Re: Advice about buying a 7904


On Fri, May 29, 2020 at 12:50 AM, Reginald Beardsley wrote:

which is the best choice from a long term support basis, 7904 or 7904A?

what plugins? 2x 7A19 and 7B92 + ???
Hi Reginald,

Advantages of the 7904A over the 7904:
- Newer, so possibly less failure-prone
- Easier to service because of split construction
- Front panel layout very much like the 7104 (or is that a disadvantage?)
- Internal construction and components very much like the 7104
- More modern look (to some at least)

Advantages of the 7904 over the 7904A:
- Fewer hard-to-get components
- No fan (a huge advantage according to some)
- Backlit Vertical Mode and Horizontal Mode buttons
- More versatile calibrator, includes current loop
- Can be had for very little money

Make sure that in either case, you buy one with the Alphanumeric screen-readout, which is standard.

It's very difficult to determine the better choice on long term support aspects: Do you want a younger and therefore possibly slightly more reliable instrument with more expensive, hard-to-get components or do you want an older, slightly less reliable instrument with fewer and cheaper hard-to get components? If you have room to spare, have 3 7904's for the price of one (or 2) 7904A's.

As regards plugins: For general-purpose work, put in a 7A26 (Hi-Z, 200 MHz) and a 7A19 (Lo-Z, 50 Ohm only) or, preferably, a 7A29 (resettable overvoltage protection, continuously variable attenuation), a 7D15 counter in the A Horizontal slot and a 7B92A into the B horizontal slot. Make sure you have all the other plugins nearby (7A13, 7A22, a sampler system for > 1 GHz).

Both have a switching supply. Apart from the well-known lifetime-driven weaknesses like electrolytic (mains voltage buffer) caps and tantalum (bead) caps, I'm not aware of any structural vulnerabilities.

I concur with Dave D. re. the 7854. It's more complicated and therefore more vulnerable but it's also one great piece of equipment, especially if you use sampling plugins.


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