From: dorian <>
Date: Sun Oct 10 2004 - 01:48:25 EDT

Thanks Chris, I'll give it a try. First, I went to school for electronics. I have a basic understanding, but really no practical experience. I have an ocilloscope and a logic probe.

When I turn the game on, there is no attract mode. Also in test mode, there is no picture, except some artifacts. I put the scope on X or Y out and got what I thought was a valid signal, but there was nothing on Z out except some very small spikes. I went to Q7 and I got a signal. Q8 was turned on thereby blanking the screen, always. I traced the signal back to H10 and found "statclk" and "latch1" were always high. "statclk" comes from K10 (vector timer control) which has an input from "strobe2". I found that J7 provided these signals among others and all were high, always. I thought the vector state prom was bad so I got one from Eldorado Games. It made no difference. I checked the three inputs, OP0, OP1, OP2 to the state prom and they were stuck too. As a side bar, I could not figure how this thing got started if the inputs to the prom were from outputs based on the prom. Anyway, I have an asteroids schematic and in the circuit description for the state machine, it says the state machine gets its instructions from the MPU to access the VG ROM. I put the scope on the processor's address lines and got some square waves and got some fuzzy lines I could not resolve. Part of the problem is I don't know what waveforms to expect. So I replaced the processor with one I know works, but nothing. I am not able to do a checksum on the ROMs. I am getting stuck.
I am not even sure my approach is correct. Btw, all the voltages are correct. I cleaned and reseated all the chips. I visually inspected the board and components. I checked the soldier on the board interconnect. All of the basic stuff seems to be as it should be. Also, I am getting all the clock signals. In regular mode, the game keeps resetting, but that is understandable since there is no activity on the address lines("wdclr" from the address decoder is always high). As an example of my inexperience, the X and Y out that I thought was a good signal in test mode, when I put it on the scope in XY mode there was something but it was not the test screen.

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Received on Sun Oct 10 01:48:26 2004

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