C64r mk2 with low contrast

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Don't Panic. Please wash hands.
  • Early morning overtime :-)


    First of all, what I did with this board was to check if it's technically OK: My employees already had it checked and noted that the VIC you've included does give lower contrast than our test chips. The particular HMOS2 VIC that you've included is fine, the circuit is also fine, but the combination does result in a slightly worse picture quality. This was a new experience to me - the VIC does output all the right signals, but light colours don't quite reach the levels that other chips reach, so it's kind of "losing stamina" at higher levels. I don't see this as a defect, as the difference is minimal.


    What I did then was to try improving the limits of the output circuits by using RF transistors that go to GHz range, but the change was marginal. I did leave them in, as the frequency limitation is in the signal itself, plus the band limitations that the surrounding parts define (intentionally, that is!). Also, with a much better datasheet for these transistors MMBT5179), I had an easier time changing the linearity of the output stage to cope with the non-linearity of the VIC chip you've included.


    Long story short: Job done, and I hope I haven't changed it into a direction that you don't like. The light colours do appear lighter now, going close to the screen shots that you've printed out and included with the shipment.


    Since this is a matter of taste, I do hope that you like it. I believe that I've hit a spot where it's on the edge between "just a very good picture" and "not original any more". Let me know if we should return it with DHL or UPS.

  • Hi Jens, you are amazing! Thank you very much for all the time and effort! For your information, I have tried with 5 different VIC chips, though they all were from the 88-89 date range if I remember well. So maybe there is a bigger batch of these type of chips?

    Anyway, thank you again for looking so deep into it, I'm quite sure the new setup will work really well.

    Both dhl and ups works, take whatever the cheapest / easiest.

    Have a great weekend! 😊

  • That "22" after the date code is interesting. Did anyone ever find out what it means? Maybe it's a designation of a property of the HMOS2 chips?


    The chips we're using here to test things are 6569 (NMOS) chips, which do not exhibit such strong non-linearities.

  • If you wanna keep the vic chip for testing or other purposes from my board that's totally fine. Sadly no clue about the 22 meaning, but I think they are all from 250469 boards, Rev B maybe.

  • The last reply was more than 365 days ago, this thread is most likely obsolete. It is recommended to create a new thread instead.

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