exxos's Atari, Amiga & retro forum

Ok, so Tube bus snooper PCB on the way thanks to the very helpful @cmorley

Also bidding on another Beeb as I'm really getting pulled in by the nostalgia effect, as aside from an Apple II, my school computer lab was populated with 8 Beebs and I always wanted one, but could never afford it at the time.

And one is never enough :) otherwise why would I have so many STs!

@BennehBoy Thanks for the ROMs they arrived today :) It may be a little while before I can play with them which is annoying though!!

Annoyingly I just missed out on BBC number 3 on eBay, by just £3! Very annoying.

My pleasure, I hope they help.

BennehBoy wrote: 18 May 2021 14:14 My pleasure, I hope they help.

Me too :)

Also have a Tube snooper on the way which may help me see what code is being executed and hopefully where it's getting stuck.

@rubber_jonnie I have a spare BBC Master if that's of any interest? Bit tatty but works.

PhilC wrote: 18 May 2021 17:29 @rubber_jonnie I have a spare BBC Master if that's of any interest? Bit tatty but works.

@PhilC If @rubber_jonnie doesn't want it, I'd be interested...

@JezC ill get it booted up, cleaned and some photos done then when I'm home later this week.

@PhilC Great...no drastic rush on my part (though @rubber_jonnie might be in more of a rush).
:thumbup:

@PhilC Very kind of you to offer but the BBC Master doesn't have the same nostalgic vibes for me, it's the Model B that floats my boat.

If @JezC wants it please let him have first refusal :)

Now, fresh back from actually being able to go to a real live karate class, I'm feeling invigorated and have decided to read the BBC service manual!!

How crazy is that!!

Ok, so reading the service manual (I should have done this first) shows RAS & CAS like this:


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:


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.