28 Jul
2013
28 Jul
'13
11:09 a.m.
I am going to try to stick to techncial comments only...
Where are the online notes, available for perusal, for all the
repairs you claim to have done?
Oh, right .. just ask.
I am not asking for every reapir you have ever done. It's just that you
said you had done varios repairs and modifications to this sensor
circuit. I am interested to know what you did, and IMHO it is relevant to
this discussion.
On 2013 Jul 26, at 2:47 PM, Tony Duell wrote:
I hope you're not addressing me, I wasn't the one who simulated it.
No. This is a pblic mailing list, I am addressing whoever it was who
simulated the circuit.
In some cases I feel that simulation actually obscures an understanding
of the circuit, in that your simulation program may well give
voltages/waveforms that agree with what you measure on the real machine,
but you don't really know wy.
In your
simulation, you used resistances for the LDR of 74K dark and
I fial to see why you need to 'simulate' this circuit. The actual
comparator circuit, whether the origianl HP one iwth no feedback or
the
intended modification is simple enopug hthat you can understand it in
your head.
Apparently Rik felt like doing so. More power to him. But of course
everyone should be able to understand it in their head, right off the
bat, like Tony can. Tony's smart, why aren't 'you'?. (Anyone reading:
heavy irony intended).
If we take the H Pcircuit, it's jsut a pair of potential fividers with
the mid-points compared by an op-amp. A sort-of whaatstone bridge if you
like. One potential divider has 2 equal-value resistors and is across the
5V line, so the midpoint wil lbe at 2.5V. The other divider is the LDR
and soem other fixed resisotr, the voltage at its midpoint depends on the
resistance of the LDR.
I think it is reasonable to assuem the op-amp is 'ideal' here. Certainly
the departures from perfection are not going to make much differece.
So basically what you are looking for is when the midpoint of the second
divider crosses 2.5V.
[Yes, Brent, I am sure you knew all this and understnad it. I am not
'teaching you to suck eggs'. I am hopefully providing some information
for others]
Rik wasn't working from the simple HP circuit, he was working from
your mis-drawn circuit, which isn't quite so obvious as to how it
would behave.
As an aside, I feel it is unwise to base a modification solely on a
schematic (where revers-engineered, in a commercial service manual,
whatever. There might be several verisons of the device, there may be
errors in the schematic (as here -- but I've come across plenyu of others
as I am sure you have), whatever. To check that the actual unit agrees
with te scheamtic doesn't take long and might save a lot of time.
Earlier, I could have said: the circuit as you (Tony) drew it is so
obviously unusual for the task you should have double-checked it as
you drew it, and the error never should have made it into your
schematic. But I didn't. Out of politeness.
Yes, agreed I should have checked it. But, actually, is it totally
ridiculous as I've drawn it (meaning 'should my bogometer have hit the
end stop'>).
I know nothign about the characteristics of the LDR. So suppose it's very
high resistance. We agree that the +ve input of the op-amp is helt at
2.5V. So if there was a -ve feedback resistor as I drew and the LER was
high resistace, the output would eb essentially at 2.5V so as to make the
-ve input at 3.5V also (it would be a voltage follower, followign the
2.5V on the +ve input). As the LDR resistance dcreases, the output would
have to swing towards the -ve reail so as to keep the -ve input at 2.5V.
The (zener) diode on the outptu would prevent it going very far -ve.
So the ouput of the circuti as I drew it (albeit not the circuit HP used)
could have given a TTL signal with a suitable LDR.
So I feel there is nothing that should instantly have said to me 'This
must be wrong'.
But anyway. It is wrong. The resisotr goes form -ve input to ground, not
from -ve inptu t ooutput.
I do agree with you that in Rik's photos he seems ot ahve connected a
resistor from LSolenoid to chassis ground. Which will hae no effect at all...
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).
Where are youre results/descriptions of this? Rik has desciribed
what he
did, you have (quite probably correctly) found errors in it. But I
don't
see your work published anywhere.
Many of my repair logs are online, I have a log for the 9830, it's
not online yet and it wasn't pertinent here, nor do I know what point
you see in it here. I was responding to Rik's implication as to my
experience with the circuit, not comparing technical data. If Rik
wishes to discuss the technical aspects, I'm here.
Well, I would like to discuss the techncial aspects -- publically. It
seems a lot more valuable than us flaming each other.
I was able to discern the errors by looking at the photographs of the
board in Rik's article, which kind of suggests I have some degree of
acquaintance with the board and circuit.
I do not dispute this at all.
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 .
-tony