Re: HP 9821 - display and tape transport issues #VintageHPComputers


Tape transport is working like a charm !

All mentioned issues was caused by defective LDR.
I can also confirm that comparator output indeed needs 18K out of the leader tape.

I have started my tests by measuring resistance in a real tape condition, but with tape transport pulled out of the case
(bulb powered by 4.5V 3R12 battery, then resistance measured on LDR pins in both conditions and different cassetes)

The results:

1) Original white HP cassette:
- Leader tape - 1.35K
- Out of leader tape - 7K

2) Other white cassete (not as white as HP, and white but not transparent leader tape)
- Leader tape - 1.93K
- Out of leader tape - 5.2K (lighter brown tape)

3) Transparent case cassette (Eagle)
- Leader tape - 1.93K (less reflective)
- Out of leader tape - 8K (the darkest of all three)

Next step was cutting orange cable between LDR and PHOTO board, then add small adjustable 15K resistor on the way.

At original HP cassete, tape transport started to work normally from 11.5K set on adjustable resistor (+7K internal LDR resistance). I wonder shall I order a new LDR because there is a lot of space for later adjustment if LDR will degrade over time

All tape transport seams on screws are still untouched....what would you do in my case ? Leave like it's now or change this LDR ayway ?

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