Ok, so reading the service manual (I should have done this first) shows RAS & CAS like this:
- RAS_CAS.png (36.35 KiB) Viewed 3003 times
Scoping both CAS 0 & CAS 1, I can see something on the Fisher-Price scope that looks very similar to what I should see.
RAS on the other hand is just not behaving at all. It is a flat line, with some sort of frequency on the output pin.
RAS is driven via IC44 and IC43:
- CAS_RAS.png (96.89 KiB) Viewed 3003 times
IC44 Pin 2 receives a 4Mhz signal and pin 3 receives an 8mhz signal, both from IC6. I believe these signals are arriving, and you'd expect there to be an output from pin 6 that is active low, and passes though an inverter, where I'd expect to see a signal much the same as CAS, but with lows where CAS has highs and vice versa. There is nothing, nothing at all.
The lack of RAS is troubling as based on the service manual it should be present. Given that CAS0/1 are cycling as expected and have a decent TTL signal all the way back from the chips to the 74S139, that suggests that the 4Mhz signal consumed by both sets of gates in IC 45 is good, and since IC44, the 74LS74, also consume that same signal, that leaves 2 possibilities for the lack of RAS.
1. The 8Mhz signal is bad.
2. The 74LS74 is bad and the flip flop is neither flipping nor flopping and there is therefore no TTL level change on pin6 and no RAS. Continuity between chips is fine by the way.
I have a replacement, so I will replace and see if I start to get RAS again.
I also need to check the ROMs
@BennehBoy sent me too.
otherwise why would I have so many STs!
