Poorly BBC B

discuss both original and modern hardware for the bbc micro/electron
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Doctor_Beaker
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Poorly BBC B

Post by Doctor_Beaker »

Good day.

This is my first post and unfortunately, it concerns a problem with my BBC B. If any knowledgeable persons on the forum have come across this before, or could suggest where to start looking, it would be much appreciated.

I repaired the power supply a few years ago, after which the machine was healthy and was put away in storage. When it was switched on recently, it gave its familiar 'beep', but the screen display is rubbish. Attached is a photo of the display after switch-on, which was taken on a CRT telly, using the composite output. The RF output looks the same, but I don't have a monitor which would display an RGB signal.

The machine has an Issue 7 board and was fitted with Aries B12 and B32 expansion boards and Watford DDFS, but I've removed these and the machine is now essentially, basic.

The flashing cursor does appear on the screen and it moves along as something is typed on the keyboard, but no characters actually appear. Typing say, MODE 0, alters the display (second photo), though returning to MODE 7 might throw up a subtly different display to that shown at switch-on (e.g. Ws, instead of Bs).

My electronics knowledge is rather dated, but I do have basic test equipment, such as a DMM and an oscilloscope. Unfortunately, my understanding of the BBC's operation is limited and I really don't have the time or probably intellect, to delve into the technicalities.

Thanks in anticipation.

MODE 7.jpg
MODE 0.jpg
iainfm
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Re: Poorly BBC B

Post by iainfm »

Hi, and welcome!

Going to stick my neck out and suggest that may be RAM related. The usual trick is to disable the upper 16Kb and see if it persists. If so, disable the lower 16Kb and retest.

There are a couple of test roms that can be blown if you have the capability, and can be used to do some ram testing.

Before going too far though, start with the basics. Are -5V, +5V and +12V all in spec? Give any socketed ICs a push down (observe antistatic precautions) and see what that brings.

Here's a thread with some RAM diagnostics discussed: viewtopic.php?t=7934

Here's a good video as well: https://www.youtube.com/watch?v=B0yhhTZ ... etroClinic
e668ecp
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Re: Poorly BBC B

Post by e668ecp »

Looks like a RAM issue as Iain says. Loo for link S25 which is near the top right corner as you open the case. Remove it and you will get one of the 16k banks become the only active ram, switch on and retest. Then switch off and move the link so it occupies the lower two pins, this will activate the other 16k bank, switch on and test.

Report back then
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1024MAK
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Re: Poorly BBC B

Post by 1024MAK »

That looks like a mismatch in the addressing for the RAM.

Does Control G produce a beep?

Mark
e668ecp
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Re: Poorly BBC B

Post by e668ecp »

Actually I've fully read your comments now and I don't think it's a ram issue but rather I agree with Mark regarding an addressing issue
Doctor_Beaker
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Re: Poorly BBC B

Post by Doctor_Beaker »

Thanks to all respondents for those useful comments. Working through them, here's what has been achieved today:-

Test ROMS: At present, I have no means of making those.

Power supplies: they're fine.

ICs: I've pulled the CPU, VIAs and all socketed chips to the east, cleaned the pins, sprayed the sockets with contact cleaner, re-inserted them and switched on. No change.

Prodded all the RAM chips, in the hope that any dry joints would show up. No change.

The diagnostic and video links iainfm provided were very informative: thanks for those.

The S25 link: the attached photos show what happened when the machine was switched on after that link had been moved around. The red and green blocks only appear when the link is either off or south. Subsequently typing MODE 0 doesn't alter the initial display unless the link is north, when the result was that shown in my initial post.

CTRL+G: yes, it does beep.

So, not much progress today. Am I correct in assuming that the RAM is likely to be OK, since the problem is there whichever 16k bank is selected by S25?

S25 (North).jpg
S25 (Off).jpg
S25 (Sourth).jpg
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1024MAK
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Re: Poorly BBC B

Post by 1024MAK »

Yeah, so based on the available information so far, I would say that the problem is with the video system not being able to read the correct data from the RAM, rather than there being a RAM fault or a fault with the rest of the system.

Do you have any test gear? If yes, what?

Do you have another BBC B?

Mark
Doctor_Beaker
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Re: Poorly BBC B

Post by Doctor_Beaker »

Hello Mark.

Regarding having another beeb, the answer should be yes, but I've spent the past fortnight looking for it. I know that it accompanied us when we moved house and I never scrap or sell anything which could potentially be useful, so where it could be now is doing my head in. How can you mislay a BBC?

Test gear: lacking somewhat in that department. There's a DMM, an oscilloscope, a frequency meter that sometimes works, an RC oscillator and a home-made box with a test clip that can simultaneously display the status of each pin on a DIL IC (not a logic analyzer or anything fancy like that). I have various sizes of soldering iron and a desoldering station.

Regards,
Ian
NJ_
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Re: Poorly BBC B

Post by NJ_ »

Doctor_Beaker wrote:
Tue Nov 23, 2021 10:05 pm
The S25 link: the attached photos show what happened when the machine was switched on after that link had been moved around. The red and green blocks only appear when the link is either off or south. Subsequently typing MODE 0 doesn't alter the initial display unless the link is north, when the result was that shown in my initial post.
MODE 0 will have no effect with S25 south or removed because the machine will be configured with 16K RAM and mode 0 requires 20K. It will be reporting an error ('Bad mode', I think) but you won't be able to see it! So you could try one of the modes supported by the Model A: mode 4, 5, or 6 (I suggest 4).
Doctor_Beaker
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Re: Poorly BBC B

Post by Doctor_Beaker »

Thanks for pointing that out, NJ.
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