Testing HP41C with oscilloscope

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Testing HP41C with oscilloscope

When repairing a HP41 machine, it is always interesting to know what it is happening inside - regardless of the screen being blank or not. Using an old memory module, we have connected the pins with a ribbon connector, and then this connected to the oscilloscope we can see several things: is there the positive voltage? when you press a key, does it appear on the interface?

The system is designed so that when a key is pressed, there are two lines ( ø1 and ø2) that output a synchronizing wave - delayed some time the one versus the second. This is a tell-tale that the circuit is working - regardless of what happens to the screen.

Here you can see in the case of a CL just installed (thanks Mr. Lerich). The circuit is outputting 360 Khz, much more stable than the usual 343-365 that you see on normal calculators. The ringing at the corners may be due to the fact that the cables were not terminated properly and therefore there were reflections. Also it can be seen the induction effect on the rises of one curve on the other, due to the parallel 150mm of the ribbon cable.


If we see it closer, we see the rise-time of the curve is equivalent to 25 MHZ - quite a high bandwidth, even despite the mentioned termination issues. Also the stability of the voltages seem very good. Both are testimony to Monte Dalrymple's good engineering!!


We need to get deeper in this method to be able to better debug and repair our calculators. Now we need to get a service module - or an image burned into an eprom of such.


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