Alan173
Well-known member
O.k.......where to begin. I'm pretty darn far into a Defender repair but I seem to be getting stumped the past few days and thought I would throw out a scenario for the Defender experts to chew on.
I started with two dead linear power supplies....have them now working perfect......between them 3 out of the 4 voltage regulators were toast. A bunch of cracked solder....bad fuse holders.....but after that the +5, -5, +12 are running smooth as silk and getting to all boards.
So on to the CPU. The CPU was dropping out the +12 and found out I had a 0.1uf cap in the RAM circuit dead shorted. Fixed that and +12 right back to life with everything hooked up.
I had some signals being dropped in the signal decoder circuit, the watchdog circuit, etc. and have everything corrected that I can find.
Reset line.....works perfect...cycles one second low then goes high. Watchdog circuit is fine. My 5P pin 6 was not pulsing and I traced it back to a faulty interconnect cable between the ROM and CPU board....have that replaced with a new one. With the cable replaced my 5P is acting fine along with the 4K and 6K driving it.
I went through a flow chart in one of the troubleshooting manuals for the CPU and everything in the MPU address buffers, video RAM control, video RAM timing, shift registers, etc., etc. checks fine.
The only strange thing I see is chip 4H of the video RAM Address Multiplexer is supposed to be pulsing on pins 7 and 9. My pin 9 is stuck low.....but when looking at the schematic the output is listed as NC even though the flow chart says it should be pulsing? Not sure if that pin 9 is a typo in the flow chart or not. On the schematic there appear to be no inputs at pins 10,11,12 and 13.
So.....here are my symptoms. Power up game.....neck glow.....monitor fires up.....rumble in speakers then stops......screen is black.......I get two sets of 4 LED flashes on the ROM board indicating good RAM/ROM.....and that is about it. I get an intermitant ticking sound through the speakers from time to time. I have also noticed the 5P pin 6 sometimes sits at 'High'...but then eventually seems to start pulsing after a reset?
Interesting enough....I have another power board.....ROM board and CPU and they are behaving the same way....which is totally confusing.
It seems like everything I'm seeing should at least get me into test mode on this CPU?
Any ideas what to attempt to check? Seems like I have been through every circuit at least 5 times....so I must be missing something.
I hope the explanation helps a little.....and I hope somebody is in the middle of one of these repairs so we can swap notes....
Alan
I started with two dead linear power supplies....have them now working perfect......between them 3 out of the 4 voltage regulators were toast. A bunch of cracked solder....bad fuse holders.....but after that the +5, -5, +12 are running smooth as silk and getting to all boards.
So on to the CPU. The CPU was dropping out the +12 and found out I had a 0.1uf cap in the RAM circuit dead shorted. Fixed that and +12 right back to life with everything hooked up.
I had some signals being dropped in the signal decoder circuit, the watchdog circuit, etc. and have everything corrected that I can find.
Reset line.....works perfect...cycles one second low then goes high. Watchdog circuit is fine. My 5P pin 6 was not pulsing and I traced it back to a faulty interconnect cable between the ROM and CPU board....have that replaced with a new one. With the cable replaced my 5P is acting fine along with the 4K and 6K driving it.
I went through a flow chart in one of the troubleshooting manuals for the CPU and everything in the MPU address buffers, video RAM control, video RAM timing, shift registers, etc., etc. checks fine.
The only strange thing I see is chip 4H of the video RAM Address Multiplexer is supposed to be pulsing on pins 7 and 9. My pin 9 is stuck low.....but when looking at the schematic the output is listed as NC even though the flow chart says it should be pulsing? Not sure if that pin 9 is a typo in the flow chart or not. On the schematic there appear to be no inputs at pins 10,11,12 and 13.
So.....here are my symptoms. Power up game.....neck glow.....monitor fires up.....rumble in speakers then stops......screen is black.......I get two sets of 4 LED flashes on the ROM board indicating good RAM/ROM.....and that is about it. I get an intermitant ticking sound through the speakers from time to time. I have also noticed the 5P pin 6 sometimes sits at 'High'...but then eventually seems to start pulsing after a reset?
Interesting enough....I have another power board.....ROM board and CPU and they are behaving the same way....which is totally confusing.
It seems like everything I'm seeing should at least get me into test mode on this CPU?
Any ideas what to attempt to check? Seems like I have been through every circuit at least 5 times....so I must be missing something.
I hope the explanation helps a little.....and I hope somebody is in the middle of one of these repairs so we can swap notes....
Alan