BBC Master 128 ,No function, no screen etc.

[MauriceH]

This machine just presents a low constant tone out of speaker.
Blank screen, no video activity at all.

Did already dive deep into the machine without luck getting it going.
Clock's are all there 1,2,6,8 16MHz

There is activity on address lines and Data lines.
IC22 activity on all Testpoints

Checked the RAM's , are ok, even replaced them. 4464, IC17,18,23,26
Replaced 6522, IC6-8
Replaced 6845

IC16 KBD Ic does scan Row and Colum, does activate CA2 when a key is pressed
but no activity at NKBEN and PA0-7.
So at start up to press "R"-Key has no use at all.

No activity at all at IC10 (74LS259) pins 1,2,3 (ABC) no activity just High. And "D"(13) no activity.

IC14 65SC12, RDY(2) activity, Sync (7) is there, RST does work, IRQ(4) activity
IC24 MOS, changed out, no effect.

When Power-on at random, or after couple of BREAK it sometimes enters into screen mode around 7 I think.
That is SAA5050 got active and I get a random scrumble screen, B/W or even with some colour in it.
By then RGB-out is active and so IC42 VIDPROC sends those through to RGB-out , so that IC seems to work.

Signals VS and HS does change when Master seems to enter different Modes,to some polling or nothing.
If there is some Video, there is no sync of the screen, it roles vertically with scramble picture, no char to be noticed.
As in those modes VS HS are steady and no pulses.

Signal "CURSOR" no activity at all.


Main problem I think is the no activity at that KBD-IC at output PA0-7.
As the nKBEN signal is not activated by a key-press or by 6522. (see attached)
CA2 does activate by a keypress though.

 

[Gary C]

So you have tried holding down R when powering up ? Try it with the battery disconnected. Sometimes on one of mine it takes a few times.

I take it the I/O processor is showing activity ?

I have two on the bench at the moment (one working perfectly, one almost working perfectly) so I can check how things operate to compare things against if you need it.

 

[MauriceH]

Gary,
Thanks for replay.
The keystroke does not give an effect at all. Only the Break key, but that is cause its a different circuit.
That nKBEN signal Must go Low after CPU indicates an activity due to CA2, as that is working if there is a key stroke.
nKBEN does not activate to Low.

So that way I thought , must be something with the Boot process that does not finish.
Looking at schematic it must be something to do with those 2 DMA handling ICs , IC29-33.

And indeed. IC29 BAD-Test.

As I have many Logic ICs , off-course the one I need... Nope, So I am waiting for those I ordered.

Rest of logic ICs I also took out, but all in good order.
So hopefully that was the main problem, I'll let you know.

The 6845 -IC31 was also defect , I changed out already.
Half of MA-signals (MA7-13) no response.After replace I had all of them.

Think Tomorrow the 253's will arive

 

[Gary C]

Might be worth blowing a test rom (https://github.com/tom-seddon/beeb_test_os)

I am going to try it if changing the CAS logic IC's which have oddly been swapped for 74LS types (along with multiple other parts)

KBEN is only lowered on an interrupt from the keyboard controller to take it out of free run, if the 6522 has been setup to interrupt from CA2 so is IRQ being asserted (I would imagine not) when a key is pressed ?

So its probably a check on memory during startup that its getting stuck on ? We need a breakdown of the things the MOS does on startup before it programs the sound IC and shuts it up ! Would be useful.

 

[MauriceH]

All did not help.
With both the new 74HCT253 still no luck.
Tested again the old IC 29 and really failed.
Both new are OK tested.

Main problem is IRQ is constant LOW, so active.
What/who that device cause, no idea.
Took out IC 45 (6850) (serial circuit) , still IRQ is low.
Typical that Cassette Relay is often switched or rattling some what.
If I press BREAK couple of times.
Relay should only be activated by IC48 SerProc.
Maybe, I'll take that one out also.


Also took both 6522's out, still IRQ is low.
Reading, IRQ is held low till program is ready.
Ready is High.

AT some point of many restart , sometimes machine enters in Both
IRQ and Ready to go in to High state.
AT that moment at KBD IC16 that CA2 signal is Low and not pulsing when a key is pressed.
pin 38 is though in LOW state.

Again it is of no use to press a key as no action on outputs.
Inputs of IC16 kbd is activated when a key is pressed.
So key;s them selves are scanned.

Put in a New MOS with new version (with hold 3 versions slected with a switch)
Is of no help.

Sometimes I get a scrambled screen.

 

[Gary C]

Sounds like one of the custom IC's.

It might be worth removing the mem controller and see if the CPU and IO controller bursts into life

And of course check all the links are set right.

 

[MauriceH]

Took out that IC48 (serial) and IC3 WD1770, no luck in a change of behaviour.
Took out IC20 Mem control, But then also whole data is also stopped.
With IC20 at least I see dataflow at MOS and Memory.

Think CPU is OK as I see data change
So its Vid Proc , I/O or Memcontrol
So have to find a scrap BBC128
Said.

IC15 : 724V VG7205-0719 , 2201362
IC20; 725V VG7205-0718 , 2201361