William, I'm beginning to suspect the EPROM as a problem. I'd like to rule out the 6502 first. I have a Radio Shack hand held logic problem - will that work? I assume I check on the 6502: - RESET Line - CLOCK - IRQ - READY (What does this do?) I potentially could get a oscilloscope...will that be any better? Logic Analyzer? I assume there is nothing I check on the 2708 EPROM. Are 2708 still available? I wonder if the 2708 is a drop in replacement for the 6540. I don't quite understand what you mean by replacing more than one 2K ROM. Are you saying that the ROMS in H7 (F800-FFF) and H4 (F000-F7FF) should be replaced at the same time...BTW-I assume this machined worked with this ROM in place. -Chandra FYI...so you don't get confused...I'm fortunate enought to own 2 8K Chiclet PETs. Unfortunately neither of them work 100%. This particular machine is completely dead and the other has a weird lockup problem. -----Original Message----- From: owner-cbm-hackers@dot.tml.hut.fi [mailto:owner-cbm-hackers@dot.tml.hut.fi]On Behalf Of William Levak Sent: Wednesday, February 21, 2001 9:42 PM To: cbm-hackers@dot.tml.hut.fi Subject: Re: Help...dead 8K PET On Wed, 21 Feb 2001, Martijn van Buul wrote: > Chandra Bajpai wrote: > > > > Can you help me with this problem? > > > > What I have: 2001 PET 8K...Board has 2114 RAMS + 6540 ROMs. The ROM that > > controls F800-FFFF had gone bad and someone replaced it with a 2708 EPROM on > > a little daughter card that fits on top of the ROM slot. I was told the > > computer worked great. When I turn it on all I get is random characters. > > Which means that the videoRAM isn't initialized yet. The video logic is > probably working, though. > > > So far I tried to reseat all the chips with no luck. Only other clue is the > > RED LED on the motherboard does not light up. The red LED is for a built-in diagnostic. There is a connector that goes on the user port and one that goes on the keyboard connector. With these connectors in place, the computer is turned on and it goes through a diagnostic with the output displayed on the screen. When the diagnostic finishes correctly, the red LED turns on. The connectors just jumper contacts on the user port and keyboard connector. The connectors are easy to make, but I do not have information at hand. I will look at my connectors and give you the results in the next few days. > > I don't know how to check if the 6502 is actually executing code...doesn't it > > usually start at FFFF? You need a logic probe. This is an inexpensive device that allows you to check the logic state of a single line at a time. You should check the reset on the CPU and interface chips and the select line on the ROMs. > It fetches a start address from FFFE and FFFF indeed. The EPROM replacement > is a possible cause indeed. However, there might be a more 'low-level' Replacing one 2K ROM is not very reliable. Both ROMs in a 4K block are addressed identically. It is the ROM that selects whether it is the upper or lower 2K. When replacing ROMs, it is best to replace both ROMs in a 4K block with one 4K EPROM which can be located in either socket for that 4K block. > problem. I hope you have a multimeter of some sort. Check the 5v supply > on a few chips. I don't really know the 2001N, but later CBMs had seperate > supplies for the logic and for the video circuitry. If the logic one > is shot, you'd get results like this. Happened to me one time; a 4032 worked > perfectly before I took it home. When I arrived home, it did about exactly > the same as your PET seems to be doing. The fact that you get random characters means that the video board and the digital display logic are working. If either of those were bad, the screen would be blank. It sounds like either the processor or the ROMs at $F000-FFFF are bad. - This message was sent through the cbm-hackers mailing list. To unsubscribe: echo unsubscribe | mail cbm-hackers-request@dot.tml.hut.fi. - This message was sent through the cbm-hackers mailing list. To unsubscribe: echo unsubscribe | mail cbm-hackers-request@dot.tml.hut.fi.
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