25 Jul
2013
25 Jul
'13
2:22 p.m.
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 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.
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. 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
repaired and 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.
And about Tony, he did a lot of work making the
diagrams available for
everybody ..
It's ok to look at his diagram and if you find an error, he'll be
the first
to admit it and correct it..
And don't throw his work away, it's a first class job, if you don't
like the
presentation that's on you.
Yes, it is up to me. Try paying attention to what was said: Tony
didn't leave it up to me. If he had, I wouldn't have said anything.
Tony is very knowledgeable about certain things and if you wish to
support him and use his assistance you are welcome to do so, right up
to the point you support him when he's being an ass. I have been on
the list for well over a decade now. I have experienced Tony's
attitude many times. The best I can say is he's not as bad as he used
to be. And no, he won't be the first to admit he made a mistake.
But reverse engineering so much HP hardware is hell of
a job and he
helped,
by making them public, a lot of people.
He reversed engineered more the 100 items, how much did you do?
Everybody makes an error ones in a while I do, Tony does, and you??
Again, if you paid attention to what I wrote, you would note that I
made allowance for making an error every now and then in such a task.
I haven't counted how many items I have reverse engineered, but it's
many dozens, over a hundred I expect depending on what level of
complexity one is including. I have supplied the resultant schematics
to numerous people. I have also made gate-level simulations based on
those reverse engineerings, and have them running successfully.
And no, Rik, I'm not enjoying myself, it's all a nuisance. And before
criticising me, try paying attention to not just what was being said,
try paying attention to what you are doing. Normally I would be more
gracious about something like this, but when people start criticising
me when they're wrong, I feel no great obligation to be polite about it.