SAC.509LGX@e.isi.edu (01/03/89)
Does anyone know of the fix for the Original CBM Genlock that corrects the color imbalance? We are having a difficulty with the whites bleeding to pink. Thank you. Steve Brady.
hull@hao.ucar.edu (Howard Hull) (01/03/89)
The only fix I know of for the original Amiga Genlock (the "1300") is a fix for difficulties with external synch or tearing at the top edges. This fix is described in Amazing Computing Vol. 2, #11 (1987) page 88 in a half-page vertical column titled "Do-it yourself Improvements to the Amiga Genlock" by John Foust. In a nutshell, it says "Find an integrated circuit labelled 'U11'" which has 16 pins. From there, you must look for a resistor "labelled R55" banded "yellow -- violet and red." Gotta be 4.7K ohms, I reckon. Remove this resistor. That's all. The metal shield you have to remove in order to get at the circuit board is held in place by small twisted tab sheet metal extensions in slots. They must be flattened with a pair of pliers for removal of the shield, and then must be re-twisted when the shield is put back in place. Be gentle, since you may need to go in there again some time, and you can only bend these about a dozen times before they'll fall off. :-) On the off chance that your problem is really chroma (as you said), there is another Amazing Computing article, Vol. 2, #7 (1987) by Oran Sands the 3rd. This article discusses things that are wrong with the Amiga 1000 chroma resistor matrix used to support the Motorola MC1377P chip. The article is nicely illustrated, and shows lots of applicable information from measurements, as well as the section of the circuit board wherin resides that "villain" resistor (R140). He says if your Amiga was bought prior to June 1986, you need to replace R140 with a 470K ohm 1/4 Watt resistor (yellow -- violet and yellow). If your Amiga 1000 is a later model, then merely cut the lead of R140 that is nearest the MC1377P chip (down by pin 20, opposite the pin 1 dot). That fixes the angular relationship of the color phases. Now then, there is also a problem with the output level v.s. impedance matching to a 75 ohm video coax. It looks like the Amiga engineers matched the 75 ohm line (which is what I would have done - I don't like video ghosts, either). Unfortunately, though, the voltage output from the chip with this arrangement is a little more than what would be considered appropriately standard by, say, a TV transmitter. To fix this problem (which shouldn't bother a monitor, by the way) you need to find and replace another resistor (R9, with purple and green stripes - it will be 75 ohms, and I'll bet there's a third stripe [black] as well). Replace R9 with a 100 ohm resistor of equivalent wattage (R9 also looks like a 1/4 Watt part to me). The author of the article goes on to point out that the 1300 Genlock also uses the MC1477P chip, and says that the engineers used a 1.5 megohm value (equivalent to an open circuit for all practical purposes) for R140's equivalent R in the Genlock. However, he goes on to note that the hue control "on the back" (of, I presume, the 1300 Genlock) wretches with things in about the same manner as did R140 in the Amiga 1000. He implies that one can adjust things so that all colors are correct "except for one". Not any one in particular, just the one you weren't looking at when you adjusted it! In your case, I surmise that must be magenta-ward of red... However, one particular setting of this hue resistor should put you, as he says "within 3 degrees" of where it ought to be, as does the R470 fix in the Amiga 1000 (which otherwise is 7 degrees out of phase alignment, 97 in place of 90). The author goes on to note that the Amiga 1000 has one other deviation from the standard, and that is that it does not have the proper amount of "setup", or pedestal, which in the black and white days was 7.5 IRE units. He says this was a standard put in place because the video equipment of those days could not go to black. (I remember that if it did that, it lost track of where the blanking edges/levels were, and then it couldn't restore properly on dark images, and THEN it would go drifting off toward white - with the result that the _transmitter_ class C output stages would lose their bias and the tower shack guy would be treated to watching the plate currents go zipping off above 20Amps (this is at several thousand volts), and THAT would trip the final stage breakers, forcing a reset cycle at the transmitter...) Well, it appears that modern equipment is more robust, and 0 IRE is allowable for black - and gives more dynamic range between dark and light for the same peak-to-peak signal amplitudes. So the author says, in another nutshell, not to worry, Commodore, we'll take the "setup" just fine the way it is. Howard Hull, HAO/NCAR hull@hao.ucar.edu
EW%ccv.UIA.AC.BE@cunyvm.cuny.edu (Karel De Bruyne/Electronic Workshop UIA Belgium) (10/16/90)
Hello there, Is there anyone who can tell me something about genlock problems with the latest revisions of the A2000. - Only rev 6.0 ? Are these problems solved with rev 6.2 ? - Only external genlock, or videoslot-genlocks too? - All types of external genlocks, or some types not ? I hope these questions make sense, since I don't know anything about genlocks- problems, and have to ask these questions for a friend. thanks, Karel De Bruyne EW@CCV.UIA.AC.BE or EW@BANUIA52.BITNET Computer Centre University of Antwerp Belgium