Tektronix 2225 scope

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Sat Jun 12 16:13:36 2004

> I'm on holiday this week and I've borrowed the spare Tek 2225 scope to
> fiddle with. Can anyone give me a head-start on this beastie so I can go
> over my dead TRS80 M1 and see what's actually working? I'm a total scope
> novice so I'm searching for online tutorials on general scope use but if
> anyone's got experience of the 2225 that'd be great :)

I've never used a 2225, but I've used other 'scoeps. And actually I don't
find a 'scope that useful for general computer troubleshooting. However,
as that's what yuo have, I will give some explanations.

Firstly the real basics. I am sure you know a 'scope plots a graph of
voltage against time (For the pedants, there is often also 'XY' mode that
plots one voltage against another, but anyway). There are 3 main sections
of the 'scope to consider

1) The Y amplifier. This takes the incoming signal and processes it so it
can drive the CRT. The main user controls are the Y-shift (moves the
entire trace up and now), Sensitivity (a multi-position switch,
calibrated in V/cm, which sets the size of the trace) and the input
coupling swtich (DC, AC (couples the signal via a capacitor), ground
(turns off the input signal, so you can see where the 0V level is on the
screen).

For most computer trobleshooting use DC coupling (you _don't_ want ignore
the average value of the signal) and set a sensitivity of 2V/cm. That
will give a trace 2.5cm high for most logic signals of course.

2) The timebase. This is the section which moves the beam across the
screen at a constant rate. Again, you have an X-shift control (moves the
trace left and right). A speed control, calibrated in s/cm sets how fast
it moves -- the setting for this depends on the signal you're trying to
display).

3) THe trigger. This starts each sweep of the timebase at hopefully a
similar point of the input waveform so you get a steady trace (it is
almost impossible to make useful measurements from a non-steady trace!).
You will hav a trigger source selector (try setting this to the Y input
in use, at least to start with) and a trigger level control (turn this
until you get a stable trace).

OK, now for some TRS80 M1 troubleshooting. I assume you have the
schematics -- if not you're going to have problems..

I would start by using a voltmeter to check the 3 supply rails -- +5V,
+12V, and -12V. A good place to find these is on the pins of the DRAM
chips.

Assuming they're all OK, check the clock waveforms. Pin 6 of the Z80 is a
good place to start (1.7MHz square wave IIRC, so try setting the timebase
to 0.5us/cm if you can). And look at the outputs of the video divider
chain with the 'scope. Again you should get steady square waves on all of
them.

Now look at the CPU address pins. You can't get steady signals on these
(at least not if the machine is being somehat sane) -- they're not
repetitive signals. But you should get TTL level square waves on them. If
any look odd -- particulrly if you see illegal logic levels, then be
suspicious. And the same for data lines.

What is the fault with this machine, BTW?

-tony
Received on Sat Jun 12 2004 - 16:13:36 BST