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1980s_john
June 14th, 2011, 03:32 AM
Hi,

Having checked out the PSU, I have now tried connecting a PC to the serial port (1200 baud!). not a sign of any output, so I went back to basics and removed all the cards other than the CPU and serial cards, and followed the 'check out' procedure from the manual (thanks to Mike Holley for website).

I started probing with an oscilloscope, this showed the baud rate generator was working and correct clocks seen on peripheral bus and saw the 6800 appeared t have plenty of activity. I then traced the O2 clock on the bus. It appeared OK on the main bus, but didn't appear on the peripheral bus. This would cause the serial card to stop working! I traced the O2 signal and think the 7402 at IC5 is faulty, so my next job will be to replace this and re-test. Need to cut it out and de-solder & replace with a socket, I think a 74ALS02 will substitute.

I'm not looking forward to testing the memory cards, these are full of soldered in RAM chips - replacing one of these is not my idea of fun!

Regards,
John

glitch
June 14th, 2011, 03:48 AM
The 74ALS family is a good choice for 7400 series replacement...less current draw means the power supply will be happier, and everything will stay cooler!

If you have a lot of problems with the RAM cards, you might consider building your own using a modern SRAM like the 6264 (8K x 8 ) or 62256 (32K x 8 ). It's a good way to get up and running quickly (that's how I got my first S-100 system up...still use the 8K RAM card I built!) and if you decide to stick with it, you'll draw a /lot/ less current (400 mA for my board vs. 1.5A for my IMS 8K board).

sjones
June 19th, 2011, 04:11 AM
"f you have a lot of problems with the RAM cards, you might consider building your own using a modern SRAM like the 6264 (8K x 8 ) or 62256 (32K x 8 )."
I agree with you. That's the best thing to do, but if not sure in doing this, they might want to consider consulting those pc technical support (https://www.techienow.com/) out there for faster solution.

1980s_john
June 20th, 2011, 11:59 PM
Hi,

I changed out the 7402 on the MP-B motherboard to a new 74ALS02 (in a socket). This cleared up the clock signals going to the peripheral bus (although the 'square' wave had steps in the top and bottom of the signal, I think this is because the bus isn't terminated). Connected up a PC and miracle - now get the $ prompt from the SWTBUG monitor - great to finally see it! I am now able to explore further, having made sure that caps lock is on as lower case commands are ignored. I can view and change memory locations, and view the registers. I tried loading a program but I messed up the start address, time now to read the manual.

I tried two of the 8K memory cards, the bottom 4K of each seemed OK, but the top 4K of each had problems with bit 0 - odd, but a puzzle for another day. Still a lot more to do, but it's fun bringing this system slowly back to life again. Being all TTL makes getting to the chips relatively easy - I'm sure if I get stuck someone here will suggest what to try next, so thanks!

Regards,
John

glitch
June 21st, 2011, 03:16 AM
Excellent -- good work! It's completely normal for the square waves to look like garbage...it depends a lot on the slew rate of the circuit and whether what it's hooked to (sounds like long bus lines, in this case) will allow the circuit to ring. It's fun fixing old TTL systems...a big part of why I like working with S-100 stuff is the ease of troubleshooting and repair.