9 Aug
2012
9 Aug
'12
3:40 a.m.
-----Oorspronkelijk bericht-----
Van: cctalk-bounces at classiccmp.org [mailto:cctalk-bounces at classiccmp.org]
Namens Jon Elson
Verzonden: donderdag 9 augustus 2012 6:57
Aan: cctalk at classiccmp.org
Onderwerp: HP 98xx series cassette EOT-sensor fix : war story
trick.
Date: Wed, 8 Aug 2012 23:22:20 +0200
From: "Rik Bos" <hp-fix at xs4all.nl>
To: "General Discussion: On-Topic and Off-Topic Posts"
<cctalk at classiccmp.org>
Subject: HP 98xx series cassette EOT-sensor fix : war story
Message-ID: <001001cd75ab$e8416670$b8c43350$(a)xs4all.nl>
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Lately I acquired some HP 9865 cassette units and a HP 9821A three
cassette drives suffered from 'slow running'
I found out the LDR's used in the EOT sensor are causing this
behavior, the used LDR's are from the NSL314 type which is practically
unobtainable.
>
> The NSL314 is a IR-sensitive CDSe type LDR, replacing it with a
> standard low resistance LDR doesn't do the trick, the LDR needs to
> have a resistance lower then ~5k at low light.
>
> The most of the cassette drives also suffer from bad sensor bulb,
> replacing the bulb by a white LED doesn't give enough light to do the
The combination white LED and new LDR isn't working because the LDR
resistance isn't getting low enough.
The sensor amplifier circuit needs to be modified to work correct, I
did this by creating Wheatstone's bridge by adding a grounding
resistor to the LDR input.
Adding a series resistor of 10k to LDR input and changing the feedback
resistor from 18k2 to 100k made the amplifier work correct with the
combination white LED and 'normal' CDS type LDR.
Why wouldn't you use an IR LED?