BBC B repair help

[Merrickd]

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

[danielj]

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.

[tricky]

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.

[Elminster]

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?

[flaxcottage]

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.

[daveejhitchins]

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

Hope you get your B sorted and working!

Dave H

[1024MAK]

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

[Merrickd]

Thank you everyone for your replies. I have taken a look at the suggested items.

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.

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

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.

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.

[Coeus]

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]

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?

[Merrickd]

Quick update. I replaced the ULA and SA5050. Still only getting a cursor in mode 7. All other modes are normal.

[Kazzie]

As Tricky mentioned, the cursor is always drawn by the ULA, even in mode 7. So it appears in your case that either the SA5050 isn't generating any display output, or the ULA isn't receiving and processing that data.

Have you checked for activity on the three lines (R,G,B) that carry the display data from the SA5050 to the ULA? They're pins 22, 23, and 24 on the SA5050, leading to pins 9, 11, and 13 on the ULS respectively. They're pulled up to 5V via 1K resistors (R99-101), but should still show some activity.

[1024MAK]

Check/test R99, R100, R101 and R103 (all 1k ohms). Both the resistance when the machine is off, and the voltages when the machine is on (at each end of each resistor).

It’s also worthwhile testing the +5V/Vcc supply voltage and 0V/GND pins on the relevant chips. Use a multimeter on DC voltage to test between the +5V/Vcc and 0V/GND pin(s) on each chip one at a time. This tests both the +5V/Vcc and 0V/GND pins at the same time.

Mark

[Merrickd]

R99-101, R103 all check out close to 1k. The RGB lines between the ULA and SA5050 are good too. Voltage on the RGB pins of the SA5050 are measuring about 0.3 volts with the machine turned on, so there appears to be no output from this IC.

[1024MAK]

But what is the voltage at the other end of the resistors? Is there a +5V supply voltage present?

Do IC5 and IC15 have both a good connection to the +5V supply and to GND?

Mark

[Kazzie]

To that end, IC15 (74LS273) should have 5V supply on pin 20 and 0V on pin 10, and IC5 (SA5050) should have 5V on pin 18, and 0V on pin 1.

Additionally, on my circuit diagram, the SA5050 should have pins 2 and 27 pulled down to 0V, and pins 3, 11, 15 and 28 pulled high via R103 (1K) to 5V, for correct operation.

[Merrickd]

There is 5 volts on the other ends of R99-R101. It appears that the SA is pulling it down to 0.3 volts (if I remove it, the voltage jumps to about 5 volts at the RGB pins on the SA socket). IC5 and IC15 both have a good supply voltage.

To that end, IC15 (74LS273) should have 5V supply on pin 20 and 0V on pin 10, and IC5 (SA5050) should have 5V on pin 18, and 0V on pin 1.

Additionally, on my circuit diagram, the SA5050 should have pins 2 and 27 pulled down to 0V, and pins 3, 11, 15 and 28 pulled high via R103 (1K) to 5V, for correct operation.

I'm getting the correct voltages here too.

[Kazzie]

There is 5 volts on the other ends of R99-R101. It appears that the SA is pulling it down to 0.3 volts (if I remove it, the voltage jumps to about 5 volts at the RGB pins on the SA socket). IC5 and IC15 both have a good supply voltage.

To that end, IC15 (74LS273) should have 5V supply on pin 20 and 0V on pin 10, and IC5 (SA5050) should have 5V on pin 18, and 0V on pin 1.

Additionally, on my circuit diagram, the SA5050 should have pins 2 and 27 pulled down to 0V, and pins 3, 11, 15 and 28 pulled high via R103 (1K) to 5V, for correct operation.

I'm getting the correct voltages here too.

All good information to add into the mix.

You say that the RGB lines all read 0.3V, I presume that's with a DC voltmeter (which will be taking an average over time). Does your logic probe show any activity on them?

Edit: for that matter, do you see activity on the SA5050 on pins 4-10 (data input), and 12-14, 19-20 (various control signals) with your logic probe?

[Merrickd]

The probe shows no activity on the RGB lines (they read as low). Pins 4-10, 12-14, and 19-20 on the SA all read as pulsing.

[Merrickd]

Not sure what happened, but the machine now has a new problem. No video output at all now and only one beep from the speaker at startup. Test ROM also shows no video too.

Edit: IC24 (74ls138) failed again. I replaced it again and got the cursor back.

[Kazzie]

My strongest suspicion now would be the SA5050, if not for the post quoted below. (Looking at the datasheet, the SA5050 can't be configured to give black text on black background, so it's not a misconfiguration in that sense.

Quick update. I replaced the ULA and SA5050. Still only getting a cursor in mode 7. All other modes are normal.

When you say replace, do you mean "removed and reinserted" or "removed and inserted a replacement" ? I had read it as the latter, but if it's the former, I'd be getting another SA5050 next.

As a side note, pin 21 os the SA5050 is the unconnected monochrome output. If there's no sign of life there either, I'd be getting very suspicious of the SA5050's health.

[Merrickd]

Here's an update. I checked everything over again and found a bad gate on IC40 (74ls00). Replacing that gave me a normal boot prompt.

So it's fixed right?

Not yet. I went ahead and installed a 1770 floppy controller kit in it. On boot it still shows the correct boot message, but it does not say "Acorn DFS" or anything like that at startup. Typing *ROMS produces the error "Bad Command". Can someone tell me if this is normal behavior? There is not a floppy drive connected just yet.

[1024MAK]

It will only report DFS etc if there is a DFS ROM fitted and this ROM finds a working disk interface chip (it does not matter if a drive is connected). You do have to use the correct version of DFS for the actual floppy disk controller chip (and if a 1770/1772 board, the correct version for that design of board).

For the model B, the "*ROMS" command is not implemented in either the OS or on the DFS ROM. So unless another ROM chip provides a "*ROMS" command, the model B will either go looking for a file of the same name using the current filing system (for example on an attached floppy disk, press ESCAPE to get out of this) or raise an error.

What does *HELP show?

Mark

[1024MAK]

So this is one of my machines. It is a BBC model B. A 5¼” drive is often used with it, but is currently not connected.

The start up screen, and what happens with *ROMS and *HELP

The Intel P8271 FDC chip in my main Beeb

The ROM chips in my main Beeb (I had removed the sideways RAM module before taking this photo).

Mark

[Merrickd]

Thanks for the pictures. I have a 1770 kit installed that is supposed to work with either Acorn DFS 2.26 or Watford DFS 1.54. I currently have an Acorn 2.26 eprom installed at IC88. The *HELP command just displays "OS 1.20".

[Kazzie]

I know that previous versions of DFS/DNFS (including the one pictured by Mark) interrogate the hardware and disable themselves if it's not detected. (This bit me in the behind with an econet installation where one pin missed the socket.)

Are you confident that you've followed all the instructions, S9 is open, and that all the chips are installed securely?

[1024MAK]

Could you take some photos and upload them here (use the ‘Attachments’ tab below the text edit box). We are interested in the area of the board where the disk interface chips / circuitry is located. And the area of the ROM chips.

Also, is the socket for position IC88 known to be good? That is do you have another ROM chip that is not a disk filing system to try in it? If you don’t have any other ROM chips, you could move the BASIC ROM chip into IC88.

Have you tried your Acorn DFS2.26 ROM in another socket?

What type of chip is your Acorn DFS2.26 ROM? Is it an EPROM chip, a ROM chip or a EEPROM/Flash ROM?

• Mask ROM - Read Only Memory. Masked ROM chips have no window and an Acorn part number printed on it.
• EPROM - Erasable Programmable Read Only Memory. EPROMs have a window on top that should be covered with a paper label or UV proof tape. Types that will work in a model B include 2764, 27C64, 27128, 27C128, 27256, 27C256.
• OTP - One Time Programmable. OTP EPROM chips don’t have a window and look like a masked ROM chip, but just have the chip manufacturers details and part number. The part number is very similar to the EPROM part numbers.
• EEPROM - Electrically Erasable Programmable Read Only Memory. Also known as E2PROM. These also have no window. They look like a OTP EPROM, but have a different part number.
• Flash ROM (also known as Flash RAM or Flash memory) - like EEPROM but often faster to program and you cannot reprogram individual bytes, only ‘sectors’. They look the same as EEPROMs, but have different part numbers.

Mark

[MartinB]

To confirm that the DFS rom is at least being 'seen', type in and run the following short program. (Note that it is deliberately quick and dirty for use with this specific test and not meant to be used for other applications.)

If you see the DFS logo (or part thereof) in the list (and there will also be some garbage characters), it shows the rom is capable of being read.....

Code: Select all

 
   10 FOR Y%=15 TO 0 STEP -1 
   20  PRINT Y%;" ";
   30  FOR M%=8 TO 17
   40   ?&F6=M%:?&F7=&80
   50   B%=USR(&FFB9) AND &FF
   60   IF B%>31 THEN PRINTCHR$(B%);
   70  NEXT M%
   80  PRINT
   90 NEXT Y%

EDIT : Just added one line to print the rom slot ID with the rom title characters and also changed line 10 to include all 16 slots so on a non-expanded Beeb, we may also see the 'ghost' rom reports.

[Merrickd]

Could you take some photos and upload them here (use the ‘Attachments’ tab below the text edit box). We are interested in the area of the board where the disk interface chips / circuitry is located. And the area of the ROM chips.

Also, is the socket for position IC88 known to be good? That is do you have another ROM chip that is not a disk filing system to try in it? If you don’t have any other ROM chips, you could move the BASIC ROM chip into IC88.

Have you tried your Acorn DFS2.26 ROM in another socket?

What type of chip is your Acorn DFS2.26 ROM? Is it an EPROM chip, a ROM chip or a EEPROM/Flash ROM?

Mark

I have attached some pictures of the board.

The socket at IC88 appears to be OK. If I put the BASIC ROM in, it works fine. The DFS ROM does not seem to work in any of the sockets though. The DFS ROM I am using is a 27c256 EPROM (with the code doubled up to fill the entire ROM). I also tried making another DFS ROM using a 27c128 but it doesn't work in this board either.

To confirm that the DFS rom is at least being 'seen', type in and run the following short program. (Note that it is deliberately quick and dirty for use with this specific test and not meant to be used for other applications.)

If you see the DFS logo (or part thereof) in the list (and there will also be some garbage characters), it shows the rom is capable of being read.....

Here is a screenshot of the output from this program.

[MartinB]

That shows that your DFS rom is faulty - we should see "DFS2.26" but the "F" of "DFS", the "." of "2.26" and the "6" of the "2.26" all have the bottom bit erroneously set to give your "G", "/" and "7". There are likely to be many repeat occurrences throughout the rom and hence, your DFS rom is not being recognised as valid by the OS.

I also tried making another DFS ROM using a 27c128 but it doesn't work in this board either.

This puzzles me though - where did you get the 2.26 image from for the second rom you programmed? Did you copy the first rom? How did you program the second? I can’t imagine two roms are faulty in the same way - if you run my ditty with both roms, do you get the same results? With either rom, if you repeatedly re-run the ditty, is the result always the same?

Do you have a non-DFS rom to try as a sideways rom? Say Wordwise or something you could program?


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