Troubleshooting a perplexing problem...

After all of the new developments on the CoCo scene, I decided to set up a CoCo to play around with. My re-pack is in no condition for this yet, so I pulled my 3 CoCo3's out of the closet and picked one of the 512k models. Wouldn't you know it, it has developed an issue. Seems it doesn't like the FD-502 FDC I have that works fine with my other CoCo3's (and 2's and 1's).

The descriptions of the problem can be found in the MaltedMedia mailing list under the following subjects:

Re: [Coco] Weird CoCo3 problem

and

Re: [Coco] Weird CoCo3 problem - AAAARRRGGGGGHHHHH!!!!

I have checked the power supply and pretty much ruled it out; all of the waveforms match the CoCo3 Service Manual.

I started checking the signals and comparing waveforms and had a few issues with some. For ALL of the pictures below,

execpt for one and it's noted, I set the volts/div and time/div as specified in the CoCo3 SM.

The pictures below are for the guys helping me diagnose the problem (good luck to us all):

This is the waveform of TP2, ECLK - supposed to be a 5vp-p square wave

Here's TP3 - QCLK - again, supposed to be a 5vp-p square wave, and apparently according to the SM, time shifted a bit - these two signals are identical.

Here's TP4 - /RAS - Supposed to be 5v with negative pulses. To save bandwidth, TP5 - /CAS is the exact same waveform. These two signals, according to the SM, are supposed to be virtually identical to each othe - but, negative going pulses and slightly different in timing.

This last one is one Christopher Hawks told me to look at - P3 of IC9. It's supposed to be 5v with negative pulses. However, it's the EXACT same as TP2 & 3, this picture I was using a different Time/Div setting. Pins 1 & 2 of IC9 are identical to P3. P6 of IC9 is a logic HI at 5vdc, P9 of IC9 is close to 2vdc.

Looking at everything as a whole - there's a better than average possibility I'm wrong - but, with all of the malformed waveforms coming from the GIME I'm wondering if that isn't the problem. I've cleaned and reseated it, but, for E and Q to be so different from the pics, and the same signal being on pins 1,2 & 3 of the 138N chip (IC9) it has me wondering. I'm hoping someone else can point me in a direction to either prove or disprove that. Tho the computer is not displaying any of the tell tale signs of GIME problems (graphics issues, sparklies,. etc...)