Display issue on my spare beeb

for bbc micro/electron hardware, peripherals & programming issues (NOT emulators!)
Post Reply
JoolsH
Posts: 526
Joined: Mon May 21, 2012 11:46 am
Contact:

Display issue on my spare beeb

Post by JoolsH » Sun Oct 29, 2017 2:18 pm

The display went on my BBC micro a few years ago - I was playing Chuckie Egg on a really hot day and the display just disappeared.

RobC kindly gave me a replacement ULA chip to try yesterday (thanks!), but I've just tried it today and unfortunately it's the same.

I tried hooking it up to my bigger CRT TV (over RGB), and I do see something.

Mode 7 on boot looks really dim. There's some blocks on the right, and if you look carefully you can see horizontal lines, some blocks on the left, and the cursor:
mode7.JPG
Mode 6 looks like this:
mode6.JPG
Mode 5 is just a block of white:
mode5.JPG
Anyone got any ideas on what this could be? Retroclinic are selling replacement 6845 chips for a fiver on eBay, worth trying one of those?

User avatar
1024MAK
Posts: 8008
Joined: Mon Apr 18, 2011 4:46 pm
Location: Looking forward to summer in Somerset, UK...
Contact:

Re: Display issue on my spare beeb

Post by 1024MAK » Sun Oct 29, 2017 2:38 pm

It may be worth a try, with swapping the 6845. However, it is doing something, as you do appear to be getting some kind of display. If the 6845 was completely failed, there would be no DRAM refresh (all programs including the OS code would crash) and you would get no picture of any kind (black screen).
It could be that the 6845 is fine and the problem is one of the many 74LSXXX series chips that switches the RAM address lines.

Have you loaded a game that plays music, does it work okay? If yes, then the RAM is most likely all okay.

Mark

JoolsH
Posts: 526
Joined: Mon May 21, 2012 11:46 am
Contact:

Re: Display issue on my spare beeb

Post by JoolsH » Sun Oct 29, 2017 3:18 pm

I don't have a disc drive here unfortunately - I tried loading a game off tape with no success, but that can be finnicky at the best of times even if you can see what's going on, so that doesn't prove anything. I can type simple BASIC sound commands in and they work.

RobC
Posts: 2328
Joined: Sat Sep 01, 2007 9:41 pm
Contact:

Re: Display issue on my spare beeb

Post by RobC » Sun Oct 29, 2017 3:24 pm

How is the invert link (S26) setup? Just based on the mode 6 output, it might be worth swapping S26 over to see if that improves things. Having it set to invert would also cause mode 7 to be dim.

If you can, it might be worth posting some pictures of your Beeb PCB (both sides) just so that we can see if it has the "invert" mod for the first batch of VLSI ULAs.

User avatar
vanekp
Posts: 555
Joined: Thu Nov 30, 2000 7:09 am
Location: The Netherlands
Contact:

Re: Display issue on my spare beeb

Post by vanekp » Sun Oct 29, 2017 3:53 pm

there is a mod you have to carry out to stop the display inverting if you have replaced an old video ula with a new one:- On 15/10/2007 23:22, Daniel Beardsmore wrote:
> I imagine most Beeb fans will be aware that the original Ferranti video ULAs
> on the Issue 4 and earlier would cook themselves and end up displaying all
> sorts of weird visual defects like snow, stripy colours, bad default palette
> entries (entry 3 in mode 2 showing red instead of yellow) and just general
> garbage on the screen.

Yes, they're supposed to have a heat sink -- and with that, they're OK
(until the white gooey heat sink compound dries out or oozes away, and
needs replaced).

> My first ever Beeb had this, and when I'd sold it to my cousin I later tried
> replacing the Ferranti ULA with a VTI 2069 from an Issue 7. While modes 0-6
> were fine, it seemed to be incompatible with the SAA5050 causing Teletext
> text output to be horribly smeared on his colour television and barely legible.
>
> I'm presently running an Issue 4 that also had a fried ULA, replaced again
> with one from a (broken) Issue 7. I have no TV; I'm using a Microvitec Cub
> screen and mode 7 displays perfectly.
>
> According to Jon Ripley's Wiki that led me here, the VTI VC 2069 is a
> straight drop-in replacement for the Ferranti predessor, but clearly
> something changed between the Issue 4 and 7 that makes the VC 2069-based
> ULA, or the software therein, incompatible somehow. I'm just curious as to
> whether anyone knows what this is about?

It was supposed to be a drop-in replacement, but the early ones (VC2023)
had a problem in that they didn't correctly switch settings between
teletext and non-teletext modes. To fix that, you had to remove a
jumper (S26) and solder a small wire between the VidProc pin 27 (=centre
of S26) and one of the select lines for the memory buffers (IC10 pin 1,
which is the inverted TTX teletext signal). The official fix put that
wire under the board, which caused some headscratching when people who
didn't know that swapped one VidProc for another (or for a ULA).

The difference is not the BBC board issue, but the VidProc version.

Does the BBC start up with a double beep and you only have no display?
have you tried other video outputs ?
Peter.

User avatar
CMcDougall
Posts: 6306
Joined: Wed Feb 02, 2005 3:13 pm
Location: Shadow in a Valley of Scotland
Contact:

Re: Display issue on my spare beeb

Post by CMcDougall » Sun Oct 29, 2017 4:02 pm

^it won't be that , as Mode7 would be good, & other modes bad, or vice versa...
I would go with the ram LSchips per Mark, I have a M128 spare in same boat, tried changing the 2 screen chips to no avail.
If CTRL-G beeps & *T. , *. Clicks the tape motor, then it lives , you just can't see it :x
ImageImageImage

JoolsH
Posts: 526
Joined: Mon May 21, 2012 11:46 am
Contact:

Re: Display issue on my spare beeb

Post by JoolsH » Sun Oct 29, 2017 4:13 pm

RobC wrote:How is the invert link (S26) setup? Just based on the mode 6 output, it might be worth swapping S26 over to see if that improves things. Having it set to invert would also cause mode 7 to be dim.

If you can, it might be worth posting some pictures of your Beeb PCB (both sides) just so that we can see if it has the "invert" mod for the first batch of VLSI ULAs.
There doesn't seem to be any jumper attached to S26 at all. I haven't unscrewed the board yet, but see attached for pic of top of PCB, at least.
CMcDougall wrote:If CTRL-G beeps & *T. , *. Clicks the tape motor, then it lives , you just can't see it :x
Yep, that's the case here, that all works.
Attachments
bbc_pcb_top.jpg

JoolsH
Posts: 526
Joined: Mon May 21, 2012 11:46 am
Contact:

Re: Display issue on my spare beeb

Post by JoolsH » Sun Oct 29, 2017 4:29 pm

vanekp wrote:have you tried other video outputs ?
I gave the RF output a try on my old portable, but couldn't get it to tune into it.

User avatar
CMcDougall
Posts: 6306
Joined: Wed Feb 02, 2005 3:13 pm
Location: Shadow in a Valley of Scotland
Contact:

Re: Display issue on my spare beeb

Post by CMcDougall » Sun Oct 29, 2017 4:33 pm

If no jumper then it will have the mod on back of board, & you will have a VC2023 at IC6, so that's normal, hence why it worked before it did not like Chuck ie Egg...
ImageImageImage

User avatar
vanekp
Posts: 555
Joined: Thu Nov 30, 2000 7:09 am
Location: The Netherlands
Contact:

Re: Display issue on my spare beeb

Post by vanekp » Sun Oct 29, 2017 4:54 pm

Sounds like the BBC is running if you can do things only the display is wonky, would not think its a ram problem then, seems more like it will be something in the video output section that's gone, and think without a multi meter and scope it will be hard to fault find.
VideoSection.png
Are you using the RGB output ?
Peter.

JoolsH
Posts: 526
Joined: Mon May 21, 2012 11:46 am
Contact:

Re: Display issue on my spare beeb

Post by JoolsH » Sun Oct 29, 2017 5:18 pm

Yeah, using the RGB output.

Not the end of the world if I can't get it working - it's only my spare machine after all.

RobC
Posts: 2328
Joined: Sat Sep 01, 2007 9:41 pm
Contact:

Re: Display issue on my spare beeb

Post by RobC » Sun Oct 29, 2017 5:58 pm

Just to be absolutely clear, is the ULA pictured the one I gave you yesterday or the original?
CMcDougall wrote:If no jumper then it will have the mod on back of board, & you will have a VC2023 at IC6, so that's normal, hence why it worked before it did not like Chuck ie Egg...
Just want to check that the mod has been done as the original ULA might have been a different type...
+

User avatar
CMcDougall
Posts: 6306
Joined: Wed Feb 02, 2005 3:13 pm
Location: Shadow in a Valley of Scotland
Contact:

Re: Display issue on my spare beeb

Post by CMcDougall » Sun Oct 29, 2017 6:04 pm

^but the ula is at IC7, and that does not matter at all , with the 3x different types chips at IC6 , just the jumper /mod is reqd :)
Last edited by CMcDougall on Sun Oct 29, 2017 6:06 pm, edited 1 time in total.
ImageImageImage

JoolsH
Posts: 526
Joined: Mon May 21, 2012 11:46 am
Contact:

Re: Display issue on my spare beeb

Post by JoolsH » Sun Oct 29, 2017 6:05 pm

The one in the pic is the original.

RobC
Posts: 2328
Joined: Sat Sep 01, 2007 9:41 pm
Contact:

Re: Display issue on my spare beeb

Post by RobC » Sun Oct 29, 2017 6:34 pm

CMcDougall wrote:^but the ula is at IC7, and that does not matter at all , with the 3x different types chips at IC6 , just the jumper /mod is reqd :)
Doesn't the mod invert the output in modes other than teletext (to counter the fault in the VC2023) by connecting the ~TTX signal to the VIDPROC's ~INVERT? Fitting a correctly functioning ULA to a modified machine would cause the display to be inverted. This is why I decided to ignore the ~INVERT signal when designing VideoNuLA and replace it with a jumper on the board...
JoolsH wrote:The one in the pic is the original.
What revision is the ULA that I gave you yesterday?

JoolsH
Posts: 526
Joined: Mon May 21, 2012 11:46 am
Contact:

Re: Display issue on my spare beeb

Post by JoolsH » Sun Oct 29, 2017 6:38 pm

RobC wrote:What revision is the ULA that I gave you yesterday?
The one you gave me is labelled:

622 V B589
VC2069
201647
(c) ACORN COMPUTER
KOREA-A

The behaviour on both ULAs seemed to be the same.

Post Reply