26 Jul
2013
26 Jul
'13
3:35 a.m.
-----Oorspronkelijk bericht-----
Van: cctalk-bounces at classiccmp.org [mailto:cctalk-bounces at classiccmp.org]
Namens Brent Hilpert
Verzonden: donderdag 25 juli 2013 23:22
Aan: General Discussion: On-Topic and Off-Topic Posts
Onderwerp: Re: HP 9830A always turns on with blank display
On 2013 Jul 25, at 2:48 AM, Rik Bos wrote:
Yes, Rik, I read your whole article, and I don't see the point or
necessity of
the
It has
had the consequence however, that the modifications Rik Bos
made in his
9830 tape drives were unnecessary and the article he wrote up about
it is itself largely in error because it was based on a false
premise.
Are you enjoying yourself ?
As far as I can see you're right about the 18k2 resistor, but there is
more.
The 100k is changed from a feedback resistor to a parallel load of the
inv.
input and the Au is changed from 10 to infinitive.
I've to look into it, maybe just removing the 18k2 resistor does the
trick also.
If you read the whole article, you'll see it's about the difference in
behavior between normal LDR's and CDSe LDR's The last ones are almost
unobtainable and sensitive to IR, standard LDR's are sensitive to
visible light.
The problem is they react different, you can't just replace a CDSe
with a standard LDR that doesn't work.
Also there is a big difference in resistance between light and dark
between the two types, that's why you have alter the EOT-circuit.
It needs to be more sensitive for smaller changes, and as it seems you
can just remove the 18k2 resistor and not replace it. changes you made.
The original circuit has a voltage divider for the LDR formed with the
18.2K
resistor in the lower half and the LDR to +5V, feeding
the negative input
of the
741, with the positive input fixed at 2.5V. The result
is the 741
(comparator) will
trip when the LDR resistance passes through
18.2KOhms.
By intention, you have changed the 18.2K resistor (as it really is, not
per
Tony's
diagram) to 6.8K and added a 100K feedback resistor
and 10K series
resistor on
the neg. input. This would actually reduce the gain of
the 741, as well as
lower
the LDR resistance (increasing the illumination
required) at which the 741
trips.
Except that's not really what you did. The 100K resistor is actually going
to
ground (re the error in Tony's diagram). In
addition though, the 6.8K
resistor
which you intended to be the lower half of the voltage
divider, you have
misplaced on the circuit board. It should be soldered on the two pins
below the
two pins you soldered it to. As it is, you have it
soldered into the
solenoid circuit
where it is doing essentially nothing.
The real result of your mods is a voltage divider formed with a 100K R to
ground,
and a 10K R in series with the LDR to +5. The 741
should trip when the LDR
resistance is around 90K. The whole thing is a far cry from what you
intended.
In your simulation, you used resistances for the LDR of 74K dark and 4K
illuminated. That should have worked just fine with the original HP
circuit,
although it wouldn't work with the way you have
actually modded the board.
repaired
and
I wrote it because it's a working (tested)
solution for an actual
problem.
AND you can't just replace the CDSe type with a normal off the shelf
LDR that doesn't work, I tried..
So be careful before just shouting something without testing it!
I'm not shouting, I am being careful - far more than you, and I have done a fair degree of experimentation with these
sensors (in one instance
replacing the lamp/LDR pair with an IR LED/ phototransistor pair, and it
didn't
require any changes to the comparator circuit).
Considering how many mistakes you've already made, I put no faith in your
claim about the CdSe vs CdS making a difference of consequence, not
without
further details about your test conditions and
outcomes.
Measure your new CdS dark/illuminated resistances in-situ, with a cassette
in
place and the cassette door closed, and unplugged from
the 741/comparator.
Try simulating that with the proper HP circuit configuration.
You're dealing with at least 4 circuits:
- the proper, original HP circuit
- the way you think the original HP circuit was, based on Tony's
incorrect
schematic
- the modified circuit you intended to implement
- the circuit you actually did implement with your on-board mods
All 4 of these are different. You need to retract your article before
other people
hack up their 9830 circuit boards unnecessarily, go
back to scratch and
reassess
everything you've done.
Oh, I'm going to rewrite the article, no worries..
But what pops up are two small questions:
- For how long did you know about the error in Tony's diagram ?
- And why didn't you contact us privately, when you became aware of those
errors ?
That's where I question your intentions or ethics if you want.
And that's also the last thing I have to say about this.
-Rik