Tektronix 7104 or 7904A - Page 1
Forum section: electronics / testgear

author: Kidon
author_rank: Regular Contributor
timestamp: July 01, 2020, 10:15:58 pm
content: Hi,Given the opportunity, would you rather have a Tek 7904 or a 7104, and why?The reason I’m asking is because I have been offered both and I can’t seem to be able to decide. I know the 7104 will, at some point, develop an issue with the Micro Channel CRT. It’s only a matter of time; it’s apparently inevitable. Other than that, any other reason(s) you would choose a 7904 instead of a 7104? I have a 7623 already, and quite a few others. I don’t particularly need any of the 2, but the price is fair.Thank you for your input.PS: I don’t mean to hurt anyone’s feelings, but I personally prefer analog scopes, and would not trade them for any digital ones. I don’t need, nor am I attracted by the bells and whistles they offer, and I can do the math myself when I need to. I don’t need a scope to do that for me. This post is for people who like, care, and still use analog scopes regularly, even if not always. I understand and respect that. So please no answers like: “Why don’t you get a 4-channel modern-technology Rigol or something, bla-bla-bla etc...”

author: wn1fju
author_rank: Frequent Contributor
timestamp: July 01, 2020, 10:26:19 pm
content: I have a 7904 at home and a 7104 at work. The 7104 always seemed kind of fragile to me and I was almost afraid to use it. My 7904, although rated to 500 MHz, will easily do 700 MHz with the right plug-ins, and is built like a tank. Plus, with the 7104, you will be tempted to obtain the very best plug-ins for it, and that may be an extra expense for you.Big and beefy the way old analog scopes should be. I could probably fit six "modern" digital scopes inside the volume of my 7904.

author: Wolfgang
author_rank: Super Contributor
timestamp: July 01, 2020, 11:41:04 pm
content: Quote from: Kidon on July 01, 2020, 10:15:58 pmHad an 7104 for some years, without any CRT issues. To make use of the bandwidth, you need special vertical and timebase plugins. But then, its anawesome scope, no problem at all. 7904 is 400MHz, 7104 makes 1GHz straight. Needed for false pulse work.

author: 0culus
author_rank: Super Contributor
timestamp: July 01, 2020, 11:58:32 pm
content: Or??? That is some kind of blasphemy...I have both. A 7904A and a 7104, to be precise. I wouldn't mind having a 7904 because it has some features that are handy that were deleted from the 7904A, like a current loop calibrator for current probes. However, the 7904A is easier to work on because it uses the split case design. My 7904A is my workhorse lab scope...I keep it on a scope-mobile. I also have a decent selection of plugins for it, which store on the cart nicely.As to the 7104, as long as the mcp isn't pre-destroyed for you, it should last a long time in hobbyist use. Just be careful to not crank the brightness way up when it isn't needed (the protection circuit helps, but it won't fully protect you from stupidity) and you'll be fine. Definitely want 2x 7A29 vertical amps and 7B15+7B10 timebases for full bandwidth operation. I'd like to find some 7A29s for mine with the variable delay line option, but those are scarce it seems. I use the 7104 when I need the speed that I can't get with the 7A19 vertical in the 7904A. I also have a pair of P6201 FET probes that I use with the 7104; they complement the scope perfectly (and there's probe power on the rear panel of both scopes, intended for these probes). Very handy for looking at higher speed digital signals in projects. Be warned that there are a few 7k plugins thatmust neverbe used with a 7104 because they will cause damage to the mcp. TekWiki does a decent job of calling them out, iirc.

author: David Hess
author_rank: Super Contributor
timestamp: July 02, 2020, 02:25:45 am
content: As 0culus points out, the 7904 and 7904A are very different instruments. The 7904A and 7934 are based on the 7104 design.I would say go with the 7104. Even with lower bandwidth plug-ins, the MCP screen is useful because of its super high writing rate. Further, the 7904 is much older so more likely to have age related problems.

author: 0culus
author_rank: Super Contributor
timestamp: July 02, 2020, 03:32:10 am
content: I'll also point out that, in either scope, the 7A29 is a far better high bandwidth 50 ohm amplifier than the 7A19. It uses an ingenious circuit breaker system instead of unobtainium fuses for input protection, and it has a vernier volts/div.

author: edavid
author_rank: Super Contributor
timestamp: July 02, 2020, 04:16:27 am
content: The 7104's performance is amazing, but I find the primitive CRT protection timer to be quite annoying. It requires endless tweaking of the intensity knob to keep the trace from blanking out. The fan is annoying too.On the other hand, my 7904s are notably unreliable, always having some sort of power supply failure. But, after listening to a 7904A fan for a while, I start to think I should fix the (fanless) 7904s againNow that we know OP is asking about the 7904A, I'll say if you have to pick one, pick the 7904A.

author: syau
author_rank: Frequent Contributor
timestamp: July 02, 2020, 06:01:01 am
content: Quote from: 0culus on July 01, 2020, 11:58:32 pmTry to go through the TekWki but not able to find the type of plugin not to be use on 7104.

author: srb1954
author_rank: Super Contributor
timestamp: July 02, 2020, 10:21:14 am
content: If you intend using the scope for a general purpose bench scope I would steer away from the 7104 as the beam protection timer can get quite annoying. You really only need the 7104 if you are measuring very high-speed waveforms and need to resolve timings in the hundred's of picoseconds.

author: 0culus
author_rank: Super Contributor
timestamp: July 02, 2020, 02:51:48 pm
content: Quote from: syau on July 02, 2020, 06:01:01 amI can't remember which one(s), sorry. It's not any of the standard vertical amps or timebases though. Some of the more specialty plugins that you're probably unlikely to use these days, but it's worth mentioning that one should be careful and make sure the plugin you want is safe to use with a 7104!

author: 0culus
author_rank: Super Contributor
timestamp: July 02, 2020, 02:53:14 pm
content: Quote from: srb1954 on July 02, 2020, 10:21:14 amI don't find it annoying at all.As David Hess said, the very high writing rate is useful beyond just looking at 300ps edges.

author: David Hess
author_rank: Super Contributor
timestamp: July 02, 2020, 03:49:25 pm
content: Quote from: syau on July 02, 2020, 06:01:01 amVector scanning plug-ins like logic analyzers and spectrum analyzers are not recommended for the 7104 because they write to the same areas of the screen for long periods.

author: edavid
author_rank: Super Contributor
timestamp: July 02, 2020, 03:52:59 pm
content: Quote from: 0culus on July 02, 2020, 02:51:48 pmThere's nothing mysterious about it, it's just the ones that display a trace in the same location for a long time, like the 7D01, 7D20, 7L5, 7L14...It's also better to minimize use of the readout, so plugins that absolutely require readout (7D15) should be avoided.

author: PaulAm
author_rank: Frequent Contributor
timestamp: July 02, 2020, 04:44:25 pm
content: 7904s may be easier/cheaper to find parts. I ended up picking up a 7904 mainframe at a swap for $25 as a parts mule since mine had started ticking. Turned out the new frame was perfect so just did a plugin swap.QuoteYeah, what Oculus said

author: MadTux
author_rank: Frequent Contributor
timestamp: July 02, 2020, 05:36:35 pm
content: 7904A and 7854 and 7603 for me.7904A for fast stuff with 7A13/26/29, 7854 for slow things, most often equipped with 7A22, the digitizer is quite useful for slow nonrepetive things that would otherwise disappear on a purely analog scope.7603 to work with 7CT1N curve tracer or 7D20.

author: Kidon
author_rank: Regular Contributor
timestamp: July 02, 2020, 06:14:03 pm
content: Quote from: 0culus on July 01, 2020, 11:58:32 pmHi,I have 1x 7A29 and both 1Ghz plugins already. That’s nice to hear it’s not “that” fragile. But I’m tempted to feel the same way as you, it seems more delicate to work on the 7104.Oh by the way, I was referring to the 7904A, not the 7904. I corrected the title.

author: Kidon
author_rank: Regular Contributor
timestamp: July 02, 2020, 06:17:09 pm
content: Quote from: wn1fju on July 01, 2020, 10:26:19 pmI don’t mind that. If they don’t make coffee, go buy groceries, and bring the kids to school, I don’t care about digital scopes at all. Their bells and whistles and color displays is of no interest to me.

author: Kidon
author_rank: Regular Contributor
timestamp: July 02, 2020, 06:23:40 pm
content: To all of you, thank you so very much for your inputs. I appreciate it.I still don’t know which one to get... I probably have too many already...

author: Wolfgang
author_rank: Super Contributor
timestamp: July 02, 2020, 06:48:18 pm
content: Get both and another one as a spare. Happy TEA

author: 0culus
author_rank: Super Contributor
timestamp: July 02, 2020, 07:08:32 pm
content: Quote from: Kidon on July 02, 2020, 06:14:03 pmPersonally, I'd say if you have the space try to get both. They are a killer combo of wideband analog 'scopes that will serve well formanymore years if well cared for. For bench work, I very seldom reach for a portable scope anymore.

author: TimFox
author_rank: Super Contributor
timestamp: July 02, 2020, 07:15:38 pm
content: When I started out at work in 1982, the Tektronix sales rep explained to us about the fragility of the 7104 vs. the 7904. He said that is why the normal test equipment rental guys never rented out 7104s, since they always came back with burn.

author: wizard69
author_rank: Super Contributor
timestamp: July 02, 2020, 08:31:21 pm
content: Seriously I'd get both!!! You never know when one might fail. Beyond that good analog scopes are not made anymore so being sticky when it comes to good ones is not a bad idea. I should note that I don't know of anybody making real analog scopes anymore, could be wrong here.

author: Kidon
author_rank: Regular Contributor
timestamp: July 02, 2020, 09:39:36 pm
content: Quote from: wizard69 on July 02, 2020, 08:31:21 pmIf I could, I wouldn’t be having this dilemma...

author: james_s
author_rank: Super Contributor
timestamp: July 02, 2020, 10:06:08 pm
content: The 7000 series are super cool, I still remember seeing pictures of them when I was a kid thinking how amazing it would be to have one. These days I can't think of any reason to justify owning something that big that can't do anything I can't do with my TDS700 but I'm glad to hear some people are still getting use out of them. If I had infinite bench space I'd totally set up an analog corner with one of those as the centerpiece.

author: Electro Fan
author_rank: Super Contributor
timestamp: July 03, 2020, 06:12:05 am
content: I’d defer to David Hess and others on this but in my experience (knock on wood) the MCP is not more fragile it’s just more fiddly because of the time out screen saver feature. I’m not sure about the 7k series but on the 246X the MCP is also somewhat smaller than the non-MCP - but it’s still a mighty fine performing screen. I’m guessing if the screen size and/or screen saver don’t bother you (which they probably won’t) that you will enjoy the extra bandwidth of the 7104. The 7904 and 7904A are exceptional scopes but if the price and condition is comparable on the 7104 that would seem to be the better option given the bandwidth. With either of the 7k scopes you are considering and the right plugins I think you will be amazed at the ability to measure rise times.

author: med6753
author_rank: Super Contributor
timestamp: July 03, 2020, 11:24:39 am
content: If you're only interested in being an “appliance operator” then the 7000 series isn't for you. Think about it, many of them are approaching a half century old. They WILL require repair at some point. Even if you spend stupid money for a completely restored unit there's no guarantee.But if you are willing to invest some troubleshooting time, and cash, they are an excellent choice. I picked up my 7904 with a 7A26, 7A24, and two 7B80's for $100. Of course the PSU was in tick mode. One bad tantalum on the -17V bulk line and it was up and running. But I decided to re-cap the entire linear section of the PSU. Then turn my attention to the noisy attenuator switch decks on the vertical plug-in's. After that was done it was fully functional and only required a few tweaks.For a few months. The PSU was back in tick mode. This time it was the HV kicking out the supply. Long story short it tuned out the CRT itself developed a leakage path to ground. Only solution was replacement of the CRT. $75 plus shipping from Ebay and back in business. This was a very atypical fail but it just gives you an indication of what can go wrong with these old beasts.

author: Kidon
author_rank: Regular Contributor
timestamp: July 03, 2020, 12:10:28 pm
content: Quote from: med6753 on July 03, 2020, 11:24:39 amI own a 7623, and I’ve had some others a while back.Thanks for the advice though.