Berzerk ZPU Board Repair Help

joneddy

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I know several of you have written post about this repair but I am stumped and i need a little help to hopefully get me moving in the right direction. I have been working on this board for a week and i can't get it past one beep and the LED stays on continuously. I treated the board for a small amount of alkaline damage to the usual area under the battery using a toothbrush and white vinegar and rinsing thoroughly with distilled water. I used a fiberglass pen to scrape off the residual crud and clean up the traces and transistors, resistors, and chip legs. Following the manual for trouble shooting the self-tests I have replaced the Z80 with a new one from Bob Roberts and replaced chips 7G and 8G along with all of the transistors in the reset circuit. I did have to run a couple of jumpers from 7G due to a couple of pads evaporating when I was desoldering the old chips. After all of this I was able to test the reset circuit and verify that it is working correctly using my logic probe on pin 26 of the ZPU(on start up it is low and then switches to high, when I press the reset it switches to low and then high also) . I replaced the flip flop divider 8A and transistor Q12. Still no change. I have checked continuity of the chips on the address bus(6B,7D) and the LED latch (7G,3A,8G). I am still working on continuity checks for I/O Decode circuits chips(5A,4A,6A,7E). My questions to you who have been down this road before are:
What else can I check for besides continuity before I just start replacing these chips? Does anyone have tables showing what the correct voltages should be on the pins of these chips? What is the likely hood of ROM chip(s) being bad and how can I test them?
Please any help would be greatly appreciated!
Thanks,
Jon
 
Here is an update, I swapped out the address bus(6B & 7D) and no change still on beep and the light stays lit. I changed out the ZPU to see if it made a difference, it didn't. I continued to check continuity of the chips that the manual said could be the problem with my multimeter but everything checked out. Please any help would be appreciated.
Thanks!
Jon
 
Anyone? How can I test the ROMs to make sure they are good?
Thanks,
Jon
 
If you have an EPROM programmer you can copy the ROMS and compare them with romident. Otherwise you'll have to get another set of ROMS to swap in.
 
I have an extra board set that unfortunately has alkaline damage as well. I am not sure if the ROMs are good on it either though. I guess I'll try swapping them one at a time. Does anyone know how much power should be supplied to the pin 6 of the Z80 from the flip flop divider 8A? I am reading around 3V does this sound correct?
Jon
 
Could corrosion under the header pins below the battery cause the issue I am having?
 
I have an extra board set that unfortunately has alkaline damage as well. I am not sure if the ROMs are good on it either though. I guess I'll try swapping them one at a time. Does anyone know how much power should be supplied to the pin 6 of the Z80 from the flip flop divider 8A? I am reading around 3V does this sound correct?
Jon

What are you testing pin 6 with? A multimeter? With a multimeter I get 2.4volts which I guess is an average but the only way to test it would be a logic probe or oscilloscope.

On pin 11 I get 5.00 volts if that helps.
 
FYI- I have a logic probe but honestly have little to no experience with it. If you detail what you need tested I may be able to get you readings from my working Berzerk.
 
Sorry, I get around 3V with the multimeter. I have a logic probe that I can check it with when I get home. But all that will show me is whether or not it is high or low correct?
Jon
 
Sorry, I get around 3V with the multimeter. I have a logic probe that I can check it with when I get home. But all that will show me is whether or not it is high or low correct?
Jon

I believe so. And if your game is not booting up at all I'm not sure that it will necessarily tell you what is wrong. Though like I said, I know little about troubleshooting with a logic probe. But I can take measurements for comparison.
 
Have you checked voltages at all the labeled test points on the board to make sure they are all close?
 
Yes I have checked the voltages at the test points. I have also checked on the chips to make sure they are getting the correct power to them.
 
Thats about all I would know what to do as well. Maybe I would check the pins for any shorts to ground. Any experts out there want to chime in?
 
An unfortunate welcome to the broken Berzerk club.

My situation sounds similar to yours, except I was getting 7/8 beeps.

Checked every cable for continuity, reseated every socketed chip and multi-metered the hell out of the thing checking the voltages at every checkpoint and on the ICs themselves.

Finally pulled the ZPU and VFB boards and they are with a local tech now who will hopefully be able to figure out what is wrong.

Sorry I'm no help, but just wanted to send some sympathy and well wishes that you find the problem.
 
When I press the reset button should the LED go out at all? I know when I press the reset button pin 26 on the Z80 goes low and then back to high. Does this mean the reset circuit is working correctly?
 
I just fixed a friends that was getting 6/8 beeps. It ended up being a header pin that is on the bottom right connector , pretty much directly under where the battery was. It was corroded where it is soldered into the board. It failed a continuity check. I ran a trace wire around for a quick fix and sure enough it booted up with all 8 beeps . Make sure you have continuity on all pins. Most of them looked good and i really had to look close to see it.
Dave
 
FYI- I have a logic probe but honestly have little to no experience with it. If you detail what you need tested I may be able to get you readings from my working Berzerk.

If it wouldn't be to much trouble would you check the logic state(H,L) for the pins on the ZPU, 7G, 8G, and 3A for me?
 
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