BBC B keyboard issues

[ed731pdh]

Bit of a poser this one, and other than scoping the PSU (next thing to be done) I'm struggling.

BBC model B, issue 7 keyboard. Turn on and powers up ok with no issues. After about a minute or so of operation the keyboard starts to become erratic in operation, missing key strokes etc. Get to around near five minutes and the keyboard is doing it's own thing, with erratic data appearing on screen. 22uf keyboard cap was changed but to no avail. I've attached a schematic of the keyboard to this post.

I'm starting to consider a noisy power supply but before I go delving further has anybody come across this fault before and if so what was the issue?

Thanks.

[Skywave]

I have no experience of that fault, but you may well be correct in suspecting the PSU. The symptoms strongly suggest that the fault is heat related. For the purposes of elimination, can you power the keyboard from an external bench-type PSU ?

Al.

[Geaney]

since there seem to be some IC's on the keyboard itself you could try spray freezing them once the fault is present to see if the problem lies in the keyboard or elsewhere.

Cheers, John

[ed731pdh]

The only repairs to the PSU were the replacement of the X class caps on the mains side of the circuit due to the usual going poof trick with lots of smoke. The secondary side is original, was inspected for leaking caps with nothing found. The five volts line is a little low, running at 4.8 rather than bang on 5 but I wouldnt have thought .2v would make a lot of difference. However, this is a digital circuit and it may be that it needs to be bang on 5 to operate correctly.

[Skywave]

Logic ccts.at exactly +5v. to operate correctly: unlikely to be that critical. I've met lots of PCs where the "+5v" can be anywhere between 4.8v and 5.2v without any indications of malfunction.
The first question that needs to be answered is this: is it the PSU or the kbd. where the fault is located?

Al.

[tiv48]

i would be incind to check the suply rails with a scope to se if there are any spikes.
rob t

[ed731pdh]

After finding the time to trapse into the nearest town for supplies I turned back to the the BBC. Some interesting findings.

1) The PSU has now decided to stab at +-4.97 volts. Still have to pop a scope onto the rails.
2) Cold start turn on, all appears to be ok, keyboard works.
3) Press break to reset, screen clears to initial page as per cold start.
4) Press any key, speaker erupts into sound, high pitched whistle with sound from another note, dependent on key pressed, screen shows correct key operation.
5) Press ESCAPE and all noise ceases.

On a whim, swapped the two 6522s over with the same result. Both therefore can be exonerated, unless both have the same fault. Checked resistors on 555 reset crcuit, all read correct at 1mr. May need to scope reset voltages to see if clean. Spraying around with freezer reveals no obvious heat issues.

Get scope out tomorrow and have a fish.