OMG! Issue 2 Beeb, Serial #000732 is alive!

discuss both original and modern hardware for the bbc micro/electron
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semtex
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OMG! Issue 2 Beeb, Serial #000732 is alive!

Post by semtex »

To celebrate Beeb@30... and to do something while waiting from my Raspberry Pi, I unboxed my BBC Micro after 23 years in storage!

I switched it on... beeeeeeeeeeeeeeeeeeee... it didn't work.

I was gutted to discover that the fault-finding thread on this board hasn't really materialised but I used a copy of the BBC Micro Service Manual to help with diagnostics.

I checked all the power supply voltages and they were perfect (unfortunately... that would have been too easy). I then removed all unnecessary stuff (all ROMs except OS and BASIC, Opus disk controller and IC4, IC7, IC69 and IC73). It still didn't work. I swapped link S25 to check the different 16k RAM banks but still no luck. A logic probe on pin 40 of the 6502A confirmed it was permanently High, even when the BREAK key was pressed and pin 7 was permanently low.

Things weren't looking good... but it was a comment on this forum that solved the problem. Having already removed IC69, someone mentioned to remove IC3 (which is also a 6522) and replace it with the 6522 from IC69.

Tada (sorry, that's from a Mac Classic! I mean Beeee-bup!) It worked!! Awesome!

But alas the story does not end there and this is where I need some help. There are still a couple of problems. Obviously I need a new 6522, but I beleive that really only impacts the user port, printer port and some games. I also noticed that Q2 is physically broken so the cassette port probably won't work. These aren't big issues and parts will solve those.

My 2 main issues are...

1) When I plug my Opus Disk controller (WD2791 based) into the socket for IC78 the Beeb will not power up but emits a single and continuous beep. I'm not sure if the fact that IC3 is missing is the cause of this. Can anyone make any suggestions?

2) The other problem is that keys "N" and "RETURN" do not work. All others are fine. The RETURN key not working is obviously an issue!! Looking at the circuit diagram I cannot spot an obvious connection between these two keys, though CTRL-N and RETURN both have the same keycode (&0D). Putting a continuity tester across the key contacts for these keys on the keyboard PCB suggests that both key switch buttons have failed. Does anyone know if replacement key switch buttons are available or are these two keys linked and maybe it is a circuit issue?

3) This is a minor issue which really just requires research on my side. I now live in Canada. The Beeb is powered from a "proper" step-up transformer. I thought video was going to be a problem until I discovered that a simple BNC-Phono adapter allows the BNC output to be connected to a composite video input (B&W only). But I'm wondering if connecting the Beeb RGB output to component video is possible?

Thanks in advance for any help. I'm really looking forward to hearing Kenneth Kendall talk again, to playing Revs, Aviator, Castle Quest, Repton etc and making music with the Music 500 system. Good times ahead :-)

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poink
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Re: OMG! Issue 2 Beeb, Serial #000732 is alive!

Post by poink »

Can't really help you on much of that, but I've seen people suggest switch contact cleaner for problematic key switches. I think the procedure is squirt a little in and then pound the key a bit!
If that doesn't fix it, I think Mark (retroclinic) is in a position to supply spares.
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paulv
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Re: OMG! Issue 2 Beeb, Serial #000732 is alive!

Post by paulv »

There are a few of us with spare keyswitches hanging around the place but it depends on the keyboard as to which type you need. A photo of the key switches would be useful to solve that issue for you..

I'd speculate that IC3 being missing is nothing to do with your Opus Disc Controller causing the Beeb to fail to start correctly and if it isn't the Disc Controller (which is the prime supsect at this point) then it could be one of the supporting logic chips in the disc interface area that is the problem. I had a couple of issues with one of my issue 4 beebs that were related to the logic chips rather than the disc controller itself one of which was a faulty socket that was causing the Beeb to intermittently not initialise the DFS correctly...

Did the Opus disc controller do stuff outside of the normal DFS type systems and do you need it to be able to read the Opus formatted discs or would a replacement 1770 disc controller do the job for you?

Paul
station240
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Re: OMG! Issue 2 Beeb, Serial #000732 is alive!

Post by station240 »

semtex wrote: 3) This is a minor issue which really just requires research on my side. I now live in Canada. The Beeb is powered from a "proper" step-up transformer. I thought video was going to be a problem until I discovered that a simple BNC-Phono adapter allows the BNC output to be connected to a composite video input (B&W only). But I'm wondering if connecting the Beeb RGB output to component video is possible?
If you have a monitor/TV with a SCART socket, then its just a simple matter of making/obtaining off ebay, a BBC to scart cable.

Note that while CRT displays will work, LCDs etc are a bit hit and miss due to signal processing. Actually the same problem exists with many signal format conversion boxes too.
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MarcT
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Re: OMG! Issue 2 Beeb, Serial #000732 is alive!

Post by MarcT »

semtex wrote: 3) This is a minor issue which really just requires research on my side. I now live in Canada. The Beeb is powered from a "proper" step-up transformer. I thought video was going to be a problem until I discovered that a simple BNC-Phono adapter allows the BNC output to be connected to a composite video input (B&W only). But I'm wondering if connecting the Beeb RGB output to component video is possible?
There is a link(S39) on issue 4 boards and above to add colour burst to the composite video out, but a mod can be done on earlier boards. Quoting from the BBC Service Manual section 7.1:
On previous issue boards it is necessary to introduce a 470pF capacitor from the emitter of Q9 to the base of Q7. This capacitor would have to be soldered directly on to the circuit board.
That said, you can probably buy/make an RGB to SCART cable (just needs a few resistors), as it would seem that your display can handle 50Hz PAL.
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regregex
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Re: OMG! Issue 2 Beeb, Serial #000732 is alive!

Post by regregex »

On the disc controller side, check the 4 MHz clock (on pin 3 of the header) and the interrupt outputs (pins 8 and 11). The latter may be being held up and perpetually interrupting the startup routine. Also check the pins for tarnish or bending in as the header Opus uses is prone to these. AFAIK the controller is separate from IC 3.
Btw that controller is rather rare, what does it look like and what is the driver ROM called?

Greg
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