(Topic ID: 273017)

Stern-200 Memory Failure

By Toastin

3 years ago



Topic Stats

  • 7 posts
  • 3 Pinsiders participating
  • Latest reply 3 years ago by Toastin
  • Topic is favorited by 2 Pinsiders

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  • Meteor Stern Electronics, 1979
#1 3 years ago

Ok, I'm going to have to call in help on this one. I'm working on a Stern-200 mpu, yes had battery damage, all cleaned up, reset section rebuilt, new machine pin sockets at U8, U11, U13, U14. Board was locked on prior to the work. Jumpers set for U2+U6. Working with known good CPU,PIA,U7 and 5101 chips as well as nvram options. I'm getting the board to boot, 2 flashes and fail. I've burned Leon's version 4 ROM and get the pulse as expected. Then when hitting S33 I get the control light on dim and the mpu board light off, indicating bad U8 memory. I've checked voltages at the chip and I have some abnormal voltage, but can't find what's pulling them down. All good except pin 5, should be 1.5v, I'm 2.35v, pin 7 should be 2.5v I'm getting 1v, pin 20, I'm getting 2.4v instead of 3.5-4v. these voltages are the same back at the CPU (U9). I have checked for shorts on all pins on U6+13. I have good continuity on all pins listed on pinwiki except U8-20 to U18-7, but do have it to U14-10 like the schematic says I should. Removing both PIA chips and all memory chips I get the same voltage reading so it's not them pulling it down. At the A0-A7 resistors I have the 5v on the right side of R-129, but 1v on the left side I swapped the resistor just because, no difference. Any thoughts on what to check next? Thanks, sorry for the long post, wanted to provide as much info as possible.

#2 3 years ago

Yeesh not fun. sounds like multiple problems. I dont do much board work anymore but Ill throw out a few things.

first, the address line R129 pull up being pulled low. I dont have a schematic in front of me, but if I remember right, the only thing that should pull down an address line is MPU U9, correct? on first hunch, bad U9 or more likely a short on the bus. I would measure resistance on the address line resistors to ground with the power off. compare a couple others to R129. if its different, start removing chips that use A7 till it goes up. if not, then youll have to start cutting traces to isolate the short on A7.

you could have a bad logic gate causing U8/U13 to be selected when theyre not supposed to be. that causes all kinds of crazy stuff.

I have found U8 and 13 need to be a matched pair and high speed at that. I think they are r/w together as one chip.

forgive my ignorance on the test chips you mentioned. I was a board tech in the 80's and we used the Bally AID system and the stern equivalent (faded memory) to troubleshoot bus problems.

#3 3 years ago

Thanks! I'll dig around the A7 lines, I'm thinking a logic gate is where my problem is at, I already replaced (twice) U14 no difference. I don't see any other logic gates IC's A7 goes thru. I've installed nvram from nvram.weebly.com designed for the Stern-200 just to make sure I removed non paired/slow U8/U13 pair issues.

#4 3 years ago

"I have good continuity on all pins listed on pinwiki except U8-20 to U18-7"

I would also try to track that down, visually follow the traces between those 2 points...i suspect it could be a broken trace or solder joint somewhere, possibly even under a chip. Good luck!

#5 3 years ago

frunch I don't believe U8-20 actually goes to U18-7, according to the Schematics it goes to U14-10, and I have continuity to there. I followed the entire trace and don't see how it could get there. I have several -35 and -200 MPU's and none of them have continuity to U18-7 and all boot up and work fine. I going back to nothing except Leon's rom in U6 and the CPU and check voltages again.

#6 3 years ago

Right on, just seemed odd. I figured if you ran down all the other leads and came up empty-handed that might be something to try. That's weird, though! Not sure why they would have included that in pinwiki. For that matter, i found the same info in Clay's guide as well...

Hopefully you'll get to the bottom of this soon!

#7 3 years ago

OK,
Just an update, I lifted Pin 16 of IC9 (CPU) A7 line, I used an alligator clip and a spare lead to bypass the majority of the MPU and get the signal to U6 to boot Leon's v.4 rom. success. I then pulled the lead off the board, and could get a pulsing read off the A7 line right from the CPU. I removed All IC's except U6 and U9 at this point and it still drops to 1v once connected to the board. I now suspect this might be due to a failure at U7 stopping the test code there. U7 only is addressed by A0-A6, so A7 could be low on design. This is leading to R/W line being the issue, but it being able to read U6 to boot has me baffled.

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