BBC Model B No2. Will it live again!

Any 8bit repairs, images, cool things, upgrades etc.
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rubber_jonnie
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Re: BBC Model B No2. Will it live again!

Post by rubber_jonnie »

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!
Collector of many retro things!
800XL and 65XE both with Ultimate1MB,VBXL/XE & PokeyMax, SIDE3, SDrive Max, 2x 1010 cassette, 2x 1050 one with Happy mod, 3x 2600 Jr, 7800 and Lynx II
Approx 20 STs, including a 520 STM, 520 STFMs, 3x Mega ST, MSTE & 2x 32 Mhz boosted STEs
Plus the rest, totalling around 50 machines including a QL, 3x BBC Model B, Electron, Spectrums, ZX81 etc...
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Re: BBC Model B No2. Will it live again!

Post by rubber_jonnie »

@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.
Collector of many retro things!
800XL and 65XE both with Ultimate1MB,VBXL/XE & PokeyMax, SIDE3, SDrive Max, 2x 1010 cassette, 2x 1050 one with Happy mod, 3x 2600 Jr, 7800 and Lynx II
Approx 20 STs, including a 520 STM, 520 STFMs, 3x Mega ST, MSTE & 2x 32 Mhz boosted STEs
Plus the rest, totalling around 50 machines including a QL, 3x BBC Model B, Electron, Spectrums, ZX81 etc...
BennehBoy
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Re: BBC Model B No2. Will it live again!

Post by BennehBoy »

My pleasure, I hope they help.
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rubber_jonnie
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Re: BBC Model B No2. Will it live again!

Post by rubber_jonnie »

BennehBoy wrote: Tue May 18, 2021 2:14 pm 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.
Collector of many retro things!
800XL and 65XE both with Ultimate1MB,VBXL/XE & PokeyMax, SIDE3, SDrive Max, 2x 1010 cassette, 2x 1050 one with Happy mod, 3x 2600 Jr, 7800 and Lynx II
Approx 20 STs, including a 520 STM, 520 STFMs, 3x Mega ST, MSTE & 2x 32 Mhz boosted STEs
Plus the rest, totalling around 50 machines including a QL, 3x BBC Model B, Electron, Spectrums, ZX81 etc...
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PhilC
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Re: BBC Model B No2. Will it live again!

Post by PhilC »

@rubber_jonnie I have a spare BBC Master if that's of any interest? Bit tatty but works.
If it ain't broke, test it to Destruction.
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JezC
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Re: BBC Model B No2. Will it live again!

Post by JezC »

PhilC wrote: Tue May 18, 2021 5:29 pm @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...
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Re: BBC Model B No2. Will it live again!

Post by PhilC »

@JezC ill get it booted up, cleaned and some photos done then when I'm home later this week.
If it ain't broke, test it to Destruction.
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Re: BBC Model B No2. Will it live again!

Post by JezC »

@PhilC Great...no drastic rush on my part (though @rubber_jonnie might be in more of a rush).
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Re: BBC Model B No2. Will it live again!

Post by rubber_jonnie »

@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!!
Collector of many retro things!
800XL and 65XE both with Ultimate1MB,VBXL/XE & PokeyMax, SIDE3, SDrive Max, 2x 1010 cassette, 2x 1050 one with Happy mod, 3x 2600 Jr, 7800 and Lynx II
Approx 20 STs, including a 520 STM, 520 STFMs, 3x Mega ST, MSTE & 2x 32 Mhz boosted STEs
Plus the rest, totalling around 50 machines including a QL, 3x BBC Model B, Electron, Spectrums, ZX81 etc...
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Re: BBC Model B No2. Will it live again!

Post by rubber_jonnie »

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

RAS_CAS.png
RAS_CAS.png (36.35 KiB) Viewed 2150 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
CAS_RAS.png (96.89 KiB) Viewed 2150 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.
Collector of many retro things!
800XL and 65XE both with Ultimate1MB,VBXL/XE & PokeyMax, SIDE3, SDrive Max, 2x 1010 cassette, 2x 1050 one with Happy mod, 3x 2600 Jr, 7800 and Lynx II
Approx 20 STs, including a 520 STM, 520 STFMs, 3x Mega ST, MSTE & 2x 32 Mhz boosted STEs
Plus the rest, totalling around 50 machines including a QL, 3x BBC Model B, Electron, Spectrums, ZX81 etc...
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