Help me choose between 2 'scopes

[eKretz]

Hi all, I have been offered the chance to get either a Tek 754A or 540C for a good price. They are both 4 Ch, 500Mhz, 2 Gs/s scopes with InstaVu. The 754 has a color screen, and the 540 is a mono. Similar options, which would you choose and why?

[c4757p]

The 754 also has more sample memory (500kp vs 50kp), and apparently a properly color-graded display. If the prices are similar, I see no reason to prefer the 540, unless somebody else knows something surprising about their reliability or something.

[eKretz]

I was under the impression that the scopes were both the same in that respect, since both have the extended memory option. This datasheet comparison shows the 754C, but i think the 754A is the same. (Comparison is on the Specifications tab).

http://www.tek.com/datasheet/instavu%E2%84%A2-acquisition-oscilloscopes

[Wuerstchenhund]

Be careful, TDS754A and TDS754C are not the same! The last letter shows the generation of that model, and newer generations will usually have different specs than their predecessors.

Here's the spec for the TDS754A:
http://www.tek.com/datasheet/digitizing-oscilloscopes-instavu%E2%84%A2-acquisition

and here for the TDS540C:
http://www1.tek.com/Measurement/Products/catalog/tds540c/specs.html

The TDS754A does 2GSa/s in 2 Ch config and 1GSa/s in 3/4ch config while the TDS540C only does 1GSa/s with two channels and 500MSa/s with three or four channels.

[nctnico]

Having a color display is nice but I never liked the color displays on the TDS500/600/700 series scopes. They use a monochrome CRT and a shutter to make the colors. If you move your head you can see the traces move a bit. An LCD replacement is a very good idea. OTOH the monochrome scopes have a neat feature the color ones don't have: they show the previous trace in grey. This is nice for some measurements.

[Wuerstchenhund]

Having a color display is nice but I never liked the color displays on the TDS500/600/700 series scopes. They use a monochrome CRT and a shutter to make the colors.

They were not great (albeit color CRTs have their own issues) but these scopes have a standard VGA output, and an old 15" TFT monitor still makes a nice scope display 

An LCD replacement is a very good idea.

They are, but the ready to fit kits I've seen are insanely expensive.

OTOH the monochrome scopes have a neat feature the color ones don't have: they show the previous trace in grey. This is nice for some measurements.

You could do something similar with short time persistance (which if I remember right the TDS754A should have).

And for using multiple channels or using math, a color display is much more useful than a monochrome one.

[eKretz]

Well, it looks like I will soon have a 754A. I borrowed to get this scope, I hope it serves me well! I inquired about the electrolytic caps being changed and he said the acquisition caps on the newer scopes like 754 are changed to tantalum and ceramic, and the only electrolytics are on the cpu board. Here was his direct response:

"Neither the TDS754A nor the TDS540C have electrolytic caps on the acquistion board, they are all ceramic and tantalum. There are a few electrolytics on the CPU board, but I do not bother changing them because I have never seen them cause a problem on these newer scopes. And the risk of causing other damage to traces, etc. is greater than any benefit of changing them. The problem electrolytics were predominatly found on the TDS544A, TDS644A, and the old TDS540 (no suffix) scopes. Tek fixed the cap problem on the newer models of scopes like the 754A and 540C."

Is this not correct?

[AndyC_772]

I can certainly confirm that the old TDS540 is full of leaky caps on the acquisition, logic and front panel boards. The TDS754D only has a couple of SMT electrolytics on the serial/parallel interface board, the rest having been designed out and replaced with tantalum and/or ceramic.

As to exactly when the change occurred and which models contain which parts, I'm afraid I can't be any more specific.

[eKretz]

Is it safe to say that the electrolytics are easy to recognize? Don't they usually have metal bodies? I also found this while googling:

"I've posted the non-cap models before but the search engine here is so poor... The 500B/700A/684A/600B or later had fewer electrolytic caps, at first none on the acquisition board, then they went away on the processor board, and finally the front panel. I THINK, I saw one printer option board with tantalums but even the very late models have smd electrolytics."

http://www1.tek.com/forum/viewtopic.php?f=5&t=4863

[nctnico]

traces, etc. is greater than any benefit of changing them. The problem electrolytics were predominatly found on the TDS544A, TDS644A, and the old TDS540 (no suffix) scopes. Tek fixed the cap problem on the newer models of scopes like the 754A and 540C."

I have two older TDS500/TDS700 scopes from 1995 and although they have some eletrolytic capacitors, they don't leak.

[eKretz]

Well good, it looks like one less issue to worry about then. You converted your scope to lcd, right nctnico? Was there an issue with the original display? Or did you just want the LCD instead of the shutter curtain setup? Is there a difference in readability or accuracy of the displays?

[nctnico]

Both scopes I bought got an issue with the display so I created a TFT display replacement with the help of a CPLD for some extra timing signals and the color palette conversion. Another option is to get a 6.5" VGA display with a converter board. That can be connected to the external VGA connector.