Model B - MODE 7 character display issue - bit 5 always zero?!

[kieranhj]

Hey hardware peeps,

I recently procured a dusty classic Model B from a local loft. After getting it safely recapped, and plugging in an MMC ROM (as it was tape only), I turned it on today and got very confused:

The bottom line is me typing ABCDE..XYZ. The machine works perfectly in every other mode but 7, oh the irony! It looks like bit 5 of the character display is always set to 0, so P (ASCII 80) becomes @ (ASCII 64) and so on. This gets very confusing very quickly but the machine does run. Does anyone have any idea what's going on here? Do I likely need to replace the Teletext chip? Is that even possible.

Thanks as always!

[kieranhj]

Or bit 4 if you're counting from 0.

[simoni]

If you have a scope, I would start by checking IC15 (74LS273); if not replace that first since they are easier to source than the SAA5050.

A scope on pins 4 & 5 of IC15 and pin 8 of the SAA5050 would show you if D4 is the issue.

[kieranhj]

Thanks Simon. Sadly I'm limited in the hardware department so no scope. I will see if the hardware team at work can take a look, otherwise it might have to wait for generosity of a kindly soul at the next ABUG meet up I can get to.

[dominicbeesley]

If IC15 is socketed try taking it out giving it a clean, straightening the pins and putting it back in again...

[kieranhj]

If IC15 is socketed try taking it out giving it a clean, straightening the pins and putting it back in again...

Alas IC14 is socketed but not IC15. I took it out and resat anyway but no avail. Apparently we do have a scope at work with the hardware team but I'll need to figure out when I can lug my aging Beeb on the train into London.

[1024MAK]

A logic probe (which is much cheaper and much smaller) that has a pulse detection LED will at least tell you if the logic level is changing between logic zero (low) and logic one (high). Or if the logic level is "stuck". See this post.

Have you ran any BASIC memory test programs? Or loaded any games? If yes, did they run without problem (excepting the MODE 7 display corruption)?

Mark

[kieranhj]

A logic probe (which is much cheaper and much smaller) that has a pulse detection LED will at least tell you if the logic level is changing between logic zero (low) and logic one (high). Or if the logic level is "stuck". See this post.

Have you ran any BASIC memory test programs? Or loaded any games? If yes, did they run without problem (excepting the MODE 7 display corruption)?

Mark

Thank you for the tip (and link) - I will procure one as I'm sure it will be useful for my hardware adventures.

I plugged in my TurboMMC card and all the games work fine - I just can't decipher any of the game titles as the built in menu system on disc 0 is all in MODE7!

[kieranhj]

Something like this? http://www.maplin.co.uk/p/logic-probe-fy73q

[1024MAK]

Yes. It looks like that one at Maplin does have a pulse detector LED, but no buzzer. It will do the job, but you have to watch the LEDs carefully.

The type with a buzzer can be used without having to always watch the LEDs.

There do not appear to be as many types / makes of logic probe as there once was. Less remain I presume

Mark

[kieranhj]

Found this on Amazon with buzzer, can get it Prime next day. At least it's made in UK.

[1024MAK]

More info here, including a download of the instruction manual .
So, until I read the instructions, it was not clear if it could give an indication of pulses, but the manual say it can, so go for it

Mark

[kieranhj]

If you have a scope, I would start by checking IC15 (74LS273); if not replace that first since they are easier to source than the SAA5050.

A scope on pins 4 & 5 of IC15 and pin 8 of the SAA5050 would show you if D4 is the issue.

So today is my hardware day (see 32-bit thread of no joy on my A3020 ) But I have logic probe in hand and have checked the pins you recommended. Results are IC15 pin 4 is pulse, pin 5 is set low. SAA5050 pin 8 is set low. What might this suggest?

Any recommendations for next steps or tests much appreciated!

[kieranhj]

More info here, including a download of the instruction manual .
So, until I read the instructions, it was not clear if it could give an indication of pulses, but the manual say it can, so go for it

Mark

Thanks for recommending the buzzer Mark - this is definitely helpful. I can verify this probe works fine enough for hardware n00bs like me on a Beeb.

[kieranhj]

If you have a scope, I would start by checking IC15 (74LS273); if not replace that first since they are easier to source than the SAA5050.

A scope on pins 4 & 5 of IC15 and pin 8 of the SAA5050 would show you if D4 is the issue.

So today is my hardware day (see 32-bit thread of no joy on my A3020 ) But I have logic probe in hand and have checked the pins you recommended. Results are IC15 pin 4 is pulse, pin 5 is set low. SAA5050 pin 8 is set low. What might this suggest?

Any recommendations for next steps or tests much appreciated!

Ah, so looking at the SAA5050 datasheet pin 8 is D5 (character data input D1-D7) which is indeed held low, so bit 5 is always 0 as far as the teletext chip is concerned.

Looking at the 72LS273 datasheet pin 4 is an input, which is pulse, whilst pin 5 is output but set low. Would this suggest that IC15 is not working correctly and needs replacing?

I guess I'm going to have to ask my wife to bake another cake for my soldering guy at work as this will need to come off the board.

[1024MAK]

Yes, it looks like one section of IC15 (74LS273) is faulty.

Mark

[kieranhj]

Thought I would close this loop, as they say. So I got a replacement 74LS273N from eBay (~£2) and bribed my work colleague with some homemade mince pies for a soldering lesson + to remove + replace IC15. Back home and MODE 7 is working as expected, yay!

Thanks again to Mark & everyone for their advice and assistance. I now have a working Model B, a logic probe and a much better understanding of chip datasheets.

[leenew]

Good work!

Lee.

[1024MAK]

Well done

Mark

[poink]

At least it's made in UK.

Sorry, but I'm afraid it's not. From the listing, emphasis mine:

All our products are responsibly sourced both from trusted ethical partners across the world PLUS (uniquely as an automotive tool brand) our own manufacturing base in the UK.

The tools that are made in the UK are more specialist things; like engine timing tools for specific vehicles. These are made by Eldon Tool (a subsidiary in Sheffield), and from what I can tell, it's purely precision mechanical engineering - there's no electronics manufacturing capability.