28 Jul
2013
28 Jul
'13
1:15 p.m.
The original LDR seems to be unavaialble. I do have
what I believe to be
the part number for it, I can look it up if you want. I believe it was
made by a company called 'Quantarol' or something similar. No, this is
not in any HP manual that I've seen, but a very similar mechanism was
used in a thing called a Racal Thermionic Digideck P72 and I have the
component-level manual and parts lists for that. In the absense of
further information it would be reasonable to use that as a starting
point.
However, I did try looking up tyhis part number on google and various
data sheet search sites and came up with nothing.
So, since these LDRs do fail, finding suitable substitues which need only
minor modifications to the circuit would be a Good Thing. That is waht I
am asking about
How did I wrote the article..
At some moment I had 3 drives with problems, not running fast at seek and
rewind.
I searched for the LDR on part number and on family, and found out HP used
CDSe LDR's which are primarily sensitive for IR and not for visible light.
This is why the bulbs in HP 98x0 machines burns on low light level, from an
engineering view this understandable less change for trouble with leaking
light etc..
But CDSe LDR's aren't available in the size/housing uses by HP, so I choose
a standard LDR with a low light-on resistance.
Keep in mind here the LDR must react on reflected light only, the cassette
housing is the reflector here.
The original CDSe LDR's have a light on resistance of less the 1K and less
then 18k when used in the calculator.
I found LDR's with a light on resistance of less than 4K, but using them in
the calculator didn't work because they aren't sensitive to visible light.
So I replaced the bulb with a 3.5mm white LED and resistor, it didn't work
looking at Tony's diagram I thought well let's make a normal amplifier of
it.
And there thing were going wrong, I must say I'm a little colorblind in the
red and brown spectrum, and on a dark evening in December I modified the
first EOT circuit .
And it worked, well let's do another one worked also, the last one I didn't
do because it was part of a test machine that I only use to test boards.
I communicated this with a friend of mine, who asked me to write it up..
I did I even draw some diagrams to add to the story, I'm using
MultiSim/Ultiboard so let's simulate and make some screenshots..
I also wanted some pictures so I'll modify the last one to so I can make
some pictures.
Testing yes it works, of cause it works ;)
After ending up my article and sending to two people from review, please
check it.
I got one back with spelling and grammar corrected.
So publish it, may be it is off some use for someone.
When Brent started his flame I checked the diagram and the pictures and
found he was correct there was a fault in the diagram.
So I have to correct my article, I also opened one of the first modified
machines to look how I did the modification..
The first one had all the components on the right place but I used a 68K
instead of 6k8 resistor.
Undoing the modification and replacing the 18k2 (100K) with a resistor of
39k-56k will do the trick.
The amplifier circuit becomes a little more sensitive and can use normal
LDR's in combination with a white LED.
If someone knows a better fix let me know..
I made some new pictures and will rewrite the article in the next few days.
A small remark about my email address, it's in the header of every
cctalk/cctech message I've send...
-Rik