On 02/03/2013 07:58 PM, Rob Clarke wrote: > On 03/02/2013 19:03, Gerrit Heitsch wrote: >> >> The luma pin needs a pullup to work. It will only sink current but not >> supply it. The datasheet specifies a 500 Ohm pullup. The usual circuit >> also contains a diode (1N4148 for example) between VIC and pullup. > Hmm, I assume the pullup is in the modulator on this board. I just > powered it on without the vic present and there's nothing pulling it up > so that could be the problem. Maybe the diode died? >>> What is the minimum requirements a VIC-II needs to generate a sync >>> signal? >> >> It should do that once you supply power and clock. There will be no >> picture, but the sync signals should be there. Can be easily seen if >> you use a real CRT monitor. When you turn on an otherwise dead C64, >> you still get a twitch from the monitor once it locks onto the sync >> signal from VIC. > I have a scope so I've been looking at the signal on the pin itself. Well, you could bend the pin from the socket, add a 1 KOhm pullup to +5V and take a look at the scope again. You should then see something. >> Also, on the 250425: >> >> - +12V are present at Pin13 of the VIC? > yep >> - Clock signals are present (dot clock and color clock!)? > yep, both >> - Does the board contain any MOS-TTLs? (small ICs with MOS logo > yes, it has a 7711 instead of a 74ls139, a 7712 instead of 74ls08 and > 7708's instead of the dram mux's. Why do you ask? Because the MOS-TTLs are known to fail. Quite often when I come about a dead board, all I need to do is to replace the 77xx with the proper 74LSxxx and I have a working C64 again. They also fail in interesting ways... Those were a stopgap anyway when Commodore couldn't source the 74LSxxx, they made their own replacement. From the numbering using HMOS-I. Gerrit Message was sent through the cbm-hackers mailing listReceived on 2013-02-03 20:00:03
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