Up and Running

[bugman]

Any ideas where I should start to look for a problem? Some of the ICs were difficult to insert, and some of the leads had oxidation on them. I was thinking of starting by re-seating some of them.

[Geoff Harrison]

Originally Posted by bugman

I was thinking of starting by re-seating some of them.

That's where I would start. Also, make sure your power supplies are not taking a while to stabilize, but that's a long shot. It might be instructive to pull the CPU chip and power it up. If the front panel is immediately stable then something is not halting the CPU on start up. If it isn't stable when the CPU isn't there then perhaps the front panel itself is flaky. You should be able to examine and modify memory from the front panel even without the CPU.

Edit: Well, maybe not. I checked that the front panel worked without the CPU before posting that and it worked fine, then I checked it again after and it doesn't work, so take that statement with a grain of salt. I'll try it again and let you know what I find.

Geoff.

[Geoff Harrison]

Well, I guess I don't know what I'm talking about. I can't get the front panel to do anything without the CPU now, so forget what I said.

Geoff.

[bugman]

Thanks, I'll do some troubleshooting tonight when I arrive home. I remember that my incoming 16 volts from the PSU is more like 14.7. I could also try tweaking it closer to the speced 16 volts.

I noticed that yours was over 16 volts.

[Geoff Harrison]

Ok, I knew I wasn't (completely) crazy. You can run the front panel without the CPU, but you have to pull BA (CPU pin 7) high. I removed my 6800 and connected CPU pin 7 to pin 4 (IRQ) which is tied high through a 3.3k resistor, and my front panel works fine.

Does anyone know why it worked for me once and then stopped? BA is driven through CMOS buffer PP, can a CMOS buffer whose input is floating drive its output high some times and low other times, at random?

Geoff.

[bugman]

I followed your lead in a different way. I plugged the 6800 cpu directly into the motherboard without the 32k expansion. All symptoms dissappeared, and it runs fine. So, its something with the cpu to expansion BRD connection, expansion BRD to motherboard connection, or some other problem with the expansion card. I need to leave home again, but I'll continue this evening. Any other thoughts based on this new info? I was carefull about seeing the pins making contact to the socket in the motherboard.

[Geoff Harrison]

Well that's encouraging. Unfortunately, I don't have a memory expansion board, so I'm not going to be much help there. Maybe Grant will have some suggestions when he logs in.

Geoff.

[bugman]

The board works by installing the cpu into its socket. Then the mini board has through connectors on the bottom that plug into the original cpu socket on the motherboard. If I am reading the info correctly, I have identified that pin 34 does not make the connection through the socket into the motherboard by using my meter. This would be easy to fix. Of course I need to confirm that this isn't by design. I'm reading the schematics to try and determine. I'm sure Grant will be able to confirm.

[Geoff Harrison]

Ah yes, that would be a Bad Thing. That's the Read/Write line from the CPU. I don't see anything on the circuit diagram that's not meant to pass through to the motherboard.

Geoff.