At least on IBM desktop pen plotter was actually a slightly modified HP machine. If yours looks like an HP, then you may find an HP carousel will fit HP has that propeerty too :-) As I have said many times, the most important piece of test gear is your brain :-). More seriously a 'scope (or a logic analyser or...) will not fix the problem fro you. What it will do, if used correctly, is provide evidence that will help you to track down the problem. In what? Tekky used a lot of their onw very custom chips (both analogue and digital) in their 'scopes. If you're talking about the EPROM programmer, then often a chip with a sticker is a programemd something-or-other. A programmed PAL, or EPROM, for example. Peeling off the sticker might reveal what the unprogrammed part was. At which point, given a suitable programmer [1] you can try to read it out. An EPROM can always be read out, of course, but other device ay have security feautres that prefent this. [1] Of course you end up with the problem 'how do you read the chips _in_ the EPROM programmer' :-). A friend with a programmer comes in handy here... Contrary to what they tried to teach me at school, a 'scope does not have infinite input impedance. It ha a finite resistance (in this case 1M Ohm) and a a non-zero capacitance in parallel with it (in this case 22pF or 47pF) -- which in part is due to the stray capacitance that comes from the connectors, PCBs, etc. In other words, the bare 'scope has the same effect on the circuit as connecting a 1M reissotr in parallel with a 47pF capacitor between the point you are looking at and ground. As soon as you add a probe, the capacitance will increase due to the capacitance of the coax cable connecting the probe to the 'scope. But there is a trixk. Suppose you connected a 9M reisstor in series with the probe at the device-under-test end. OK, the 'scope would on;y see 1/10th the signal (for a DC signal, see later...), but the loading on the circuit, particulalry the capctative loading would be reduced. And you could just turn up the Y gain toe compensate. That's why many probles are '*10 attentuator'. But there's a problem. If you conenct just a resistor like that, it will form a low pass filter with the 'scope input capacitance. Any waveform with high frequnecy components -- like a square wave -- will be horribly distorted. Now, there's a trick for that too. You connect another capacitor in parallel with the probe resistor. It turns out that if the time constant (RC product) of the 'scope input and of the probe resistor/capacitor are the same, then you have no problems. That's why a *10 proble has a little tweaker on it. It's a trimemr capacitor in paralell with the resistor which toy use to adjust that time constant. You clip the proble on a fast rise square wave signal nnd adjust the trimmer to get a sharp edge with no overshoot. Of course for even less loading of the circuit, and for very high freqeuncy signals, you use an 'active probe' THis has a buffer amplifier inside the probe itself, which presetns almost no load to the circuit under test, but which can drive the cable back to the 'scope. Nowadays you use FETs (or FET_based ICs) for this., I still have a probe, good to 800MHz or so, with a triode valve inside (!). Not it's not all that large. -tony

--- On Sat, 5/1/10, Chris M <chrism3667 at yahoo.com> wrote: Awesome score! I've been looking for one of those for years. It's a fixed frequency analog RGB monitor, and was used by several flavors of workstations (IBM, SUN and HP, I do believe). Awesome picture on that thing. -Ian

On Saturday 01 May 2010 03:36:59 pm Chris M wrote: No... It means that the input impedance of that connector is 1 Mohm. And that it has the equivalent of 22-47pF of capacitance across it. The reason it's important to know this is that if you want a waveform to display properly you need to compensate for that in the design of the probe that's attached to that input. Usually it's a 10X probe (meaning it attenuates the signal by a factor of 10) and has a series resistor of 9Mohm or so, plus some capacitance in the cable. There's typically a trimmer capacitor in the probe somewhere to adjust this so that square waves are actually shown as being square. Too much one way and the corners get rounded off, too much the other way and there's overshoot... -- Member of the toughest, meanest, deadliest, most unrelenting -- and ablest -- form of life in this section of space, a critter that can be killed but can't be tamed. --Robert A. Heinlein, "The Puppet Masters" - Information is more dangerous than cannon to a society ruled by lies. --James M Dakin