8-bit acorn hardware • Dead BBC B - Blank Screen, constant Beeeeeee

Hey folks,

Looking for some sagely advice!

The story so far:

As some may have read in my 'Introduce yourself' post the other week; I pulled my old BBC B (Issue 7) out and booted her up for the first time in 35-odd years.

Initially, it vaguely booted OK (double beep, then quiet) but had a garbled screen with incorrect characters when I typed. I did manage to blindly type in a program to play random beeps, and this worked. Up to the point that the PSU went 'PoP!'...

Replacement capacitors ordered from RetroClinic (thanks Mark - super service!) and installed, and the PSU was all good once again.

Whilst waiting for the caps to arrive, I took the opportunity to pull IC2 and IC6, check their pins, give them a quick IPA wipe down, then reinsert them (carefully!).

Now, trying to boot, the Beeb makes long, continuous "beeeeeeeee", the screen is blank (black, no cursor or text), the Caps Lock LED is on, and the keyboard completely unresponsive...

Things I have tried - pretty much in order, but to no effect):

1. Checked the voltages on the board at each VCC insertion point - all at 5v except VCC3 at 4.87v. The -5v is exactly that....
2. Pulled all the extra ROMs, leaving the OS (1.20) in IC51 and BBC Basic in 101.
3. Tried playing with S25 (originally North, South, off)
4. Swapped IC3 and IC69 (R6522P)
5. Replaced IC14 (SN74LS245N) as it's socketed and cheap to buy (put in a 'big' order with CPC )
6. Checked VCC on IC1 (6502), IC51 (OS ROM), IC101 (Basic ROM), IC3 (6522), IC69 (6522), IC2 (46505), IC6 (VC2023), 74LS<various>. All good.
7. Checked frequencies at IC1 (6502) - 2MHz and IC6 (VC2023) pins 4-7 show 1, 2, 4 and 8MHz respectively.
8. Followed MACK1024's excellent "Minimalist Beeb Guide"
9. Removed and replaced the IC51 socket (some of the other ROM sockets were iffy, so...) - I'll change the other sockets in due course - and reinserted the OS ROM

Now a bit stuck...

I have a decent multimeter that will also measure frequencies, but, sadly, I don't have an oscilloscope or logic analyser...

I'm thinking of replacing IC2 (HD46505SP) given its role and it too is in a socket and can be bought from Mark for not much dosh...

Any other advice or pointers? I've seen mention of replacing IC26 (SN74SL139AJ), but I'm wary of ordering more ICs in dribs and drabs (unless someone points me somewhere more useful, the usual suppliers charge an arm and a leg for delivery (or have minimum quantities that are daft for me!)). I'm also wary of desoldering too much - already worried I've caused more harm than good so far!

Thoughts?

Statistics: Posted by Bruisah — Thu Feb 01, 2024 10:04 pm

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