Model B Video gone fuzzy/corrupt and blurry

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Elminster
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Model B Video gone fuzzy/corrupt and blurry

Post by Elminster » Sat Apr 11, 2015 7:50 pm

The Victim
——————
BBC B Series 4. Recently done Caps upgrade and 1772 DFS 2.26 upgrade and all was well. Was testing a RAMROM board and left the machine running while I was reading a manual .......

The Incident
——————
All of a sudden and there was a change of pitch to the general whine. Looked up and screen was all fuzzy and unreadable.

Restarted it and goes through power cycle okay. You can type ‘vdu 7’ and it will beep, can type ‘mode <n>’ and the screen will change to a different colour/format of fuzzy. (See attachments, the first one is at boot, I forget which modes the others were from).
IMG_1888.jpg
IMG_1889.jpg
IMG_1890.jpg
IMG_1891.jpg
Had a look at http://homepages.paradise.net.nz/mjfoot ... Faults.txt


The suspect
—————-
And sounds like this (?) :
Fault: Machine appears to initialise Ok but SYNC on picture is incorrect.
Fix: Not talking to VIDEO ULA. Check VIDPROC signal.
Check IC21 74LS00 Pins 4,5,6 for VIDPROC in and out.
Check IC41 74LS02 pins 2,3,1. Check Pin 1 for output.
Check data lines on VIDEO ULA.
Check IC25 74LS20 pins 9,10,12,13 and 8.

Or the ‘Trouble shooting the BBC Micro’ says that background and text are influenced by IC2, IC5 and IC6.

The verdict
————---
Does that sound about right? It isn’t the physical connector as does it for both RGB and TV connector.

Now bearing in mind I have my brand new scope (and only read 10 pages of the scope primary that MartinB send me) how to I go about testing it? No hurry I have two other machines and loads to do on my to do list. So treating this as a learning exercise.

Comments, ideas, suggestions, tutorials etc.

Thanks.

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1024MAK
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Re: Model B Video gone fuzzy/corrupt and blurry

Post by 1024MAK » Sat Apr 11, 2015 7:57 pm

First, if you have chips that are known good that you can swap, try changing the video processor/ULA (use the same type) and then the 6845 CRTC. Try the suspect chips in a known good working Beeb.

Then, and only then do you attack with a 'scope...

Oh, and check the supply voltages to all these chips.

Mark
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Elminster
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Re: Model B Video gone fuzzy/corrupt and blurry

Post by Elminster » Sat Apr 11, 2015 8:02 pm

I have no spares of anything at present so would need to buy, beg, borrow and steal.

I dont really want to start pulling stuff out form the working B+, or putting broken bits in it if I can help it.

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Re: Model B Video gone fuzzy/corrupt and blurry

Post by 1024MAK » Sat Apr 11, 2015 8:36 pm

Elminster wrote:I have no spares of anything at present so would need to buy, beg, borrow and steal.

I dont really want to start pulling stuff out form the working B+, or putting broken bits in it if I can help it.
Well, swapping chips between machines is what I and many other people do. I do this at work (as instructed by the equipment manufacturer) and at home.

It is very unusual for a chip that operates off a single 5V logic supply to damage other chips. Same applies to a board that only has mostly a +5V logic supply (the -5V is not used for, or used anywhere near the video-proc/ULA or the 6845 CRTC).

Do read the thread about taking some sensible anti-static precautions first.

Mark
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Elminster
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Re: Model B Video gone fuzzy/corrupt and blurry

Post by Elminster » Sat Apr 11, 2015 8:57 pm

It wasn't the theory, I was thinking more that I haven't check what is socketed and what is solder. Also not looked at how different B+ is from B.

I know from recent fitting of the 1772 to the B and B+ they aren't the same.

I could buy another Model B but I would have to explain that to my wife.

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Re: Model B Video gone fuzzy/corrupt and blurry

Post by DutchAcorn » Mon Apr 13, 2015 5:49 pm

You're in luck; I have a spare one I can send you for postage (£1,-). I just checked that it works. If it fixes the issue you can keep it, otherwise I'd like it returned. :D

Please PM me with your address if you'd like to have it (and allow a week or so for it to cross the Channel).
Paul

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Re: Model B Video gone fuzzy/corrupt and blurry

Post by Elminster » Mon Apr 13, 2015 6:19 pm

DutchAcorn wrote:You're in luck; I have a spare one I can send you for postage (£1,-). I just checked that it works. If it fixes the issue you can keep it, otherwise I'd like it returned. :D
A spare Model B?

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Re: Model B Video gone fuzzy/corrupt and blurry

Post by DutchAcorn » Mon Apr 13, 2015 6:24 pm

Elminster wrote:
DutchAcorn wrote:You're in luck; I have a spare one I can send you for postage (£1,-). I just checked that it works. If it fixes the issue you can keep it, otherwise I'd like it returned. :D
A spare Model B?
I need to be more specific. :roll:

I meant a spare video ULA.
Paul

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Re: Model B Video gone fuzzy/corrupt and blurry

Post by Elminster » Thu Apr 16, 2015 1:47 am

Ah. Thanks for the offer. I might get back to you after I have finished sorting my B+ out. Too many projects not enough time.

Regards

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Re: Model B Video gone fuzzy/corrupt and blurry

Post by Elminster » Thu Apr 23, 2015 11:39 am

An untested Beeb turned up today.

Turns out it is a working B Issue 7, with no econet or disc. Very dirty.

So needs a clean, new caps and then I can get to work try to get to the ottom of the missing video. (Have another Beeb B turning up soon that has got blown caps in it, got carried away).

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Re: Model B Video gone fuzzy/corrupt and blurry

Post by richardtoohey » Thu Apr 23, 2015 7:34 pm

Elminster wrote:An untested Beeb turned up today ... another Beeb B turning up soon
We need to find and stop these dangerous people sending you computers before you get in trouble with SWMBO! :lol:

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Re: Model B Video gone fuzzy/corrupt and blurry

Post by Elminster » Thu Apr 23, 2015 7:57 pm

Yes. Would be good if they didnt keep raiding my bank account though.

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Re: Model B Video gone fuzzy/corrupt and blurry

Post by Elminster » Sun May 03, 2015 7:42 pm

Swapped IC 6, 7 and 2. But no effect. I have all 3 version of ULA spread across a series 4, early 7 and late 7. I tried both series 7 ULAs as the Early one has the Acorn wire fix. ( I read it on beebwiki).

Will have another look later after next job is to fit econet to late series 7.

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Re: Model B Video gone fuzzy/corrupt and blurry

Post by Elminster » Fri Aug 10, 2018 2:40 pm

Had a few mins to look at this today, since last looking at it in 2015!

Swapped out all the socket video chips, with know goods. Piggy backed all the 74LS mentioned above except the 74LS20 one, which I dont have a spare of.

No difference. Force to 16k machine, no difference, but issue in all modes (did the sas 5050 and6845 for completeness).

Probably onto multimeter and scope next, as run out of IC to swap. Pretty sure I tried typing in a sound command and it works last time, but will double check.
Last edited by Elminster on Fri Aug 10, 2018 2:44 pm, edited 1 time in total.

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Re: Model B Video gone fuzzy/corrupt and blurry

Post by Elminster » Fri Aug 10, 2018 3:57 pm

I have ICE-T plugged in. Though I might as well prove it was not an issue with the 6502. And behaves the same

The screen stabilzers when I halt it. When I run the mem test for the 0000 to 7FFF everything returns as passed.

Interesting if I now update mem location 716C with C5 I can make the screen fill with 'E', if it has C4 it is full of 'D' and C6 gives me an 'F'.

Did do it after a reset, but if I run the mem test again from 7100-7200 after blindly typing 'mode 6' it writes a letter to the screen. (same letter to every part of the screen). No idea if this means anythign, just playing really. But I guess it proves some bits are working, just not sure what!

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Re: Model B Video gone fuzzy/corrupt and blurry

Post by Elminster » Fri Aug 10, 2018 5:11 pm

So just to prove it is in mode 6 at switch on. I just break in to the cpu

f 6000 6fff 58

And I get a screen of 'X'. (Hex for 58)

Not convinced that means it is working and in mode 6 though, as you can type 'mode x' and try filling rane you expect mode to be part of mode and it doesnt work.

But filling 6000 to 6fff at start up always seems to work.

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Re: Model B Video gone fuzzy/corrupt and blurry

Post by Elminster » Wed Aug 15, 2018 10:50 am

As per other threads.

ICET proved no issue with 6502 and passed all memory tests on 32k of DRAM

After messing about tested output on composite and RF and both have the same display issue, so not a problem with RGB connector.

Seem to have a second issue where machine is freezing. This seems to take a while when machien is cold and gets more frequent as machine warms up, trying to ignore this until video fixed. You can see this wehn using scope or running live decode with fx2pipe/logica analyser (which (seemingly) shows it stopping in random places. something to check later on). This is a pain though as have to keep 'breaking' to bring machien to life to test video issue.

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Re: Model B Video gone fuzzy/corrupt and blurry

Post by Elminster » Thu Aug 16, 2018 8:22 pm

The joys of not so great paper schematics. Been scratching my head to why ic25 does seem to do what it says in schematic, on any Beeb, only to realise when viewed on the online version you can see it actually says ic26

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Re: Model B Video gone fuzzy/corrupt and blurry

Post by hoglet » Fri Aug 17, 2018 9:11 am

It might be worth doing a few more tests with ICE-T on the video memory.

I would try the following:

1. <return> on the ICE-T to enter command mode, the do RD 0355 to establish the current screen mode, then C to continue (for the rest of these thoughts, I'll assume you know how to use the ICE-T commands)

2. Blind-type Mode 0 on the Beeb

3. Repeat (1) to establish the mode has changed to mode 0

4. Use the ICE to fill 5800-7FFF with &00 - does this have any effect on the screen? (before and after photos?)

5. Do a CRC on 5800 to 7FFF. Repeat CRC three times, wait one minute, repeat CRC three times. Are they all the same?

6. Use the ICE to fill 5800-7FFF with &FF - does this have any effect on the screen? (before and after photos?)

There are some clues in your previous photos. If you look carefully, you can see a stable cursor, which means most of the video circuitry is OK, the 6845 is generating correct vsync/hsync signals and border blanking, etc.

I would guess the problem is:
a) In the video address generation: IC2 (6845), IC 8-11 (81LS95).
b) In the video data path IC6 (Video Processor), IC14 (74LS245)

As you have already swapped IC2 and IC6, I think (b) is more likely, given the photos you have posted.

I would be suspicious of IC14 or it's control signals, as the interference/noise looks to me like the 6502 clobbering the video data.

You could check IC14's control signals (pins 1 and 19) are what you would expect.

Then I would be tempted to cut it out, fit an IC socket, then fit a new replacement. This part is very timing critical I believe, so do replace with the same type.

Dave

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Re: Model B Video gone fuzzy/corrupt and blurry

Post by Elminster » Fri Aug 17, 2018 10:48 am

hoglet wrote:
Fri Aug 17, 2018 9:11 am
It might be worth doing a few more tests with ICE-T on the video memory.

4. Use the ICE to fill 5800-7FFF with &00 - does this have any effect on the screen? (before and after photos?)

6. Use the ICE to fill 5800-7FFF with &FF - does this have any effect on the screen? (before and after photos?)
I played with this before, but no photos, yes I can effect the screen.
There are some clues in your previous photos. If you look carefully, you can see a stable cursor, which means most of the video circuitry is OK, the 6845 is generating correct vsync/hsync signals and border blanking, etc.
Yes, you can get the cursor to move by hitting return (or can now I swapped the 6522 back)


I would guess the problem is:
a) In the video address generation: IC2 (6845), IC 8-11 (81LS95).
b) In the video data path IC6 (Video Processor), IC14 (74LS245)
As you have already swapped IC2 and IC6, I think (b) is more likely, given the photos you have posted.

I would be suspicious of IC14 or it's control signals, as the interference/noise looks to me like the 6502 clobbering the video data.

You could check IC14's control signals (pins 1 and 19) are what you would expect.

Then I would be tempted to cut it out, fit an IC socket, then fit a new replacement. This part is very timing critical I believe, so do replace with the same type.
I have replaced or piggy backed all the video circuit chips, except IC14 as dont have spares of that one (about the only one I dont have spares of so wouldnt surprise me if that was the one broken). On my massive spreadsheet of things IC14 is also one chip I havent got to testing yet.

Edit: My list contains (for one reason or another)
IC8, IC9, IC10, IC11, IC12, IC13, IC6 (Vid ULA), IC2 (6845), IC5 (SAA5050), IC14, IC15, IC40 (CLK), IC43 (CLK) , IC32 (SCRAD), IC39 (SCRAD), IC21, IC26, IC41, IC48

It gets longer all the time as Video circuitary seems to grab the odd gate from other glue logic chips in the system. Some chips on list, like SAA505, I am ignoring currently as unlikely.

Definely not the 6845, as that one was easy to run in an issue7 and it works fine. And vice verse in that putting it in broken machine. IC6 is a bit more tricky as I dont have any spares of the type used by Issue 4, only the later fixed ones, so results not really conclusive.
Last edited by Elminster on Fri Aug 17, 2018 11:07 am, edited 1 time in total.

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Re: Model B Video gone fuzzy/corrupt and blurry

Post by Elminster » Fri Aug 17, 2018 1:45 pm

Actually I have some 74LS245N I bought a while ago for something else. Piggy backing has no effect. Although a bit of an assumption the spare actually work.

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Re: Model B Video gone fuzzy/corrupt and blurry

Post by hoglet » Fri Aug 17, 2018 2:03 pm

Elminster wrote:
Fri Aug 17, 2018 1:45 pm
Piggy backing has no effect.
Piggy backing works in quite a narrow set of failure cases, i.e. specifically when an output has gone open circuit, or is very weak. As the ICE-T can read/write video memory, we known that is not the case here.

IC14 is a very critical component in a Model B, as it isolates the video data bus from the CPU. If it is in any way "out of spec", all manner of weird faults can arise. And sometimes even if the part is within spec! For example, some people have reported that replacing with a HCT or a ALS didn't work. I worked out the worst case RAM timings a while back, and this part of the Beeb is definitely working on a wing and a prayer!

Have you checked pins 1 and 19 on a scope?
Last edited by hoglet on Fri Aug 17, 2018 2:05 pm, edited 3 times in total.

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Re: Model B Video gone fuzzy/corrupt and blurry

Post by Elminster » Fri Aug 17, 2018 2:38 pm

hoglet wrote:
Fri Aug 17, 2018 2:03 pm
Have you checked pins 1 and 19 on a scope?
Not yet. I am still away away from scope. I was still checking continuity.

Turns out IC14 is socketed on the issue 7, so tested my picky back chip in there and that works. So at least I know I piggy backed with a known go LS254. 6.5 chips done, 13 to go. I may have tested at some point, I was forgetting what I had and hadnt done, which was why I have ended up putting in a spreadsheet and just systematically working my way through. But I could cheat and skip to ic14 and the scope.

While I was at it I retested the 6845 in the issue 7, which works fine, as I expected it to.

I also retest the ULA. This is slightly tricky as the Issue 7 has a VC 2023/201647, my 2 spare ULAs are VC 2069/201647 and the issue 4 has the original heat sink ULA 5C094E.But if I use he inverted switch on the Issue7 I get a white screen with text on the issue7 with the issue 4 ULA. The 2023 does seem to work at all on the issue 4 (assume due to link wires) But weirdly the 2069 behave the same on the issue 7 and issue 4 as the issue 4 ULA. As far as I can tell with out have another machine that use the original 5C094E (or hacking about with link wires) the ULA is working.

I didnt bother to retest the SAA5050, and nothing else is socketed that looks relevant.

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Re: Model B Video gone fuzzy/corrupt and blurry

Post by Elminster » Fri Aug 17, 2018 3:13 pm

Probably will unsolder on mass if need be. I.e. put in sockets for IC8-IC15. But that is a next week job. channel 1 is pin 19, channel 2 is pin 1. (counterintuatively)

So pin 19. High is 4.16V - amp = low = 0.3v

pin 1 High 4.16v with amplitude of 4.01.

You can probably guess all that as both channels are 2v per division

Edit: Or were th probe the other way around. Hmm, need to remmeber to put the little colour rings on.
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Re: Model B Video gone fuzzy/corrupt and blurry

Post by Elminster » Fri Aug 17, 2018 4:44 pm

And to answer question in Logic Thread

RST & /RST are flipping on power up and soft reset, but /RSTA is only ever low on power up. It doesnt drop to low. Neither of them are getting stuck in the wrong place when the system hangs. And the reset is correct on the 6502 as well. So I think the reset circuit etc is probably a red kipper, and can be ignored, at least for now.

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