BBC Model B - Issue 7 problem - Solved

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1024MAK
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Re: BBC Model B - Issue 7 problem

Post by 1024MAK » Thu Jul 25, 2019 8:48 am

What issue board is in your machine?

In early issue boards, there are some errors which affect some configuration links.
All the following information assumes you have an issue 4 or issue 7 board.

If you have a look at the minimal configuration you will see that IC7 is not needed for the rest of the machine to work.

The parts needed for a 8271 floppy disk drive controller are detailed in the Acorn BBC service manual. See
http://chrisacorns.computinghistory.org ... nuals.html or
http://www.8bs.com/othrdnld/manuals/essentialsbbc.shtml or
http://www.computinghistory.org.uk/downloads/5285.

Aside from link S9 being fitted (if IC78, 8271 is not installed), I can’t think of anything else in the floppy disk drive section that would cause this kind of problem. Note that the serial number may tell us if your Beeb came with a floppy disk drive interface from the factory. Machines that were supplied as cassette only computers would mostly have had a wire as a link for S9.

Can you also check link S2, and report back on its state, as well as letting us know if IC89 is fitted. These are part of the Econet circuitry. Again if Econet was not fitted at the factory, S2 is likely to be (or have been) a wire link.

Can you measure the +5V / VCC voltage at all three supply points on the main board. Test by connecting your meter leads between each PCB +5V / VCC terminal and each PCB 0V / GND terminal rather than on the spade connector that the red and black wires are connected to. Report back you actual voltages.

Mark
Last edited by 1024MAK on Thu Jul 25, 2019 11:29 am, edited 4 times in total.

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Re: BBC Model B - Issue 7 problem

Post by Vyper68 » Thu Jul 25, 2019 9:26 am

Thanks for the replies, I am away for a couple of days but will take a photo and measure the voltages when I get back home.

This is an issue 7 board and sadly the serial number is missing from underneath. I have ordered a MMC kit from RetroClinic to see how it performs with software and hopefully the new LM324 will sort the garbled sound saving and not loading I will have it by then.
The problem with it starting does feel like a power/timing problem not memory moving the required jumper makes it a 16K machine for both alternate settings.
Last edited by Vyper68 on Thu Jul 25, 2019 9:41 am, edited 2 times in total.
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Re: BBC Model B - Issue 7 problem

Post by Vyper68 » Fri Jul 26, 2019 1:15 pm

OK, here is a photo of the motherboard. Things to note are I have fitted a jumper to S9 was blank "open" before, I removed the BNC Video connector temporarily while the board is in and out of the case. I have started the process of fitting turned pin sockets to the ROM sockets to replace the 30 year old ones on there. Fitted new 47nf Electrolytics and the 3 smaller ones to the bottom left. IC 33 has been replaced as has the 555 IC.

Any obvious things missing would be greatful for the input.

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Re: BBC Model B - Issue 7 problem

Post by DutchAcorn » Fri Jul 26, 2019 2:16 pm

What happens if you remove the system via? Does it boot then (with continuous beep and no keyboard function)?
Paul

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Re: BBC Model B - Issue 7 problem

Post by Kazzie » Sat Jul 27, 2019 12:35 pm

Thanks for the picture. The empty sockets all look like those used by the 8271 Floppy Controller add-on to me. (The socketless locations in the upper right / north-west are all for the Econet add-on.)
Vyper68 wrote:
Thu Jul 25, 2019 3:37 am
Also does having 1.2V coming from pin 11 mean this chip IC 7 has failed? And do I need another?
I popped the lid off my Beeb and measured the voltages of all pins of my IC7, to have as a benchmark for comparison.
Mine's an issue 7 board, fitted with a Ferranti ULA 2C199E-7 (date code 8407) in IC7.
The cassette and RS423 interfaces are both known to work.

Code: Select all

       ----\/----
1.06 -|1 o     28|- 4.94
0.95 -|2       27|- 4.08
1.28 -|3       26|- 2.20
1.36 -|4       25|- 2.21
1.22 -|5       24|- 3.02
2.11 -|6       23|- 0.08
1.53 -|7       22|- 3.05
1.31 -|8       21|- 0.07
3.46 -|9       20|- 0.19
1.91 -|10      19|- 4.44
0.07 -|11      18|- 2.15
0.92 -|12      17|- 4.47
0.07 -|13      16|- 0.19
0.03 -|14      15|- 0.07
       ----------
(Voltages measured relative to 0V of VCC1 power connection, machine booted and idle at BASIC prompt)

If you could measure the voltage on your IC7's pins, we can take a look and see if anything critical is awry (other than pin 11 which you've already told us about).


With regard to your booting issue, what happens if you hold break down while the machine is off, turn it on, and then release the break key immediately?
Last edited by Kazzie on Sat Jul 27, 2019 12:37 pm, edited 2 times in total.
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Re: BBC Model B - Issue 7 problem

Post by Vyper68 » Sat Jul 27, 2019 1:35 pm

It's a bit of a moot point right now. I fitted two new 6522's I had spare and it seemed to boot 90% although it would often say Lanuguage? instead of Basic except if I pressed Break.
Then I flicked the power on and got a Black screen and a wail and no amount of pressing break brought it back to life this time. So I have had enough for now and I'll have to send it away for repair ( reccomendations for UK repair facilities ) when I can afford to. I just seem to be on a hiding to nothing right now and feel pretty cheesed off. Maybe a little break away from it and I'll have another go. I do think the CTRC might have let go as I saw some out of sync text for a little while before the Black screen.
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Re: BBC Model B - Issue 7 problem

Post by tricky » Sat Jul 27, 2019 4:43 pm

Sorry to hear that, I feel the same most times I get out some non-working beebs, but if I get one working out of three, I'm happy.

I think that the general recommendation is Mark Haysman at http://www.retroclinic.com/.

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Re: BBC Model B - Issue 7 problem

Post by Kazzie » Sat Jul 27, 2019 10:35 pm

Sorry to hear it's gotten the better of you (for now). A step back and a breather can be a good choice with troublesome machines.

If you decide to have another go at it, pipe up and we'll be happy to help you. Otherwise, Retroclinic is the main recommendation for repair services that you'll find on this board.

In the meantime, just in case, I'll make a quick note here: the two blue jumpers of S21 may be worth a look, to check they're making good electrical contact. The upper one carries the select signal for the OS ROM, the lower one has the select signal for BASIC and other Sideways ROMs. If the upper one is loose or missing your machine won't boot, a problem with the lower one will give you the "Language?" prompt.

Come to think of it, the jumpers you have on S21 are the only blue ones in your box, the rest are the normal black. We already suspect that a previous owner has removed a floppy controller; the S21 jumpers are often removed when fitting expansions such as a Sideways ROM expansion card. If the previous owner removed one of these too, and had to get some spare jumpers from their bit box, they might not be a perfect fit.
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Re: BBC Model B - Issue 7 problem

Post by Vyper68 » Tue Jul 30, 2019 4:22 pm

I have been having another go today and made some progress i think. I checked all the voltages again and they are good, one of the 6522's was bad so that was swapped out for a new one. Still a black screen with no keyboard response. I fitted the OS Test and it did exactly like it was before ( I am starting to wonder about the EPROM as I burned another as a replacement OS 1.2 and that doesn't work either ). My point is the CTRC and ULA's must be working for this OS Test ROM to behave the way it does. It also doesn't need the break key to start it's sequence either. If i leave it alone it starts up itself.
Query:- could a failing OS ROM cause the problems I had with the POST hanging until I pressed the BREAK key and do you think it's worth trying to source a real one to try?
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Re: BBC Model B - Issue 7 problem

Post by Kazzie » Tue Jul 30, 2019 5:57 pm

Hi there.

How did you know that one of the 6522s had failed: was it just the case that pressing break didn't get your machine to boot, or was there a different fault?

Mask ROMs (the type that are fully encased in plastic, and programmed during manufacture) are usually pretty hardy, and don't fail very often. You say you burned a copy of MOS 1.2 to your EPROM; how did your machine behave when you fitted it? (Was it the same need to press break to boot, or did it not work at all?) If they behave the same, I'd say it's likely the fault is elsewhere.

I didn't pick up on the fact that the Test ROM executes its program without needing a break press. That may be an important distinction.

:arrow: Did you try the Minimal Configuration that 1024MAK suggested? How did the machine behave?

:arrow: A video of your machine's display output running the Test OS ROM, and a description of the LEDs' behaviour could also be useful.
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Re: BBC Model B - Issue 7 problem

Post by tricky » Tue Jul 30, 2019 6:41 pm

I don't know where the code diverges, but on RESET, the beeb checks the state of the system via to decide whether to do a power-on RESET of an after BREAK reset.
Have you compared the "OS-test" ROM running in your beeb to running in an emulator?
Booting them side by side, possibly recording with a phone/video camera might give some insights.
PS Sorry that I haven't got back to the test ROM to make it more user friendly and to actually add some diagnostics beyond just exercising the system.

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Re: BBC Model B - Issue 7 problem

Post by 1024MAK » Tue Jul 30, 2019 6:44 pm

There is a fundamental difference between the OS ROM code and the Test ROM code.

The OS ROM does lots of stuff, which assumes that the machine is fully functional (both RAM and the various standard fitted hardware, such as the system 6522 VIA and the keyboard circuitry).

The Test ROM code takes a completely different approach, it assumes only that the CPU and the ROM are functional. It does not depend on any working RAM and similarly it does not rely on the various hardware I/O systems.

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Re: BBC Model B - Issue 7 problem

Post by Vyper68 » Tue Jul 30, 2019 6:56 pm

OK I see, thanks for the insight into how the OS ROM works,

Regards the 6522 it was only showing 1.5V on the power side and replacing it with a new one produced the normal 4.89V so I took that as it wasn't working. I don't have a lot of diagnostic kit.

I will take a video of it running the OS Test ROM tomorrow.

The BBC behaves the same either with the original ROM or the EPROM fitted - no difference. Just a Black screen ( TV flicks into RGB mode but no video ) the Keyboard has the cassette motor lit and if I press break a few times the Caps Lock light comes on but still nothing on screen and and the Shift Lock doesn't come on.

The reason my hopes were raised was the Beeb was able to output colour and that made me hope the CTRC was not ruined.
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Re: BBC Model B - Issue 7 problem

Post by Kazzie » Wed Jul 31, 2019 9:10 am

Here's an extract from the assembly code of the MOS, from http://mdfs.net/Docs/Comp/BBC/OS1-20/.

Note that whenever the CPU gets reset by a low input on pin 40 (on power-up, or by pressing the break key), it always starts executing code at the same place, address &D9CD (by looking up the contents of addresses &FFFC and &FFFD, and jumping there)

Code: Select all

**************************************************************************
**************************************************************************
**                                                                      **
**      RESET (BREAK) ENTRY POINT                                       **
**                                                                      **
**      Power up Enter with nothing set, 6522 System VIA IER bits       **
**      0 to 6 will be clear                                            **
**                                                                      **
**      BREAK IER bits 0 to 6 one or more will be set 6522 IER          **
**      not reset by BREAK                                              **
**                                                                      **
**************************************************************************
**************************************************************************

D9CD	LDA #&40    ;set NMI first instruction to RTI
D9CF	STA &0D00   ;NMI ram start

D9D2	SEI         ;disable interrupts just in case
D9D3	CLD         ;clear decimal flag
D9D4	LDX #&FF    ;reset stack to where it should be
D9D6	TXS         ;(&1FF)
D9D7	LDA &FE4E   ;read interupt enable register of the system VIA
D9DA	ASL         ;shift bit 7 into carry
D9DB	PHA         ;save what's left
D9DC	BEQ &D9E7   ;if Power up A=0 so D9E7
D9DE	LDA &0258   ;else if BREAK pressed read BREAK Action flags (set by
		    ;*FX200,n)
D9E1	LSR         ;divide by 2
D9E2	CMP #&01    ;if (bit 1 not set by *FX200)
D9E4	BNE &DA03   ;then &DA03
D9E6	LSR         ;divide A by 2 again (A=0 if *FX200,2/3 else A=n/4

********** clear memory routine ******************************************

D9E7	LDX #&04    ;get page to start clearance from (4)
D9E9	STX &01     ;store it in ZP 01
D9EB	STA &00     ;store A at 00

D9ED	TAY         ;and in Y to set loop counter
   	
D9EE	STA (&00),Y ;clear store
D9F0	CMP &01     ;until address &01 =0
D9F2	BEQ &D9FD   ;
D9F4	INY         ;increment pointer
D9F5	BNE &D9EE   ;if not zero loop round again
D9F7	INY         ;else increment again (Y=1) this avoids overwriting
   		    ;RTI instruction at &D00
D9F8	INX         ;increment X
D9F9	INC &01     ;increment &01
D9FB	BPL &D9EE   ;loop until A=&80 then exit
   		    ;note that RAM addressing for 16k loops around so
   		    ;&4000=&00 hence checking &01 for 00.  This avoids
   		    ;overwriting zero page on BREAK


D9FD	STX &028E   ;writes marker for available RAM 40 =16k,80=32
DA00	STX &0284   ;write soft key consistency flag

**+********** set up system VIA *****************************************

DA03	LDX #&0F    ;set PORT B to output on bits 0-3 Input 4-7
DA05	STX &FE42   ;


*************************************************************************
*                                                                       *
*        set addressable latch IC 32 for peripherals via PORT B         *
*                                                                       *
*       ;bit 3 set sets addressed latch high adds 8 to VIA address      *
*       ;bit 3 reset sets addressed latch low                           *
*                                                                       *
*       Peripheral              VIA bit 3=0             VIA bit 3=1     *
*                                                                       *
*       Sound chip              Enabled                 Disabled        *
*       speech chip (RS)        Low                     High            *
*       speech chip (WS)        Low                     High            *
*       Keyboard Auto Scan      Disabled                Enabled         *
*       C0 address modifier     Low                     High            *
*       C1 address modifier     Low                     High            *
*       Caps lock  LED          ON                      OFF             *
*       Shift lock LED          ON                      OFF             *
*                                                                       *
*       C0 & C1 are involved with hardware scroll screen address        *
*************************************************************************

   	;X=&F on entry

DA08	DEX         ;loop start
DA09	STX &FE40   ;write latch IC32
DA0C	CPX #&09    ;is it 9
DA0E	BCS &DA08   ;if so go back and do it again
   		    ;X=8 at this point
   		    ;Caps lock On, SHIFT lock undetermined
   		    ;Keyboard Autoscan on
   		    ;sound disabled (may still sound)
DA10	INX         ;X=9
DA11	TXA         ;A=X
DA12	JSR &F02A   ;interrogate keyboard
DA15	CPX #&80    ;for keyboard links 9-2 and CTRL key (1)
DA17	ROR &FC     ;rotate MSB into bit 7 of &FC

DA19	TAX         ;get back value of X for loop
DA1A	DEX         ;decrement it
DA1B	BNE &DA11   ;and if >0 do loop again
   		    ; on exit if Carry set link 3 made
   		    ;link 2 = bit 0 of &FC and so on
   		    ;if CTRL pressed bit 7 of &FC=1
   		    ;X=0
DA1D	STX &028D   ;clear last BREAK flag
DA20	ROL &FC     ;CTRL is now in carry &FC is keyboard links
DA22	JSR &EEEB   ;set LEDs carry on entry  bit 7 of A on exit
DA25	ROR         ;get carry back into carry flag
After jumping in at &D9CD, the MOS reads the interrupt enable register from the 6522 VIA (IC3) almost immediately (&D9D7). If it finds any of these are set, it will conclude that it's on a warm boot (break was pressed) and load the contents of RAM address &0258 to decide whether it should go through the RAM clear procedure or not.

After the RAM has been cleared (or skipped), at &DA03 we have a routine that sets up the devices that are connected to the VIA. It does this by setting the contents of IC32 with values written out of the VIA's Port B, in a loop between &DA08 and &DA0E. This includes setting the Caps Lock LED to ON.

Vyper68, you said that Caps lock doesn't come on when you switch your machine on, but after pressing Break a few times it will turn on. This indicates that your machine usually fails to get as far as the loop at &DA08, but it sometimes does. The only things that the MOS does (outside of the CPU) between &D9CD and &DA08 is:
  • Read from the VIA, and potentially read and test a value from RAM
  • Write to and read from RAM, to clear its contents
You've already swapped out the 6522 VIA, so that just leaves an issue with reading from and writing to RAM.

As said, Tricky's Test OS avoids reading any values from RAM (or at least relying on their contents), which would also explain why it is able to run, while the MOS isn't.

Earlier, when your machine was still partially working, you tried moving S25 to disable one half of the RAM at a time, and found no improvement. That would suggest that there isn't a fault with any of the individual RAM chips. There are other chips that control how and when the RAM communicates with the CPU and the rest of the system. The main ones (for our purposes) are the Data buffer chip (IC14, a 74ls245), the Address buffer chips (IC12 and 13, 81ls97 or similar), and some other chips that enable and disable them. I suggest that one of these is faulty, and where it previously "just about" worked when you pressed Break after powering up, it has now failed completely.

:arrow: Vyper68, before we go about testing these, what sort of diagnostic kit do you have? I'm guessing that you have a voltmeter, but if you have a logic probe, oscilloscope, or similar available, they could make the diagnosis a bit easier.
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Re: BBC Model B - Issue 7 problem

Post by Vyper68 » Wed Jul 31, 2019 9:20 am

I am waiting for the video to upload to YouTube so while that does it's thing.

I have a multimeter that is it :(

I have one of those £20 oscilloscope not much use so far.

I have been having a feel of the the Logic chips and IC8 a 81LS95 is getting really hot so maybe I have found the problem? or one of them? - Looking at the schematic anyways.

Is there a modern replacement for one of these chips as i cannot find any for sale.

Here is the video.

https://youtu.be/FtEFCSopQvE

As an aside how hot is R14 supposed to get? It is getting pretty warm then cools down a bit and then heats up again.
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Re: BBC Model B - Issue 7 problem

Post by marcusjambler » Wed Jul 31, 2019 9:54 am

Hi Vyper68

That is not operating correctly.

See original thread here :

viewtopic.php?f=2&t=10268#p139467

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Re: BBC Model B - Issue 7 problem

Post by marcusjambler » Wed Jul 31, 2019 10:18 am

Reading back through that thread.... I had a similar issue to begin with.
I eventually got it working by erasing and programming the test OS image onto an older device I had lying around

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Re: BBC Model B - Issue 7 problem

Post by Kazzie » Wed Jul 31, 2019 10:49 am

Thanks for the video. It really is worth a thousand words!

Here's a video of what Tricky's Test OS should do:
TrickyTestOSVideo.zip
(853.54 KiB) Downloaded 18 times
Yours doesn't match, so there is some fault with the video circuitry, or with the circuitry that lets the CPU put values into memory (which is later read by the video circuitry).

(Also: Was the speaker plugged in to the board, and/or was the machine beeping like the attached video?)

The first screen, which should be a Mode 7 (teletext) title screen of Frogger, shows as a sequence of 64 black squares followed by 64 white squares in your video.

The subsequent screens (flashing colours) are displayed in Mode 0, where IC8 isn't used (and should be disabled). Yours shows ten columns of alternating colour and white. Mode 0 has a horizontal resolution of 640 pixels, so these columns are 64 pixels wide. IC8 (which you say is running hot) is used as an address buffer when displaying non-mode-7 screens, as is IC9.

On the second half of your video (where the Test ROM is looping through again) it looks like the 6845 CRTC hasn't been reconfigured properly for the mode change for some reason.

There's certainly something going on here, possibly involving address line A6 (as 2^6=64), but I can't put my finger on what it is yet. If IC8 were misbehaving and not disabling its outputs, it might be overriding the outputs of the other memory buffer chips. But I can't think how it would create the images you're seeing.

:arrow: Could you use your multimeter and measure the voltage of each of the pins of IC8, so we can see what it's doing? (Measuring a DC voltage will give an average of the waveform, so we will be able to see if it's always high, always low, or averaging somewhere in the middle.) Measure them while the Test OS is running, and showing the columns instead of the squares, as we'll know the chip's output should be disabled at that time. We might ask you to go back with your pocket oscilloscope to check some of the "middle-value" pins afterward.

EDIT: IC8 is used for non-teletext display modes. Got it the wrong way around earlier.
Last edited by Kazzie on Wed Jul 31, 2019 7:30 pm, edited 3 times in total.
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Re: BBC Model B - Issue 7 problem

Post by Kazzie » Wed Jul 31, 2019 10:57 am

Vyper68 wrote:
Wed Jul 31, 2019 9:20 am
As an aside how hot is R14 supposed to get? It is getting pretty warm then cools down a bit and then heats up again.
Did you mean IC14? That chip is the buffer between the system's data bus and the RAM. It's a known point of failure, but I can't think how it could cause the screen corruption you're seeing (alternating between white and black/colour every 64 characters or pixels).

As for 81ls95 chips, I don't know if there are any modern equivalents, but there are a number of people selling old stock on eBay, if it turns out you need a replacement.
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Re: BBC Model B - Issue 7 problem

Post by Vyper68 » Wed Jul 31, 2019 11:04 am

Kazzie wrote:
Wed Jul 31, 2019 10:57 am
Vyper68 wrote:
Wed Jul 31, 2019 9:20 am
As an aside how hot is R14 supposed to get? It is getting pretty warm then cools down a bit and then heats up again.
Did you mean IC14? That chip is the buffer between the system's data bus and the RAM. It's a known point of failure, but I can't think how it could cause the screen corruption you're seeing (alternating between white and black/colour every 64 characters or pixels).

As for 81ls95 chips, I don't know if there are any modern equivalents, but there are a number of people selling old stock on eBay, if it turns out you need a replacement.
No there is a large Resistor next to the ULA with the Heatsink and it's getting really hot which I don't know if that's correct or not, suspect not even though is a big fat thing.
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Re: BBC Model B - Issue 7 problem

Post by Vyper68 » Wed Jul 31, 2019 11:06 am

Kazzie wrote:
Wed Jul 31, 2019 10:49 am
:arrow: Could you use your multimeter and measure the voltage of each of the pins of IC8, so we can see what it's doing? (Measuring a DC voltage will give an average of the waveform, so we will be able to see if it's always high, always low, or averaging somewhere in the middle.) Measure them while the Test OS is running, and showing the columns instead of the squares, as we'll know the chip's output should be disabled at that time. We might ask you to go back with your pocket oscilloscope to check some of the "middle-value" pins afterward.
I will do that and report back my findings - thanks for the help by the way it's really appreciated. I know my way around Atari 8-Bits, Orics and C64's but this is a new system for me.
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Re: BBC Model B - Issue 7 problem

Post by Kazzie » Wed Jul 31, 2019 11:29 am

Vyper68 wrote:
Wed Jul 31, 2019 11:06 am
I will do that and report back my findings - thanks for the help by the way it's really appreciated. I know my way around Atari 8-Bits, Orics and C64's but this is a new system for me.
That's just as well, 'cause I'd be starting from scratch on all of those! :lol:
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Re: BBC Model B - Issue 7 problem

Post by 1024MAK » Wed Jul 31, 2019 11:34 am

Vyper68 wrote:
Wed Jul 31, 2019 11:04 am
No there is a large Resistor next to the ULA with the Heatsink and it's getting really hot which I don't know if that's correct or not, suspect not even though is a big fat thing.
R114, the large resistor near the VideoProc / Video ULA will run hot, as the current used by the ULA flows through it.

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Re: BBC Model B - Issue 7 problem

Post by 1024MAK » Wed Jul 31, 2019 11:46 am

Vyper68 wrote:
Wed Jul 31, 2019 9:20 am
I have been having a feel of the the Logic chips and IC8 a 81LS95 is getting really hot so maybe I have found the problem? or one of them?
All the 81LS95 chips should run at a similar temperature. If one is noticeably hotter than the others, then I think it is worthwhile renewing it.

These are discontinued and there is no pin compatible chip that I know of. The function provided is available from other chips, but Acorn found the 81LS95 worked more reliably in the development machines. You may be able to find some from a European eBay seller.

See also this thread :wink:

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Re: BBC Model B - Issue 7 problem

Post by Vyper68 » Wed Jul 31, 2019 12:10 pm

Hi
So these are the voltages observed on IC8 don't know how to read them.

1=>0.07v
2=>1.93v
3=>1.37 - 1.56v
4=>1.90v
5=>1.75v
6=>1.80v
7=>0.6v - 1.72v
8=>1.92v
9=>0.6v - 2.0v
10=>0.0v
11=>1.3v - 1.14v
12=>1.93v
13=>0.6v - 2.0v
14=>1.93v
15=>0.6v - 2.0v
16=>1.80v
17=>3.60v
18=>4.93v
19=>3.73v
20=>4.93v


They seemed to fluctuate depending on the OS ROM tester clicking away. I did my best to get them accurate.
Last edited by Vyper68 on Wed Jul 31, 2019 12:14 pm, edited 2 times in total.
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Re: BBC Model B - Issue 7 problem

Post by 1024MAK » Wed Jul 31, 2019 1:41 pm

3B9A267B-6A12-4DE5-996B-C1A7469BC169.jpeg
81LS95 Pin-out
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Re: BBC Model B - Issue 7 problem

Post by Wheel_nut » Wed Jul 31, 2019 3:31 pm

#1 BBC Model B Level 3 (upgraded from Model A) + 1770 DFS + Dual Floppy
#2 BBC Model B Level 7 + 1770 DFS + Dual TEAC Floppy
#3 BBC Model B Level 3 + 8271 DFS + Watford Dual Floppy
#4 BBC Model B Level 7 + 8271 DFS + Viglen Single Floppy

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Re: BBC Model B - Issue 7 problem

Post by Kazzie » Wed Jul 31, 2019 8:29 pm

Vyper68 wrote:
Wed Jul 31, 2019 12:10 pm
Hi
So these are the voltages observed on IC8 don't know how to read them.

1=>0.07v
2=>1.93v
3=>1.37 - 1.56v
4=>1.90v
5=>1.75v
6=>1.80v
7=>0.6v - 1.72v
8=>1.92v
9=>0.6v - 2.0v
10=>0.0v
11=>1.3v - 1.14v
12=>1.93v
13=>0.6v - 2.0v
14=>1.93v
15=>0.6v - 2.0v
16=>1.80v
17=>3.60v
18=>4.93v
19=>3.73v
20=>4.93v


They seemed to fluctuate depending on the OS ROM tester clicking away. I did my best to get them accurate.
Thanks.

I've realised that I got things slightly arsy-very in an earlier post: IC8 is used for the non--teletext modes, rather than just mode 7. Sorry. :oops: That means that IC8 is enabled for a lot of the time when the columns are on-screen, which isn't quite as useful for trying to work out if the buffer is isolating its outputs (high impedance) properly. Trying to measure it when it's disabled during the mode 7 (squares) screen isn't going to be very practical, so let's work with what we've got.
  • Pins 10 and 20 are the power supply lines. All fine.
  • Pins 1 and 19 are the two (active low) gate inputs. Pin 1 is active for modes 0-6, and is clearly all the way low. Pin 19 is only pulled low when latching a "row" address from the 6845 into the RAM chips, which is roughly a quarter of the time. A value of 3.73V is perfectly fine for this, as 3/4 of 5V is 3.75V. So both gating inputs appear fine.
  • The remaining even-numbered pins are inputs to the latches, with the odd pins being their outputs. The fact that you found many of the odd pins changing value at the machine ran is sensible: when pin 19 is high, their voltage would be driven by IC10, 12, or 13. I'd expect pin 5 to be changing value, though. Pin 17 is used to control the Read/Write input of the RAM chips; because the display circuitry is only ever meant to read from the RAM, it's input (pin 18) is connected to 5V.
So, of all of those, only pin 5 is a mystery. Pin 5 is connected to A5 on the memory address bus, which would normally be associated with errors every 32 addresses (2^5=32), rather than 64. However, given that IC8 is running hotter than the other buffer chips, and you clearly have a video fault, I'd agree with 1024MAK's recommendation to replace it, and we'll see how the machine behaves after that.

You say you know your way around several other old micros. Are you confident at desoldering logic chips, or would you be wanting advice?

If we're lucky, IC8 is interfering with the output of the other buffer chips, and is the only (major) fault on the board. If so, removing the apparently faulty IC8 would let the CPU access the RAM correctly, and allow the machine to boot properly with the MOS fitted, but only to the default mode 7. (I wouldn't recommend trying Tricky's ROM without IC8, or accessing modes 0-6 with the MOS, as IC8 buffers the row addresses in these modes, which are used to refresh the DRAM. Without a regular pattern of row addresses, the DRAM will lose its contents very quickly.)
BBC Model B 32k issue 7, Sidewise ROM board with 16K RAM
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Re: BBC Model B - Issue 7 problem

Post by Vyper68 » Thu Aug 01, 2019 7:53 am

Hi Kazzie,
Yes I can desolder IC's and I am pretty good with a soldering Iron so replacing it will not be a problem, when I can get so of the right chips.
If this A5 input on Pin 5 is not as you would expect is there anywhere else you would want me to check this?

Just saw the small print on your post :D

So I will desolder it and fit a socket and try it with or without the IC.
Last edited by Vyper68 on Thu Aug 01, 2019 7:56 am, edited 1 time in total.
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Re: BBC Model B - Issue 7 problem

Post by Vyper68 » Thu Aug 01, 2019 10:43 am

Well that was a dead end I’m afraid. Makes no difference in or out with the OS ROM and BASIC fitted. same problem. Hangs on power on, pressing BREAK five or six times gets the second LED on and that's it.
It does not work in minimal boot configuration either I tried that from the get go when it packed in. So it's not the 6522's, I think the Cassette ULA is not working properly but I can live without it, again the only with minimal boot this is removed and have same problem.
I have tried a known good 6502A and that doesn't work either. The jumper setting for the RAM has no effect in either of the configurations.
So it's something on the Mainboard that has gone. What symptoms does the Heatsinked ULA present when it fails? and if the Beeb can produce a screen output of the various coloured bars does that indicate that the CTRC is working?
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