Special Hardware Failure reg. Oscillator

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Special Hardware Failure reg. Oscillator

Postby Stree » Sat May 01, 2021 1:43 pm


To all hardware specialists:

I have a very specal failure with my HP41CV:
When you use it for several minutes the reaction to any Key strike is more and more delayed. So you can not work with the instrument. When you are successful to switch it off and wait about 10 minutes you can work with the HP41 for another several minutes.
With the service manual I found the following: There are three modes: run (oscillator is running, display on), standby (oscillator ist off to save energy, display is on) and sleep (both off).See the oscilloscope screen shot:
C1 ist the key row signal which if a key is pressed going to low (at 2 div from the left). C3 is the oscillator which starts a few µs later. C2 and C4 are key columb lines, The CPU scans this lines successive to find the key which is pressed.
This is the normal state as it should be.
In the case of failure after several minutes of standby the oscillator starts with a delay. For the first with several ms, so the operator does not recognize it. But after more minutes it is growing to seconds and you can not work with the HP41 any more.
I can exclude the power supply (soldered wires from a Laboratory power supply) and the keyboard (exchange the common board and the keyboard workes.
It does not matter which key is pressed, the failure is always the same.
The oscillator is inside the CPU and has only two external components: L1 82µH and C4 150pF, I changed them but without success.

Has anybody an idea what I can do more exept of chnging the complete common board? The CPU itself is unavailable.

Best regards
Klaus Stree
Posts: 1
Joined: Sat May 01, 2021 10:49 am

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