TEK 7904 / 7A19 etc. worth getting for future possible upgrades (S-2,S-4,S-6)?

[evb149]

Hi, I know this sounds like a rather newbieish and silly question but I'm wondering if in this day and age it'd be sane to pick up a 7904 and 7A19 with associated plug ins and accessories?  I've already got a DS1054Z which is quite nice for an every day scope for most problems and I can't see the analog CRO being better or attractive in most ways. 

Having a 500 MHz system would be occasionally nice but mainly the sort of thing that has me interested in Tek boat anchors would be some of the more esoteric plug ins like the S-2, S-4, S-6, et. al. which I think would be awesome to experiment with for time domain pulse stuff like the JW-Pulser or TDR or so on but I don't know if this would be a good choice of mainframe to get with the thought of in the future maybe finding some compatible interesting plug-ins like the fast samplers etc. 

Also I haven't ever looked into the maintenance / repair of such 7000 series of old Tek instruments so I'm not quite sure of how much of a maintenance and repair headache it could pose if I got it.

So if one is looking for a good "boat anchor" mainframe to play with the sampler units in the future is it a good one to look at or should I just wait for something a bit better suited for such plugins or a bit more generally useful etc.?

Is it commonly done by advanced EE hobbyists to make compatible plug-ins to add such capabilities as TDR or sampling or such to such mainframes?  It seems like it should be simple enough given the (AFAIK) open schematics / specifications of these models but I seem to rarely have heard of people doing it.

[JuiceKing]

Yes, there are many interesting and useful plug-ins that make an old 7000 Tek scope worthwhile.  The 7904 is especially attractive because it has no fan and so it runs nice and quiet. The mainframes are expensive to ship, so if you can find one in good shape locally you have won more than half the battle. Plug-ins are plentiful on eBay and inexpensive to ship. If you are patient, you can put together an amazing capability for little outlay.

[PaulAm]

The sampler heads you mentioned go into one of the sampler plugins (7s11/7t11, 7s12, 7s14).  It's really kind of absurd, but if you're lucky you can end up with a > 10 GHz sampling scope for around $100.  The last 7904 mainframe I bought cost me $25.

You can also get a 7ct1 curve tracer plugin and there are SA plugins as well.

The 7904 uses a switching power supply that can fail into "tick-mode".  That can be a bit of a hassle to fix.

They are also a bit on the large side, but I keep them around because they're versatile and cheap.

[Electro Fan]

Yes, there are many interesting and useful plug-ins that make an old 7000 Tek scope worthwhile.  The 7904 is especially attractive because it has no fan and so it runs nice and quiet. The mainframes are expensive to ship, so if you can find one in good shape locally you have won more than half the battle. Plug-ins are plentiful on eBay and inexpensive to ship. If you are patient, you can put together an amazing capability for little outlay.

I think it is as JuiceKing says.

The 7904 will take up a fair amount of space and it's heavy, so finding one locally or one that has affordable shipping that will arrive safely in good condition and then finding a place to put it is roughly half the battle.  After that there are lots of fun possibilities.  The caveat is that we're talking half a hundred years old so it's hard to predict exactly how well and how long everything runs but back in the day Tektronix built some seriously great gear so with a bit of luck a lot is possible.

Overall, I'd say you are headed on a good path but before you officially adopt the 7904 route you might take a look at Howardlong's posts on the HP 54121T; it looks to offer some very good performance in somewhat more modern packaging.  I'm not sure about the price differences and I'm not saying the 54121T is as good or better overall than the Tek mainframe approach but it's probably worth a look.

In my experience you have to plan for about a 2:1 ratio on Tek 7000 plug-ins; for every 2 you wind up with, 1 might work just the way you want.  The good news is that if you are handy with repairs you will acquire some spare parts - whether those are the parts you will need later is harder to say. 

Among the plug-ins I'd recommend would be the 7A29 (or 7A29P), 7S12 (with a S-4 or S-6 and a S-52), and a 7B92A.  The 7A26 and 7D15 are worth a look too, and I'm sure there are plenty of others to keep you occupied.  eBay can be a good source but you have to study the pictures and writes-up carefully along with the seller's MO, and then still be ready for the 2:1 factor.

If you search in the forums here you can find a fair amount of discussion on the 7904 and other Tek 7000 series products.  Plenty of people here will tell you to save your money, time, and space for more modern gear but there are enough other Tek vintage fans around that you won't be completely lonely with a Tek vintage addiction in case you acquire it.

EEVblog legend/philosophy is that until you have something like 19 scopes you haven't hit your quota, so using one allocation out of 19 for a 7904 doesn't seem too unreasonable.

Here's one of a bunch of threads you might find interesting:

https://www.eevblog.com/forum/testgear/40-ps-rise-time/

[Tom45]

To see some not so obvious things you can do with a Tek 7000 mainframe see this app note:

http://docmesure.free.fr/manuels/Tektronix/Serie%207000/plugins%207000/7000_pi_apps.pdf

[David Hess]

I have a S-4/7S11/7T11A setup, 7904, and more so I can answer these questions.

High bandwidth at an economical price is a good reason to get a 500 MHz 7904.  The CRT with its 21 kilovolt acceleration is also useful at lower bandwidths for its sharp and bright trace although this advantage is shared with the other 4-bay 7000 mainframes since the 200 MHz 7704A that have the same acceleration voltage.

The 4-bay 7000 mainframes can display a sampled and real time display at the same time.  This can be important with the 7T11 because it is tricky to use and the real time display serves as a sanity check.

If you are only interested in the sampling capability, then I think one of the 3-bay 76x3 mainframes would be better because their linear power supplies are easier to maintain, they are simpler, and they are smaller.  The analog storage mainframes are especially useful with the 7T11 but are also more complex.  I use my 7T11A with my 7834 (sort of a 7904 with analog storage) but I would use a 7613, 7623, or 7633 with the 7T11A instead if I had one.

The 7S14 sampling plug-in might be better if bandwidth only up to 1 GHz is acceptable.  It is much easier to use although pretty much all 7S14s require some minor maintenance to replace the mercury coin batteries used cleverly as part of their sampling bridges.

Making TDR measurements using the 7T11 and 14 GHz S-4 or 12 GHz S-6 sampling heads and a fast pulse generator is amazing.  Compared to a "fast" 300 to 500 MHz high input impedance oscilloscope, you can see every impedance discontinuity with millimeter precision.  If you are more interested in TDR capability, then a 7S12 would be better than the 7T11.

The 12 GHZ S-6 sampling head is a loop-through design for TDR applications.  For general use, I would prefer the 4 GHz S-2 because it is easier to maintain and provides better blow-by characteristics than the faster 14 GHz S-4.

So:

If bandwidth up to 1 GHz is acceptable, get a 7S14 instead of the 7T11.
If you are mostly interested in TDR, get a 7S12.
If you do not already have a 7000 mainframe for the above, get a 76x3.
If you want maximum real time bandwidth for a give price, get a 7904.

If you buy any of the above and are married, prepare to do wife faction quests.

[Electro Fan]

David, thanks for the always very helpful info.  Your explanations have gotten me sorted onto a better track with Tek gear a bunch of times.

Just one more technical question - regarding wfq:  what is the official definition "wife factions quests"? Sounds important

Thx, EF

[David Hess]

David, thanks for the always very helpful info.  Your explanations have gotten me sorted onto a better track with Tek gear a bunch of times.

Just beware that all of this equipment is old and requires maintenance.  Luckily this is possible because it is also all fully documented but the sampling plug-ins are especially tricky.  Some units like the S-4 and S-6 sampling heads are basically unrepairable if damaged in most cases because they rely on tiny hybrid construction for their sampling circuits.  With time you can certainly put together a working collection for a reasonable price.

Ask over on TekScopes@yahoogroups.com or search the archives for all kinds of advice.

I do not know where you are located but one thing I recommend is watching Ebay and Craigslist for weeks to months to get an idea of what is available at what price.

Just one more technical question - regarding wfq:  what is the official definition "wife factions quests"? Sounds important

I would not know but I have heard others discuss this aspect of collecting old test equipment and it seems important.  I think maybe it has something to do with chocolate and remembering dates or something.