(Topic ID: 264624)

System 7 MPU board... no data lines


By uncivil_engineer

3 months ago



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  • 15 posts
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  • Latest reply 3 months ago by uncivil_engineer
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#1 3 months ago

So I picked up a used system 7 board that appears to be pretty clean. I am using Leon’s rom to try to get it up and running. So far I haven’t gotten the board to run Leon’s program.

The address lines are all moving as the should, and the cpu is trying to run... however I’ve got nothing on the data lines. Could a single failed PIA hold all the data lines low?

I am hoping to get this board running so I can load Firepower Deluxe on it.

#2 3 months ago

Are you trying to run it with the driverboard connected? If so, try it without (Leon's runs fine without it).

See if it's IC9.

#3 3 months ago

Yup, I’m running it without the driver board. Ic9 is the prime suspect right now. The problem is I don’t think I have any replacements.

#4 3 months ago

Ic9 checked out ok... I also noticed I don’t have a R/W line or VMA at IC 1, so the program is definitely not running.

#5 3 months ago

Ok, so still no progress. I have eliminate a lot of possibilities, but the board is still not running.

IC 8 and 9 check out ok.

Ive replaced the PIA at IC18. The outputs of the old one were floating at around 2 volts at the outputs. I have seen this in the past with bad PIAs so I went ahead and installed a socket and replaced it.

Per Clay's guide, I also looked at IC11 and IC12. IC12 was already socketed, and the 7408 checked out good. IC11 is a triple gate, and it appears to be working as I am getting signals at the inputs and pulses at the outputs.

There is a little bit of prior work on this board, but it all looks good at first glance. my next step may be to trace down all the connections for the prior work, and make sure there is not something amiss there.
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My chip tester has come in very handy today.
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#6 3 months ago

BTW, the other thing I have done is convert IC 17 to accept a 2732. My leon's test program is written to the 2732. Would this make a difference?

#7 3 months ago

I think I may have traced the issue down to the chip select signal at IC17, pin 20. It appears to be stuck high. It is supposed to be pulsing....

#8 3 months ago

The crystal looks like it got whacked pretty good. Make sure the "E" pin of the cpu is active. I think that test point near the CPU is the external cpu clock "E". Sometimes those crystal seals break open at the bottom and they get fouled up.

AMI is known to be problematic compared to other CPU brands. I'd try a new CPU since it is already in a socket.

It looks like the data buffer is in a socket and been replaced but did you look at both in and out sides? If the chip has a bad socket at enable pin the outputs may be dead.

Do you get any action at IC17 P20 when you reset the CPU. A blip low at all? If the software crashed out, that pin may be always high. The schematic im looking at is rough and hard to tell what lines are crossing over or electrically connected, but I think that enable comes from 74ls139 at IC 15. Can try with the ROM removed. I think the /CE should pulse then as the CPU scans for software when the address comes up. Check that W11 and W12 are set right for the EPROM used. Looks like those jumpers move the IC17 address around.

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#9 3 months ago

The crystal is banged up, but I still have nice wave at IC1, pins 38 and 39.

I pulled the rom chip, and I never see any pulses at pin 20 of IC17. Using the table above, I see that I am supposed to be getting inputs on pins 1,2,3 and outputs on 4,5,6, and 7. I can see the inputs cycling just fine, but I have no outputs at all on that side of the chip. I can't pull the chip til tomorrow, but am now thinking this is the problem.

Regarding the data buffer at IC9: I did pull that chip for testing. Per my little green box, the chip is fine. I am getting nothing on the inputs or outputs from that chip.

#10 3 months ago

I had problems with data shorts... I used the meter and
test continuity side by side on the bottom 40 pin connector...
I found that two bus lines had shorted together.
Once cleared... the test chip started to work...

#11 3 months ago
Quoted from uncivil_engineer:

The crystal is banged up, but I still have nice wave at IC1, pins 38 and 39.
I pulled the rom chip, and I never see any pulses at pin 20 of IC17. Using the table above, I see that I am supposed to be getting inputs on pins 1,2,3 and outputs on 4,5,6, and 7. I can see the inputs cycling just fine, but I have no outputs at all on that side of the chip. I can't pull the chip til tomorrow, but am now thinking this is the problem.
Regarding the data buffer at IC9: I did pull that chip for testing. Per my little green box, the chip is fine. I am getting nothing on the inputs or outputs from that chip.

To be clear, you are seeing pulsing on pins 1, 2 and 3 of IC15 (74139) but nothing on pins 4,5,6,7?

How about IC15 pins 13, 14, 15 (inputs) and 9,10,11,12 (outputs)?

Verify good ground at IC15 pin 8 and VCC at pin 16.

If you have good power and the inputs are pusling but no outputs are pulsing, I think you found your culprit.

#12 3 months ago

Ok, I replaced IC15, and the board now boots! I have 1 flashing LED just like I should, and moving lines on the PIAs. There is no activity on the led display, so I think I may have a bad BCD.

#13 3 months ago

So I was getting this board ready to run Firepower Deluxe on, and today I hit another snag... I have a bunch of recycled 2532s and 2716s I was going to use in the board.... However when I went to plug in my UV light to erase them, the bulb burned out
Oh well.... Hopefully it won't take long to get a replacement off ebay.

#14 3 months ago

It's always something. (Gilda Radner )

#15 3 months ago

Well here’s my final update...

I put a new 7447 chip in the board to get the segment display to work. This lead me to discover that both ram chips were bad. Once the ram chips were replaced the rest of the board appeared to be working fine.

So my new eprom eraser arrived in the mail and I set about getting a couple of 2532s ready along with a mess of 2716s and a 2732 to be loaded up with Firepower Deluxe.

After a session with my burner, I was all set...
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Then I turned it on and it worked! I was playing firepower deluxe...
A6736348-D607-446A-B756-64EEEB3F3D77 (resized).jpeg

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