Re: Advice about buying a 7904


Chuck Harris <cfharris@...>
 

Issues and features that I recall:

7904 has lots of tantalum caps, and a couple are bound to blow.
They are easy to find and fix.

7904 has a weak power supply that can't handle many of the digital
plugins, such as the logic analyzer and spectrum analyzer plugins.

7904 always ticks 3 times when it is turned on after being off for
a while.

7904 has at least one "blue" harmonica connector that will have
ossified by now. If you touch it, it will crumble to dust.

Fortunately, the so-called Dupont connectors are compatible.

7904 has a united power supply transformer that has everything
from the low voltage windings through the EHT windings on the
same core. It is reliable, but repair isn't likely.

7904A Has few tantalum capacitors.
7904A has a separate EHT transformer... but because that was too
reliable, they drove it from an AC tap on the main transformer.

Why do I care? If the main inverter goes full tilt on, the HV
and filament voltage for the CRT doubles.

Specs that matter are the same for 7904 and 7904A.

7904A has a noisy fan. 7904 has none.

7904A uses hypacon connectors on its many, many hybrids.

All this said, I prefer the 7854 as my go-to scope.

-Chuck Harris

Reginald Beardsley via groups.io wrote:

I have a 485 and a 7104. The fragile CRT of the latter discourages regular use unless needed. Reed refurbished my 485 and obviously it's a really fine instrument.

I'm mulling over getting a 7904 for more routine work because I find the overshoot on DSOs very annoying. My 1.5 GHz LeCroy has a spec of 17% overshoot. I don't consider anything above 2-3% acceptable. I've asked Tek for a step response for a 3 or 4 series 1 GHz DSO using one of Leo's pulsers or similar. After my adventures with Keysight's MSOX3104T and the Rohde & Schwarz RTM3104 I don't want to get a demo unit unless it has an acceptable step response. It will be interesting to see if I get one.

I've never touched a 7904 and have no knowledge of them other than what I've read on TekWiki.

So some questions:

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

what plugins? 2x 7A19 and 7B92 + ???

common issues (e.g things like the 576 transformer problem)?

anything else?

Thanks,
Reg



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