HP 65 - a (possibly) not so common writing problem


Greetings to all HP fans and users

I recently repaired an early HP 65, which had been suffering from the gummy wheel syndrome. I used o-rings for the card wheel (which seem to work extremely well) and the clutch was rebuilt using a small piece of wire insulating material (vinyl?), as suggested in an article available in this site. Even though all the mechanical parts now work nice and smoothly, and the calculator reads all my cards with 100% accuracy (so far), it nevertheless refuses to write, leaving the cards apparently untouched. The more common faults, such as misaligned spring switches on the back (I checked these with standard leds connected to the corresponding connectors that go from the keyboard to the CPU board), and open coils in the read/write head (I measured 45 ohms on each coil, 30 kohms between coils and ground, and a nice zero between ground and head housing) have been ruled out. Two things remain (I think): a possibly damaged tantalum capacitor (although I have been reading that these mostly affect reading, not writing) and a possibly damaged read/write chip. I guess that I'll have to desolder the capacitors in order to check them, so it would be great if you could help me choosing the most probable one.
I have one last (and maybe relevant) question: I can't remember whether the HP 65 flashes or not in case there is a writing error (like when you try to write on pre-recorded or clipped cards) and I don't have a manual to check this. Regardless of what I feed the reader (a flipped card for instance) I never get an error after the card gets through in write mode, not even if I abruptly interrupt and then resume writing using the on/off switch. If there should be an error in this case (with a corresponding blinking of the display, as in run mode) then maybe the CPU is damaged, which would make me very sad, also because my HP 65 is a rather early model (serial number starts with 1333A) with "particular" features, such as CPU board shape (slightly different from later models) and the different materials used in the mechanical parts.
It would be great if you could help me on this. I thank you in advance for your time and patience.

Helder Crespo

Porto, Portugal


Two capacitors are in series with the heads, the other filters the power supply. If it is them, you should replace them all. The power supply one (usually small blue dipped tantalum) is the one that goes bad and affects reading. It should always be replaced. If a head cap is bad, replace them both (but I have NEVER seen a bad head cap in hundreds of readers). My bet is still a flakey reader switch. It could either be the write protect or the head enable switch.

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