[Test-Equipment] Advice about used scopes

[Marco IK1ODO]

At 19.23 04/05/2007, you wrote:
>Thanks a bunch for all the info.  Disagreements are good.  Let's us see
>both sides of a point of view.  As for my use of the scope.  I do want
>to be able to look at the waveform from  radio transmitters that works
>at frequencies of up to 50Mhz.   I need to be able to design and test
>RF transformers and RF mixers and filters at frequenies up to 50Mhz.  I
>want to see what is being sent to the antenna (or more likey dummy
>load)... A 20Mhz scope may not work for that

you need at least five times the fundamental frequency of a signal to 
see a square wave... preferably ten times.
The load from a simple 10x probe is around 10-12pF, and may detune 
the circuit you are looking to. I mean, to look at real RF signals is 
not a thing to do with a 100MHz scope.


>As for measurements.  My guess is that any used scope will not be
>calibrated well and the cost to calibrate it would be more than the
>scope is worth.

No, I disagree here. I live reselling second hand test equipment in 
Europe. 95% of the equipment coming in do not need recalibration; 
many need repairs. You may basically check the vertical calibration 
with a voltage source (a battery and a potentiometer) in parallel 
with a voltmeter. The time base is also easily checked using the time 
base divider chain of your counter. People discard test equipment 
since they are obsolete, surplus to requirement, defective, beyond 
economical repair, and so on; not because out of calibration (well, seldom).

>  I'm working on getting (building actually) a
>frequency counter and a peak indicating RF voltmeter  I figure a used
>scope is for making relative measurements, comparing signals and
>looking at the shape of waveforms (is is clipping?)

Again, you need BW to see this...

>Now, after reading some downloaded manuals,  I understand the
>serviceability of the older analog  4xx scopes.  I've skimmed over the
>service manuals and I think they are well written and I might be able
>to do some simple maintenance on those "portable" scopes.

The 465 series may SEEM easy to maintain. It is well documented, of 
course; but rarely easy.


>Question:  What fails on the 7000 series scopes?

Power supplies, all plastic parts, knobs, potentiometers, contacts, 
sockets, EHT supply, CRTs, capacitors, and expecially the large metal 
film resistors.

>   I had the idea that
>maybe I could get a 7904 and a good sock of spare plug-in modules.  If
>it's the modules like the vertical amp and the time bases that go out.
>They sell for cheap.  I could buy two (or three) of everything.  But if
>it's the mainframe itself, well I guess they are cheap enough I could
>keep spares too but storage space is an issue.

If you have storage space buy a 7904A and a 7603 to be used as 
plug-in test bench.


>One more question:  The 7904 has two vertical amp bays.  Do I
>understand this right?  With an amp in each bay I could have a dual
>trace scope.  Or with a dual channel amp in one bay I could have a dual
>trace scope.  What do I get by placing two dual channel amps in each
>bay a four channel scope?.  I've also seen scopes with two time base
>units.   I've not found this covered in a Tek manual , yet.

You get four traces, and each time base may be triggered by a 
different vert plug-in, so in effect you get two dual trace 
oscilloscopes in a single mainframe. Incredible flexibility, in 
effect. But I think that even with the failing prices you have to 
spend something more than $300 for two good mainframes, plus two 7A19 
and two time bases (get more of them... ).

I don't see the 7000 as the only oscilloscope in a shop today. You 
will need a second 'scope to fix it...:-) and that has to be a reliable one.

Marco IK1ODO