.

The acceleration voltage makes a big difference.  This is apparent in the 485 portable compared to the other 400 series portables and when one of the storage CRTs switches between normal and reduced scan mode where only the cathode acceleration is doubled.

Huh, the 7704 has the same 24 kilovolt acceleration so I guess it would be on the list also although they are the oldest "modern" units and have some quirks from their early design.  Documentation is scarce for the earlier models so I have fewer notes on them.  I would not recommend them as a working oscilloscope unless you like refurbishing old instruments in detail.

[sigmatechnica]

Thanks everyone, some good info here! 
Now i just have to watch UK ebay like a hawk since 4 bay scopes hardly ever come up here  

of course the rigol and a 5k series Tek i already have just isn't enough of a collection     

[rhb]

Thanks everyone, some good info here! 
Now i just have to watch UK ebay like a hawk since 4 bay scopes hardly ever come up here  

of course the rigol and a 5k series Tek i already have just isn't enough of a collection     

In which case, you *really* need a Tek 11801/A/B/C with a selection of sampling heads, including at least one SD-24 and SD32 ;-)

But even the lowly SD-22 will let you see the calibrator signal reverberate between SMA connectors and 3.5 mm connectors.  Install four SD-22s and you can have loads of fun measuring FPGA gate delays in a DSP pipeline. For board trace length mismatches you'll need the SD-24 and if you want to know how fast that step really is, an SD-32.

[aandrew]

You've just reminded me that I have a 7904 that I've been meaning to clean up. Quite interesting beasts, those 7000-series chassis...

[PrecisionAnalytic]

I have a 7904 that I've been meaning to clean up.

Me too, I'm still trying to boost my confidence to take my 7904 out of the box and see which modules in the other two boxes are functional and need repairs.
https://www.facebook.com/groups/oldtekscopes/permalink/193781858076661/

Amazing how cautious I am with my 7904 system.  Probably because it can be repaired and improved and I need to know confidently how to do that before I let magic smoke out.

The Pacific Measurements D14 will probably get more attention since I picked up three now... actually... I have to literally pick up the third one and have one system that is actually out of the box... though I need to read into more before I power on.  Plus... I suppose because the box was already open and leaking foam out... I was more interested in opening up to take a look how fended.   Some of the manuals just came in the mail yesterday along with two detectors and a few days prior the RJ21 cable (1038-NS20/NS201 system). 

The modules for the D14 or D14A that I've found are all frequency domain and not time domain.  Might be some Tektronix 5000 modules that will work for time domain measurements... I forget at the moment and have to find the email where a gentleman explained what he knew.

Reading into, the 7904 seemed like the one I wanted or the 7103/7104 1GHz models (still want some day).

[0culus]

Once I have more space, I'd definitely love to have one of these scopes with some quality plugins. I've seen some neat ones, such as TDR. Mr. Carlson of Mr. Carlson's lab even rolled a few of his own, for 5xx series scopes.

[1audio]

I have a few Tek 7000 and 5000 plugins. They have all been really reliable as have the 5 7k mainframes and the 5K mainframe I have. However even the 7854 (which I have 3 of) is just not up to lots of what a modern inexpensive DSO can do. What I would like is a "frame" for the Tek plugins with a single ended outputs for a modern DSO. It seems one could scavenge from a dead 7k but maybe something from scratch with the essentials would be enough. Don't need the readouts or the chop/blanking/etc. I have looked but my Googlefu has not found anything. Tek made a one wide module for holding plugins but its exceedingly rare and I can't find reference to it. Some of the tek plugins have few or no current equivalents like the 7A13 and 7A22. And the samplers and the spectrum analyzers are still quite useable.

[woodchips]

Having acquired something like 70 Tek 7000 mainframes over the last few years my findings are:

None of the four slot mainframes worked, nearly all of the three slot ones did.
Avoid the R7704, Tek were obviously practicing building them, they are almost impossible to service.
The 7704A is nice in that the two halves split making servicing much easier.
The CRT on the 7613 is impressive, I prefer this to the larger 7603.
The linear PSU's are a real benefit.
Use the 7B80 or 7B85 time base in the 76x3, this gives you direct sweep speeds to 10ns without the irritating X10 expansion. Also the 7B92 but they are complicated.
Some time bases have a holdoff control, essential in my book.
Much easier to service than the portables.
The 7T11 and 7S11 with S2 etc heads gives you a multiple GHz scope which will actually trigger at those frequencies.
An analogue scope isn't the same as a digital one, probably need both.

[Johnny10]

Buy a couple different ones.
They are cheap.

More important is plug-ins and bandwidth choices.

[David Hess]

I have a few Tek 7000 and 5000 plugins. They have all been really reliable as have the 5 7k mainframes and the 5K mainframe I have. However even the 7854 (which I have 3 of) is just not up to lots of what a modern inexpensive DSO can do. What I would like is a "frame" for the Tek plugins with a single ended outputs for a modern DSO. It seems one could scavenge from a dead 7k but maybe something from scratch with the essentials would be enough. Don't need the readouts or the chop/blanking/etc. I have looked but my Googlefu has not found anything. Tek made a one wide module for holding plugins but its exceedingly rare and I can't find reference to it. Some of the tek plugins have few or no current equivalents like the 7A13 and 7A22. And the samplers and the spectrum analyzers are still quite useable.

Tektronix also made a three slot mainframe for 7000 series plug-ins with coaxial outputs but it is very rare.  One was up on Ebay a few years ago.

[edavid]

Tektronix also made a three slot mainframe for 7000 series plug-ins with coaxial outputs but it is very rare.  One was up on Ebay a few years ago.

Are you thinking of the Oregon Analog Tools ASM7?

http://w140.com/tekwiki/wiki/Oregon_Analog_Tools_ASM7

[MarkL]

Tektronix also made a three slot mainframe for 7000 series plug-ins with coaxial outputs but it is very rare.  One was up on Ebay a few years ago.

Are you thinking of the Oregon Analog Tools ASM7?

http://w140.com/tekwiki/wiki/Oregon_Analog_Tools_ASM7

Or this thing, similar but branded/re-sold by Tektronix:

  http://w140.com/tekwiki/wiki/NT-7000

I've been searching for one for years.  ASM7 now added to my search list - thanks!

[David Hess]

I was thinking of the Oregon Analog Tools ASM7.  It was designed by Tektronix engineers so close enough.

Now you have two you can look for.

Whoa, there are schematics to the NT-7000.  Yep, it is about what I expected.  The 155-0022-00 is well documented (I even have a few spares) and the discrete parts duplicate how Tektronix commonly did the differential to single ended conversion.

You could do it without the 155-0022-00 easily enough or modernize the design with modern fast operational amplifiers but I think an AD8129 or AD8130 difference amplifier could do the whole conversion with a single IC.

[MarkL]

...
You could do it without the 155-0022-00 easily enough or modernize the design with modern fast operational amplifiers but I think an AD8129 or AD8130 difference amplifier could do the whole conversion with a single IC.

That's exactly what I was going to do, and with those exact chips too (probably AD8130; I'd have to look at the output levels again).

I have several 7A13 I want to use standalone.   I bought a very tired 7633 I was going to cut up for parts and make an enclosure out of it, or try to fit the backplane into a TM503.  I ended up with a DA1855A because the project had to get done, but I'm still interested in eventually either buying or building (Frankenstein-style) an NT-7000/ASM7.

(Sorry to the OP... getting a little off-topic here...)

[David Hess]

...
You could do it without the 155-0022-00 easily enough or modernize the design with modern fast operational amplifiers but I think an AD8129 or AD8130 difference amplifier could do the whole conversion with a single IC.

That's exactly what I was going to do, and with those exact chips too (probably AD8130; I'd have to look at the output levels again).

For what it is worth, each signal output is 0.5 milliamps into 50 ohms per division so 25 millivolts per side or 50 millivolts differential per division.  That makes the maximum voltage +/-250 millivolts differential for +/-5 divisions of deflection which is well within the input voltage range of even the more limited AD8129.

This information is available in the service manuals for the 7000 mainframe standardizers if you want to review it for yourself.