BBC B repair help

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Merrickd
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BBC B repair help

Post by Merrickd » Fri Oct 12, 2018 3:42 am

Hi all,

This is my first post here on Stardot. A few weeks ago I picked up my first ever BBC computer from eBay. Since then I've been trying to get it working.

The machine is an issue 4 with no Econet or disk controller. The original fault was a continuous tone from the speaker and no image on the display.

I've been combing through posts on the forum and I've found a lot of helpful information that has gotten me this far. After much troubleshooting with a logic probe and Tricky's test rom, I've replaced:

IC 45 (74S139)
IC 24 (74LS138)
IC 14 (74LS245)
IC 39 (74LS283)
IC 66 (4816 RAM)

The computer will now boot, but there is no startup message, only a blinking cursor. Typing will advance the cursor but no characters are printed on screen. I can blindly enter the "Mode" command to change screen modes. In all screen modes except 7, text is printed correctly and I can execute Basic commands. If I switch back to Mode 7, no characters are displayed, only the cursor.

I can get a startup message to appear if I remove IC 3 (VIA):

Code: Select all

BBC Computer
Searching
File Not Found
BASIC
I tried swapping the VIA for the one at IC 69 and again with a brand new one, but the startup message does not display as long as there is a VIA installed at IC 3.

I feel like I'm close to getting this working, but I'm not sure what to try next. Any help would be appreciated.

Thanks,
-Derrick

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danielj
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Re: BBC B repair help

Post by danielj » Fri Oct 12, 2018 4:52 am

ICs 31 and 32 are immediately responsible for controlling ic 3 - I'm not saying replace, but can you check they're behaving as they should (not studied the schema long enough to work that bit out myself)? That's where I'd start looking at this point.

d.

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tricky
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Re: BBC B repair help

Post by tricky » Fri Oct 12, 2018 8:16 am

Welcome, sounds like you have made some great progress so far.
Iirc, the cursor is generated by the 6845 and in mode 7, the text by the sa5050.
Can you tell if the teletext chip is getting data, and if so, how far the signal is getting.
Does my test ROM produce the frogger screen for the first few seconds, possibly implying that the text in mode 7 has been turned off some how.

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Elminster
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Re: BBC B repair help

Post by Elminster » Fri Oct 12, 2018 8:57 am

Bit odd as there is no direct connection between ic3 and SA5050.

As Tricky said have you looked at what SA5050 is actuall doing, you might be able to trace it back? And also have you check IC15?

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flaxcottage
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Re: BBC B repair help

Post by flaxcottage » Fri Oct 12, 2018 9:05 am

For what it is worth, I had a similar problem on one of my issue 4 boards; MODEs 0 through 6 were OK but MODE 7 would not display. The fault turned out to be the video ULA. Once I changed that everything was hunkydory.
- John
Image

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daveejhitchins
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Re: BBC B repair help

Post by daveejhitchins » Fri Oct 12, 2018 11:24 am

Welcome to the Forum, Derrick . . . Enjoy . . .

Hope you get your B sorted and working!

Dave H :D
Parts: UM6502CE, GAL22V10D, GAL16V8D, AS6C62256A, TC514400AZ, WD1772, R6522, TMS27C512, AT28C256
Products: ARA II, ARA III, ABR, ATI, AP6, MGC, AP5 . . .
For a price list, contact me at: Retro Hardware AT dave ej hitchins DOT plus DOT com

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1024MAK
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Re: BBC B repair help

Post by 1024MAK » Fri Oct 12, 2018 1:19 pm

The MODE used for start-up is determined by the keyboard links/DIP switches (as are some filing systems options). See this web page. If the “system” 6522 VIA (IC3) is not present, the OS can’t read the keyboard links/DIP switches. Yes, I know that most machines don’t have any links or DIP switches fitted. But a not fitted link/DIP switch does not have the same affect as IC3 not being fitted.

So your problem now looks to be a defective MODE 7 (the teletext MODE).

The circuitry involved in MODE 7 includes IC5 (SA5050 the teletext chip), IC15 (74LS273), IC33, IC37, IC43 (all 74LS04) or IC36 (74LS10).

Also, the type of Video ULA/Videoproc chip fitted in position IC6 can affect operation of MODE 7.

So first off, can you report back which Video ULA/Videoproc chip is fitted in position IC6. What is the status of link S26 and is there a modification wire on the underside of the board in the area of IC6.

Mark
Last edited by 1024MAK on Fri Oct 12, 2018 1:23 pm, edited 1 time in total.
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Merrickd
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Re: BBC B repair help

Post by Merrickd » Sat Oct 13, 2018 4:24 am

Thank you everyone for your replies. I have taken a look at the suggested items.
danielj wrote:
Fri Oct 12, 2018 4:52 am
ICs 31 and 32 are immediately responsible for controlling ic 3 - I'm not saying replace, but can you check they're behaving as they should (not studied the schema long enough to work that bit out myself)? That's where I'd start looking at this point.

d.
These ICs seem OK.
tricky wrote:
Fri Oct 12, 2018 8:16 am
Welcome, sounds like you have made some great progress so far.
Iirc, the cursor is generated by the 6845 and in mode 7, the text by the sa5050.
Can you tell if the teletext chip is getting data, and if so, how far the signal is getting.
Does my test ROM produce the frogger screen for the first few seconds, possibly implying that the text in mode 7 has been turned off some how.
No Frogger screen. I either get a blank screen, or a white rectangle filling the top third of the screen. The data lines on the SA5050 all appear to be pulsing. The inputs on IC15 are also pulsing
Elminster wrote:
Fri Oct 12, 2018 8:57 am
Bit odd as there is no direct connection between ic3 and SA5050.

As Tricky said have you looked at what SA5050 is actuall doing, you might be able to trace it back? And also have you check IC15?
IC15 appears OK. I replaced it anyway but there is no change in Mode 7 behavior.
1024MAK wrote:
Fri Oct 12, 2018 1:19 pm
The MODE used for start-up is determined by the keyboard links/DIP switches (as are some filing systems options). See this web page. If the “system” 6522 VIA (IC3) is not present, the OS can’t read the keyboard links/DIP switches. Yes, I know that most machines don’t have any links or DIP switches fitted. But a not fitted link/DIP switch does not have the same affect as IC3 not being fitted.

So your problem now looks to be a defective MODE 7 (the teletext MODE).

The circuitry involved in MODE 7 includes IC5 (SA5050 the teletext chip), IC15 (74LS273), IC33, IC37, IC43 (all 74LS04) or IC36 (74LS10).

Also, the type of Video ULA/Videoproc chip fitted in position IC6 can affect operation of MODE 7.

So first off, can you report back which Video ULA/Videoproc chip is fitted in position IC6. What is the status of link S26 and is there a modification wire on the underside of the board in the area of IC6.

Mark
I just checked IC33, 36, 37, and 43. These all appear OK with the logic probe. I had a spare IC15 so I swapped it. No change.

My board has the Ferranti ULA. No wire soldered to the bottom side of the board. S26 is facing west.

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Re: BBC B repair help

Post by Coeus » Sun Oct 14, 2018 5:25 pm

1024MAK wrote:
Fri Oct 12, 2018 1:19 pm
The MODE used for start-up is determined by the keyboard links/DIP switches (as are some filing systems options). See this web page. If the “system” 6522 VIA (IC3) is not present, the OS can’t read the keyboard links/DIP switches. Yes, I know that most machines don’t have any links or DIP switches fitted. But a not fitted link/DIP switch does not have the same affect as IC3 not being fitted.
So you were thinking the same as me - that failure to read these DIP switches resulting in selecting a different screen mode for the initial sign message, probably mode0?

Merrickd
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Re: BBC B repair help

Post by Merrickd » Sun Oct 14, 2018 6:23 pm

Coeus wrote:
Sun Oct 14, 2018 5:25 pm
1024MAK wrote:
Fri Oct 12, 2018 1:19 pm
The MODE used for start-up is determined by the keyboard links/DIP switches (as are some filing systems options). See this web page. If the “system” 6522 VIA (IC3) is not present, the OS can’t read the keyboard links/DIP switches. Yes, I know that most machines don’t have any links or DIP switches fitted. But a not fitted link/DIP switch does not have the same affect as IC3 not being fitted.
So you were thinking the same as me - that failure to read these DIP switches resulting in selecting a different screen mode for the initial sign message, probably mode0?
This makes sense. I just tried starting it up with IC3 installed and with link 8 on the keyboard connected (mode 6). I get a normal startup message in that configuration. So that means it's just a Mode 7 fault like 1024MAK suggested. I guess this means the problem is likely in the SA5050 or the video ULA at this point?

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