Defender CPU Help Needed....

Alan173

Well-known member
Joined
Nov 14, 2007
Messages
1,537
Reaction score
98
Location
Macomb County, Michigan
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
 
Wow Alan, I don't remember Defender being B/W.:D

Touch base with Saltbreez, he loves to dig into Defender's really deep like that.;)
 
Wow Alan, I don't remember Defender being B/W.:D

Touch base with Saltbreez, he loves to dig into Defender's really deep like that.;)

LOL!!! :)

Well...my screen is still black.....so maybe that is why I continue to work on it. You mean Defender is color??? ;)....
 
Are you sure the monitor is ok ?

Put a logic probe on the outputs of 2M see if you're getting any output pulsing.
Then check outputs of 2C and 1C see if you're getting pulsing.

If you are that'd point to a problem monitor.

- James
 
Good point though..... "screen is black" doesn't say whether it plays blind or anything.
I'm assuming the game is just dead with what's been mentioned, but would like to clarify.
 
Are you sure the monitor is ok ?

Put a logic probe on the outputs of 2M see if you're getting any output pulsing.
Then check outputs of 2C and 1C see if you're getting pulsing.

If you are that'd point to a problem monitor.

- James

ONce I hit reset and get the 5P pin 6 pulsing then.....2M....pins 4, 7, 9, and 12 are pulsing nice. Also pins 5, 7 , 9 and 11 of 1C and 2C are pulsing as well.

I did a quick check early on for the monitor and verified the fuse at 601 was good, b+ voltage was at 130vdc......the HOT was not shorted and I had neck glow.

Stumped.....
 
ONce I hit reset and get the 5P pin 6 pulsing then.....2M....pins 4, 7, 9, and 12 are pulsing nice. Also pins 5, 7 , 9 and 11 of 1C and 2C are pulsing as well.

I did a quick check early on for the monitor and verified the fuse at 601 was good, b+ voltage was at 130vdc......the HOT was not shorted and I had neck glow.

Stumped.....

What make and model monitor is in it Alan?
 
Good point though..... "screen is black" doesn't say whether it plays blind or anything.
I'm assuming the game is just dead with what's been mentioned, but would like to clarify.

Yes......dead....not playing blind. I only get a slight audible tick like it is watchdogging.....but the reset circuit and watchdog circuit checked fine....
 
Have you tested the monitor with another working game such as your Ms Pac?

I have not yanked it but will soon. There seems to be so many CPU issues that cause a black screen in Defender it amazes me. In another repair post that 5P chip with pin 6 stuck high caused the same symptom (or so I thought) making the game dead...just like mine. My new interconnect cable seems to have fixed that and corrected everything upstream of 5P as well.......but still no love.

I'm not sure if a CMOS RAM failure would take out the screen as well...those are still original....
 
If you're getting pulsing on the output of the color RAM and you have no display, either all 3 of the final color output transistors are bad ( unlikely ) or blanking is permanently active.

Check 4b pin 8, that should be pulsing. It's the blanking signal, active high.

Check the at resistors r20, r18 and r16 that should pulse with the blanking signal.

Also check the sync outputs for H & V, 3A pin 8 and 3A pin 11.

If all that's working, check the monitor hookup and if it really is working.

- James
 
If you're getting pulsing on the output of the color RAM and you have no display, either all 3 of the final color output transistors are bad ( unlikely ) or blanking is permanently active.

Check 4b pin 8, that should be pulsing. It's the blanking signal, active high.

Check the at resistors r20, r18 and r16 that should pulse with the blanking signal.

Also check the sync outputs for H & V, 3A pin 8 and 3A pin 11.

If all that's working, check the monitor hookup and if it really is working.

- James

Thanks James!

I have in my notes 3A pins 8 and 11 are pulsing fine....but one thing I did forget to check is the blanking signal...(Ooooops). I'll check it out. And yep...I agree...if the blanking signal is there maybe a monitor switch to a working cab to confirm it is actually showing a picture.

Alan
 
If you're getting pulsing on the output of the color RAM and you have no display, either all 3 of the final color output transistors are bad ( unlikely ) or blanking is permanently active.

Check 4b pin 8, that should be pulsing. It's the blanking signal, active high.

Check the at resistors r20, r18 and r16 that should pulse with the blanking signal.

Also check the sync outputs for H & V, 3A pin 8 and 3A pin 11.

If all that's working, check the monitor hookup and if it really is working.

- James

Hi James,

Had a chance to do some more checking today:

3A pins 8 and 11 - pulsing fine.

4B and 4C pin 8 pulsing fine.

R20, R18 & R16 - High.
R17, R19 & R21 - pulsing.

Q1, Q2 & Q3 output - pulsing.
Q4 - High. **This should be pulsing for the blanking signal to be correct right???**
R23 - High. Same here - should be pulsing???

Confirmed all 6 pins of 1J4 have pulsing signals up and to the monitor chassis. No cracked solder.

So I have checked the blanking, page 0 decoder, vertical count buffer gate and the count 240 circuits which to me are the only culprits to affect video output...and they all check fine.

What do you think?

My 5P pin still will not start pulsing upon game start up which is strange. I can 'jump start it' by pressing the advance button on the coin door - does that make sense?

Alan
 
Last edited:
I can 'jump start it' by pressing the advance button on the coin door - does that make sense?

What do the LED's do when you do that ?

ROMs all good I assume ?

What if you unplug the I/O board ribbon cable ?
(had a Stargate do some wonky voltage drop stuff because of that once)

Just throwing stuff out there.
 
Confirmed all 6 pins of 1J4 have pulsing signals up and to the monitor chassis. No cracked solder.

That does sound like the monitor if you're still not getting any display but you are getting output on the video connector.

- James
 
What do the LED's do when you do that ?

ROMs all good I assume ?

What if you unplug the I/O board ribbon cable ?
(had a Stargate do some wonky voltage drop stuff because of that once)

Just throwing stuff out there.

All 4 LED's flash twice. This is the first Defender I have worked on but have read that indicates a RAM and ROM test that passes - is that correct? I tried disconnecting the interface board cable.....no change....
 
Back
Top Bottom