Re: Tek mainframe vs 235A

[bronson]

Electro Fan asked my opinion about my 7904 mainframe vs 2465B (back in Oct, sorry).  Here's my lament.

In an ideal world, the 7904 plus a bunch of plugins is simply an unbeatable setup.  It is very good at a lot of things, and can do just about everything.  I had slow and stable time bases, I had fast and cranky ones, I had HV amps, current amps, a super sensitive diff amp, a sampling head (never got it working), a pretty good curve tracer, and I could have had a spectrum analyzer if I found one whose price matched its limited capability.  The front panel is well laid out, feels great, and is super easy once you get the hang of it.  It's worth dimming the lights in the lab just to see the backlit switches in their full glory.  Stunning.

So why did I get rid of it and get a boring old 2465 instead?  Quick answer: lack of space and time.

First space: this thing is huge and puts out a ton of heat.  Don't stack anything on it, and get long probes and clip them so they don't drag your project off the bench.  I put my mainframe on a scopemobile which doubled my bench space but really cut into floor space.  Now try to find room for 16+ plugins that just don't stack very well thanks to their springy ESD fingers.  (Why so many plugins?  Because you need certain combos to get good readings.  You don't want to feed the 7CT1N with a fiddly fast amp, no point to the 7B10 if you only have 7A18s, etc.  You'll get a feel for which ones work well together, but first you need to obtain them...)

And time: at least 20% of my plugins tended to be in the repair pile.  These things are old and oxidized, full of tants, and a lot of signals route through the front panel (no digital encoders here).  Buy contact cleaner.  A LOT of contact cleaner.  Pray that all the plastic shafts and electromechanicals don't crack because they're impossible to source (maybe 3D printing can help).  Don't touch the trimpots because they can be hell to stabilize.  Need to probe a plugin while it's powered?  Guess what?  That takes another plugin!  Google "plugin extender", lot of options but all of them are a PITA and you end up debugging the extender as much as you do the plugin.  Fast plugins break a lot more often than the slow ones, and they're MUCH harder to fix.  And I think I remember needing to be careful of semi-high voltages running bare-leaded next to signals, easy to fry.

Back in the late 90s, when I started my 7000 series habit, I would clean up at surplus sales.  You could get a set of 7A18s and 7B53s for $40 total because these shops were sick of them collecting dust.  I found a 7A11 for $30 because the seller didn't realize it was any different (it quit working a year later).  I bid on a lot of 5 plugins and found one of them was a working 7CT1N, which alone was worth 3X what I paid.  Easy to get hooked.  By 2008, though, it was mostly dried up.  Heck, the whole surplus test equipment market seems to have disappeared from the Bay Area, and the 7000 series equipment I see at the De Anza flea market is stupid expensive or broken.

An all-in-one seems great, doesn't it?  Not really...  Any time I wanted to use the curve tracer, I had to slide all the current plugins out, slide the 7CT1N in, plus the plugins it needs, reseat a few times until the traces were stable, use it, and restore everything when done.  It's a lot more effort than just pulling a curve tracer off the shelf, and can be infuriating if you have to take apart a working probe setup.

One day I realized I was spending almost many hours keeping my equipment happy (repair, procurement, inventory management) as I was keeping my designs happy.  So, in a fit of despair, I sold the entire package for an absurdly small amount.  I couldn't bear it.

So far the 2465B is treating me well.  It doesn't respond instantly the way the 7904 did, and it isn't anywhere near as flexible, but I haven't had to debug it even once (yet).  I love that.

So, if you have tons of time and tons of space, and if enjoy figuring out if noise is in the signal or just in the scope, and reseating plugins, and if you always want to be on the lookout for more plugins, and you enjoy coming up with clever fixes, I heartily recommend the 7904.  It's simply beautiful.  I miss mine dearly.

But I'd never buy another one...

[Smokey]

I lost interest in my 7000 series mainframe project after about 2 weeks.  Every time I through I figured out why it was broken it ended up being something else.  Too much pain for something I knew I was only keeping around as a show piece.

[Wuerstchenhund]

One problem was that the modules weren't designed for being swapped on a daily basis but to allow to offer flexible scope configurations which during their useful life would mostly remain unchanged.

The place I worked in the 90's had some Tek 7000 Series scopes (mostly rack configs), and about the only time the modules were unplugged was when one died (which happened pretty regularly) or when they were sent off for calibration.

Even back then it was a very repair-intensive system.

[Martin.M]

I would never change one of my  7k for a 2k.
The spares of 2k are short before empty... and how to plug a 7L5 in that 2k? never 

my 7844 mod 78, 21


greetings
Martin

[mazurov]

If your bench is small, a 7904 (non-A, i.e. with no fan and with backlit buttons) can be placed on the floor vertically. Back in the days I have had one inside the IBM 370 chassis at work. Being an older model it won't accept newer high power plugins, a sampler being one of them.

[edavid]

If your bench is small, a 7904 (non-A, i.e. with no fan and with backlit buttons) can be placed on the floor vertically. Back in the days I have had one inside the IBM 370 chassis at work. Being an older model it won't accept newer high power plugins, a sampler being one of them.

That is not correct, a 7904 has enough power to run any plugin.  Maybe you are thinking of a different model.

However, I have found them to be the least reliable mainframe, probably because they are fanless (yes, I know there are some late model 7904s with a fan).

[mazurov]

That is not correct, a 7904 has enough power to run any plugin.  Maybe you are thinking of a different model.

My two non-A 7904 won't turn on with 7Sxx installed. I saw a discussion somewhere (yahoo groups likely) attributing this to the power supply. There are several variants of 7904 and for older ones 7Sxx plugins are not even listed in the manual. 7904A (with fan and no backlight), which I also have, takes any plugin.

[edavid]

That is not correct, a 7904 has enough power to run any plugin.  Maybe you are thinking of a different model.

My two non-A 7904 won't turn on with 7Sxx installed. I saw a discussion somewhere (yahoo groups likely) attributing this to the power supply. There are several variants of 7904 and for older ones 7Sxx plugins are not even listed in the manual. 7904A (with fan and no backlight), which I also have, takes any plugin.

My 7904s work fine with 7S/7T plugins, but they are all above SN B260000 - what are yours?

My 7904As do have lights power.  Maybe yours just has a broken regulator?

[David Hess]

I have a non-fan 7904 (B28xxxx) and it works fine with my 7S11/7T11A sampling plug-ins although I do not know why I would use them in it.  It and one of my 7834s does have problems powering a complete set of high power plug-ins but I believe they both need to have their aluminum electrolytic capacitors replaced and it is on my list of things to do.

As far as repairs, none of the 7000 mainframes or plug-ins that I have rebuilt have needed significant further maintenance.

I rarely have to change plug-ins around but I have more than one mainframe.

As far as the power issue, the 7904 is 190 watts maximum and the 2465B is 120 watts maximum.  I do not consider that a significant difference.

[TerraHertz]

Among my generation of engineers (ie old) I think there are always going to be Tek 7000 series believers. Those things are warm reminders of another time, if nothing else.

Anyway, I just received a box of 10 Tek 7000 plugins. This set I bought because there's a 7D13 and 7A22 included, and the whole set was less than those two would typically cost to buy individually. The rest are mostly for spare parts.
The 7D13 (multimeter) is for laughs. A multimeter plugin in a 7000 series scope, ha ha. Yes, it's the 'multimeter collector' syndrome at work.

[Electro Fan]

TerraHertz, nice find - Congrats.  Please keep us posted on your 7000 exploits.  EF

[mmagin]

Now that I've accumulated a dozen 7000-series plugins in various states of disrepair and a couple mainframes, I'm going through the same thing.  They just don't have the mechanical quality of the 500-series scopes and plugins, so it's not like owning an old car that you like to fuss around with.  Instead it's a world of tiny switches that jam up, mechanical linkages that break, etc.  And the interesting plugins are often still fairly rare and go for too much money in non-working condition.

I'd gone down this road because it seemed interesting and less consuming of space than the old scopes, but I think I'm going to get one of the nice pre-1970 monolithic scopes (I'm thinking a 454, space is tight) and otherwise just use my modern HP digital scopes.

[PaulAm]

At the moment, I have a 7ct1n plugged into a 7904 mainframe.  Seems like a waste, but it gets the job done.  The last 7904 mainframe I bought cost me $25.  It was supposed to be dead, but turned out to be in perfect working order.

I picked up a 454A on the local craigslist for a few bucks and it's in the repair queue.  That's a classic scope and the A variant had a larger screen.