Exidy Crossbow won't boot - FIXED
I'm getting what appears to be a "common" problem with my newly acquired Exidy 440 system. It will start up, but it will only display yellow/blue bars on the screen. It will make a painful attempt at playing some sort of sound, but it generally only spits out 60hz for a couple seconds (along with noise) and then stops. Starting in test mode doesn't make any difference.
As per some other threads here I checked for voltage drop issues at various points on the board. The incoming 5v line has an initial drop of about .2v on the main power connector (the connector has clearly gotten hot) but after that, it is the same at all points on the board. The +/-18v supply works, but has a decent bit of ripple and needs its caps replaced. The 5v supply has a lot of high-frequency noise on it.. probably should replace the big blue cap on that part of the supply as well. The 5v rail is currently at 5.18v.
I'm not sure what actually causes the blue/yellow bars - is this just how the palette ram is initialized or the proms? Or the line buffer? Does this indicate that the main CPU has not started?
I figured my best bet was to check on the clock generation circuitry.. Being a bit of a noob, I'm not sure the best way to test this. When I hook my o'scope up to the output from the 7404, I see a lot of echo in the signal and it's quite noisy. I tried to calculate the frequency it was oscillating at, and I get 4.7mhz (?!?!) which doesn't seem right at all - however, I could be doing something very wrong here.
TL;DR: What do the blue/yellow bars mean and how to I check the crystal oscillator properly?
Attached is a schematic of the oscillator circuit so you don't need to hunt it down in the full schematic.
Thanks for any help!
I'm getting what appears to be a "common" problem with my newly acquired Exidy 440 system. It will start up, but it will only display yellow/blue bars on the screen. It will make a painful attempt at playing some sort of sound, but it generally only spits out 60hz for a couple seconds (along with noise) and then stops. Starting in test mode doesn't make any difference.
As per some other threads here I checked for voltage drop issues at various points on the board. The incoming 5v line has an initial drop of about .2v on the main power connector (the connector has clearly gotten hot) but after that, it is the same at all points on the board. The +/-18v supply works, but has a decent bit of ripple and needs its caps replaced. The 5v supply has a lot of high-frequency noise on it.. probably should replace the big blue cap on that part of the supply as well. The 5v rail is currently at 5.18v.
I'm not sure what actually causes the blue/yellow bars - is this just how the palette ram is initialized or the proms? Or the line buffer? Does this indicate that the main CPU has not started?
I figured my best bet was to check on the clock generation circuitry.. Being a bit of a noob, I'm not sure the best way to test this. When I hook my o'scope up to the output from the 7404, I see a lot of echo in the signal and it's quite noisy. I tried to calculate the frequency it was oscillating at, and I get 4.7mhz (?!?!) which doesn't seem right at all - however, I could be doing something very wrong here.
TL;DR: What do the blue/yellow bars mean and how to I check the crystal oscillator properly?
Attached is a schematic of the oscillator circuit so you don't need to hunt it down in the full schematic.
Thanks for any help!
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