Should I Buy This Tektronix 7904? - Page 1
Forum section: electronics / testgear

author: m.m.m
author_rank: Regular Contributor
timestamp: October 12, 2019, 02:58:50 pm
content: HiI'm an EE student and hobbyist, and I'm planning to start my own lab. I already own a ds1054z, but then found this Tektronix 7904 oscilloscope for ~500 USD on a local website. I was wondering if it's a good deal. Note that I don't have access to eBay or other international shopping websites, so if it's a good deal I have to buy it now or it's gone and the chances of finding anything similar might be low.Also, the seller lets me do a test before buying the product, are there any common issues with this model that I need to check before buying?

author: perdrix
author_rank: Frequent Contributor
timestamp: October 12, 2019, 03:21:30 pm
content: They're excellent 'scopes, but USD500 is a bit pricey unless it comes with a warranty!David

author: Tom45
author_rank: Frequent Contributor
timestamp: October 12, 2019, 04:18:17 pm
content: The 7D01 plugin is a basic logic analyzer that you might find useful. However, it needs a couple of specialized probes that are quite rare. If the scope doesn't come with those probes then the 7D01 is pretty much useless.Do you think you will be able to find other plugins for the 7904 in the future?

author: m.m.m
author_rank: Regular Contributor
timestamp: October 12, 2019, 04:41:40 pm
content: Quote from: Tom45 on October 12, 2019, 04:18:17 pmUnfortunately, the unit doesn't come with any probes, and I guess the logic analyzer is useless nowadays since there are lots of cheap USB ones available in the market. I'm not sure about finding other plugins but there is also another seller with the 7904 that has different modules installed.She has 7A19, 7S14, and the 7B92. I asked the price for that one and I'm still waiting for her response.Update: The other one is ~600 USD, and seems a bit more abused in the picture.

author: Tom45
author_rank: Frequent Contributor
timestamp: October 12, 2019, 05:23:34 pm
content: The 7S14 is not useful by itself. So you would be getting a single trace 7A19 and a dual time base 7B92 with the second scope.Between the two choices, having 2 inputs with the 7A26 and a single time base with the 7B70 is probably a better deal.The ultimate goal would be to have 4 inputs with 2 7A26 vertical plugins, and a dual time base with 7B70/7B71, or 7B80/7B85, or 7B92. As you know your local markets better than we do, you need to decide how likely it is that you would be able to add an additional vertical plugin and time base in the future.The flexibility of adding various plugins to the 7904 is an important feature. If you won't be able to acquire any more plugins, then it might be best to not get the 7904.

author: m.m.m
author_rank: Regular Contributor
timestamp: October 12, 2019, 05:36:35 pm
content: Quote from: MasterTech on October 12, 2019, 04:57:13 pmI don't think anyone's really going to pay for the 7D01 nowadays. especially that the probe is missing.Quote from: Tom45 on October 12, 2019, 05:23:34 pmbtw if anyone could be able to get me one with more useful modules from eBay or somewhere else (you won't need to ship to IR, I could give you my friend's address at some other country) I would really appreciate that. I could pay through Bitcoins.My problem is the lack of a credit card.

author: james_s
author_rank: Super Contributor
timestamp: October 12, 2019, 07:36:56 pm
content: I think the important question I haven't seen answered yet is what do you need it for? Don't buy a big boatanchor just because it's available, buy it because you have a specific need for something it offers over the little DSO you already have. If you don't know whether you need it, you probably don't.

author: m.m.m
author_rank: Regular Contributor
timestamp: October 12, 2019, 07:56:09 pm
content: Quote from: james_s on October 12, 2019, 07:36:56 pmActually, I don't have any plans for it at the moment, I just thought if it's a good deal that I might not find later, seize my opportunity.At this moment I mostly work with microcontrollers, switch-mode power supplies, maybe some FPGA, and some low-frequency analog circuits. (Frankly, I don't yet know which part of electronics I love more)

author: 0culus
author_rank: Super Contributor
timestamp: October 12, 2019, 08:10:58 pm
content: I have the updated brother, the 7904A. I've got it on a matching scope-mobile lab cart, and it's basically my workhorse general purpose scope. The ability to use a variety of plugins is very very useful.As long as you keep in mind that this stuff is old and you may well be called upon to do some repairs, they are very nice instruments and well worth preserving.

author: m.m.m
author_rank: Regular Contributor
timestamp: October 12, 2019, 08:17:50 pm
content: Quote from: 0culus on October 12, 2019, 08:10:58 pmThank you, how is the repairing on these models? I had heard some other Tektronix scopes that had special ICs in them that blew up too often and were hard to find and replace.

author: 0culus
author_rank: Super Contributor
timestamp: October 12, 2019, 08:35:24 pm
content: I think tantalum caps are probably the biggest scourge. Unobtainium hybrid ICs tend to be less of a problem AFAIK (provided no abuse takes place). The 7904A is a lot easier to service than the 7904 simply due to the case and chassis design, which on the 7904A splits in half right under the crt and mainframe controls, allowing for better access to the insides.Forum member med6753 has a 7904 in regular use, that he repaired; perhaps he will chime in.

author: med6753
author_rank: Super Contributor
timestamp: October 12, 2019, 09:15:47 pm
content: $500 is a pretty stiff price for a 7904 even if it is a 500MHz scope. Especially with a basically worthless plug-in. $250 would be my upper limit. I was able to pick up my non-working 7904 with a 7A24, 7A26, and 2 7B80's for $100. It wouldn't power up and the power supply was in "tick" mode which is a very common issue. And the fix was a common fail point....bad tantalum capacitor in a 17 volt bulk supply.Once that was fixed the scope operated for a period of time then failed to power up again. Found out that the HV was causing the supply to shut down. Ultimate repair was a defective CRT. It had a high resistance short to ground. I was able to source a CRT on fleabay for about $50 USD. It currently is operating fine and it is a real work horse. But again....$500 is too much.

author: james_s
author_rank: Super Contributor
timestamp: October 12, 2019, 09:44:57 pm
content: Quote from: m.m.m on October 12, 2019, 07:56:09 pmThe main thing it has going for it is the high bandwidth and the flexibility of being a mainframe scope. It doesn't really sound like the sort of thing you are going to have a lot of use for honestly, unless you plan on doing a lot of high frequency analog work it's probably going to largely collect dust. Not to mention for $500 you could probably find a TDS500 or TDS700 with the same bandwidth but these are DSOs which are much more capable than analog scopes. Analog has advantages, but it's a niche thing, mostly guys who have been using them for years and are most comfortable using what they're familiar with.

author: Tom45
author_rank: Frequent Contributor
timestamp: October 13, 2019, 02:24:30 pm
content: Quote from: Tom45 on October 12, 2019, 05:23:34 pmI was wrong about the 7S14. Unlike the other 7S plugins, it is a standalone sampler with dual channels and a dual time base with delayed sweep. It has some mercury cells in the input circuits that won't last forever. So it is possible that you would need to do something about that before you could use the 7S14.So the 7904 with 7A19, 7S14, and 7B92 is actually a pretty good wide bandwidth setup. The 7A19 would have a 500 MHz bandwidth and the 7S14 two channels with 1 GHz bandwidth. However, all 3 channels are 50 ohm inputs. You would need to find another vertical plugin such as the 7A26 if you wish to have a high impedance channel.

author: 0culus
author_rank: Super Contributor
timestamp: October 13, 2019, 02:36:36 pm
content: With 50 ohm inputs, an active probe like the P6201 (which uses slip on attenuators for 10x or 100x) is useful if you want to be able to probe.

author: BravoV
author_rank: Super Contributor
timestamp: October 13, 2019, 02:39:30 pm
content: Do you guys aware that the OP lives in Iran ?Things to consider, he wants to buy and pay with Bitcoin , not to mention the shipping cost.

author: m.m.m
author_rank: Regular Contributor
timestamp: October 13, 2019, 05:19:47 pm
content: Quote from: BravoV on October 13, 2019, 02:39:30 pmI already found two sellers in my city, but I was hoping someone could send me one with a better price and options if that's possible, which I know is a longshot.

author: Bicurico
author_rank: Super Contributor
timestamp: October 13, 2019, 05:27:57 pm
content: Why do you want such a boat anchor if you already have a Rigol 1054Z?You can get a Salae Logic Analyzer for little money and a cloned one almost for free.Save the money to equip your lab with a spectrum analyzer.This of course if you are just getting test equipment without a specific need.If you do have a specific need - make your math and see if you have a profit afterwards.But generally don't touch such old gear.My opinion...RegardsVitor

author: chris_leyson
author_rank: Super Contributor
timestamp: October 13, 2019, 06:38:08 pm
content: Depending on the age of the scope the switched mode supply might need re-capping at some point and the two large electrolytics on the primary side will be difficult to source if you want the exact same size capacitor. They have screw terminals so fitting anything that uses a different style of construction is not going to be easy. I think the same goes for the 6V smoothing cap on the secondary side, screw terminals. Watch out for tantlum caps. Bacplane connectors for the plug-ins are difficult to repair if damaged. I might have a pair of 8+1 channel probes for the 7D01. The connector is a custom TEK part. The 7D01 isn't paticularly fast and the memory depth is only 254 bits in 16bit mode. Most plug-ins will need extension cables if you need to repair them. I used to own a pair of 7854 scopes with a dozen or more plug-ins and to honest I'm glad I sold them.

author: james_s
author_rank: Super Contributor
timestamp: October 13, 2019, 06:43:52 pm
content: The screw terminals shouldn't actually be a huge obstacle depending on how picky one is about retaining the original look. Modern capacitors could be soldered to brass nuts or ring terminals screwed to the original holes. If it's a classic device and one wants to retain the original look, there's always the antique radio trick of gutting the original part and installing a new capacitor inside the housing.