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Re: Problem Using a Commodore 1084s Monitor on a IIgs
Polymorph wrote:
To: Alex Freed
Alex Freed wrote:
Polymorph wrote:
Alex,
I'm just curious how you arrived at 51 Ohms. Was it through trial and
error, or some other means of deduction?
"51" looks very precise - it's just a standard value I tried when 75
proved not to be low enough. I used a scope to see what the signal
looks like. I would not use a much lower value for fear of overloading
the GS's output driver, but 51 is safe.
To elaborate:
Fixed value resistors come in certain values based on their tolerances.
For example, if a resistor is a 5% tolerance resistor, it wouldn't make
sense to make values less than 5% apart! The exact values chosen (for
the marked values--presumeably the center of the actual distribution of
values--are standardized:
10 11 12 13 15 16 18 20 22 24 27 30 33 36 39 43 47 51 56 62 68 75 82 91
and, of course, these values recur in every decade of values--the next
value is 100, then 110, etc.
Components with 10% tolerances are made in only half as many values:
10 12 15 18 22 27 33 39 47 56 68 82
So the marked value should not be interpreted as an exact value. ;-)
OK, so when I next go to the electronics store, I might pick up a 51 Ohm
resistor in addition to the 200 Ohm pot. As we're only talking a few
cents extra. Actually, it's rather embarrassing walking into an
electronics store and walking out with a 38 cent part. When I was
confronted with this problem the other day, I actually stood in the
store until I thought of something else I needed for a different project
just so that I didn't have to look the dealer in the eye when he said
"that'll be 38 cents please". ;-)
Never worry about this. If the store is concerned about their
transaction costs (which are almost certainly a few tens of cents)
then they should just give your the part! (And some will if they
know you. ;-)
The 200 Ohm pot will provide you with complete adjustment flexibility,
corresponding to *all* values of resistor, so you don't need another
fixed resistor for this.
When you wired up the 51 Ohm resistor, I assume it was in the same
manner as I tried with a 75 Ohm resistor - i.e. the resistor between
the composite sync and ground?
Yes.
I thought that's what you meant.
Could you do me a favour and measure what the voltage is between
composite sync and ground? It could be that I haven't lowered my
composite sync signal quite enough? Maybe the sync level is 0.3V P-P.
I think this is what is listed for one of the other 1084 inputs.
Perhaps this is an omission from the specs of the RGB port?
Specifying a video peak-to-peak level of 0.7v and a sync peak-to-peak
level of 0.3v corresponds to the standard composite video+sync level
of 1v (of which 0.7v is video going positive from black level and 0.3v
is sync going negative from black level).
It would not be unusual for the internals of the monitor to accept
composite sync at the same level and polarity at which it would be
found in a composite video signal.
I don't have this system connected at this time, sorry. However using
the 200 ohm pot as was suggested here should do the trick for you.
>>
No worries. Its good to hear of someone who experienced a similar
problem as myself using the 1084S with the IIgs. I wonder why most
people don't have a problem? Did later revisions of the 1084 line come
with greater tolerance to sync levels? It has me stumped how come some
1084 monitors require modification of the sync signal and others don't.
The sync level tolerance is strongly dependent on the input circuitry
of the monitor, and a very minor change would make it quite insensitive
to sync level. It would not be surprising if this were the case in
some of the monitors (which may have different manufacturers).
-michael
NadaPong: Network game demo for Apple II computers!
Home page: http://members.aol.com/MJMahon/
"The wastebasket is our most important design
tool--and it's seriously underused."