Repairing a BBC Master #3: Power checks

Now that we know the sickly BBC Master’s power supply is okay, it’s time to check whether that power is getting to all the chips on the board.

Luckily, the schematics help us here in more ways than one. On the first page is a table of all the ICs with pin numbers for their VCC and GND connections. I have this printed out and on a clipboard for easy reference.

I also have the schematic PDF imported into Notability on my iPad. Not only does this make it easy to zoom in on sections, I can also scribble notes over the image.

Tracking down where the IC is on the motherboard sometimes took a bit of hunting – there’s no readily apparent logic to the numbering. And not all of the ICs have power pin info. And as these are typically custom chips, that’s a problem. Nevertheless, I was able to ascertain that some pins on the chips were getting +5V, which is something. I’m sure there are pinouts for these chips somewhere on the interwebz and I should probably go looking for them at some point.

IC35, a 74ALS169, is listed in the table but not fitted on either this motherboard or the other one I have, so I guess Acorn decided it didn’t need that particular 4-bit binary counter.

It didn’t take very long to establish that, yes, power is getting to all the chips just fine. Of course, what they’re doing with it is another matter. This exercise doesn’t tell me if the chips are alive.

Strange event

During my probing, something strange happened that doesn’t throw any light on the problem but does at least give me hope.

It’s clearly not going to work in this state. Note the printed out IC list (left) and iPad with schematic.

I’d started the execise with the monitor switched off because I know I’m only getting a blank screen and I wasn’t about to fix anything. But halfway through I thought, ‘why not turn it on, just in case’. And I was treated to something on the screen. It was an asterisk and a cursor. So something is working, although the operating system doesn’t seem to be booted. Turning the machine off and on again made it all go away – back to a blank screen.

Either my probing, or perhaps the touching of chips, caused things to work – at least a little. I’m still thinking that there may be a bad solder joint or trace somewhere. I’ve already removed, cleaned and re-seated the few socketed chips.

Hot chips

The BBC Master had been on for a little while at this point, so I checked the motherboard for any ICs that might be running hot. IC18, a 4464, was warmer than the other 4464 chips. And IC42, the VID PROC chip, was quite hot, but not to the point where I couldn’t hold my finger on it. So nothing that seems out of the ordinary.

Oh well, on to the next step.

[BTW, I wrote this post some time ago but didn’t get around to publishing it. I’ve had an enforced absence from all things electronic but hope to be getting back into the swing of things soon, which is why I thought I’d make this live now.]



1 thought on “Repairing a BBC Master #3: Power checks

  1. Oliver

    Great to read another post. I’m sorry to hear about your life-woes. I sympathise because we’ve recently had some and I am still waiting for a chunk of free time to try out Ben Eater’s 6502 project which I first read about on your site. Take care & best wishes from the UK!


Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.