I’ve moved the game on a game a little in the last few days.
When I first tried the machine I tried to type in a four line program to output numbers so I could see if the machine was hanging or if the keyboard interface PIA was failing. At first I couldn’t type it in before the machine hung. I determined that this was overheating and found a cause.
Now I can type in the program but when I run it it crashes with changing errors. Sometimes it reports a bogus/ill formed error message. Sometimes it just hangs. If I press “Break” the machine restarts. I then type “OLD” and the program is still there. When I run it I get the same result.
I’m thinking dodgy a RAM chip. About half of the DRAM has been socketed.
I have made some progress over the last couple of days.
I looked at the power rails before and after the hang and nothing to report there really. At bit of hum (about 40mv) but it says constant.
I found that IC 69, the 6522 was getting too hot to touch. Cooling it kept the machine running a little longer. I looked at the circuit diagram and IC69 is connected to the outside world via an 8 bit buffer. I pulled the buffer and the temperature went right down. Looking at the buffer, its pins were filthy and so I cleaned them up with a pencil eraser and then some IPA. When I put the chip back in the 6522 stayed cool. Result.
I think the buffer wasn’t getting powered properly because of the dirty pins and so was taking “Phantom” power through it’s I/O lines. I’ve seem something similar before.
Now the machine is running for a few minutes before hanging. Much improved.
My plan now is to work through the rest of the chips making sure that they are not just seated well but also cleaned
I have asked for clues on the StarDot forums, a valuable source of BBC knowledge. Within hours some very good ideas were suggested.
So far I have tried cleaning and re-seating the socketed chips and swapping the 6522s but to no avail. I have measured the volages on the supply rails before and after the hanging but they don’t change.
I also tried putting a spot of IPA on the back of the chips to see which was getting hot. The hottest should evaporate the IPA quickest. I couldn’t tell but I thought the machine stayed running for longer because of the cooling effect but I’m not sure.
Spoiler alert. If you don’t want to know what happened, don’t read the title of this post. Too late.
I got home tonight to find that the new keyboard cable for the BBC Micro had arrived. I plugged it in and just a long beep. Doh!
I then did a lot of looking and checking and found that the left most pin on the motherboard had been bent over at some point. I bent it over and tried again. This time, beep, beep and a banner and prompt.
However. It hands after a minute of so. A restart and it only lasts seconds and so I’m convinced that it’s thermal. More investigation needed.
As I said in my previous post, the BBC micro was showing some signs of life but wasn’t booting. I sent off for capacitors for the PSU and they arrived. In with the screwdrivers.
Just three screws from the back and the PSU is on the bench. The unit itself is a little trickier to take apart but not difficult.
Be aware there can be high voltages on the board even when it’s out of the computer and disconnected from the mains.
The easiest way is to pop out the switch and external power connectors first. A flat bladed screwdriver will let you press the springy plastic lugs down and then they push out. Look out for the two green earth leads bolted inside.
Then the unit is out.
I think two of the caps had already been changed which just left the electrolytic at C9 (IIRC). This looked bad and was bulging so out it came.
I reassembled the machine but to be honest I wasn’t expecting a change as I think the fault is caused by the keyboard cable and not the PSU. Changing the cap was a safety measure. Hopefully the cable will arrive in the next day or two.
Now be honest. Is there anyone aged between 35 and 55 in the UK who hasn’t used a BBC micro? This little machine was massive over here. They were everywhere, Rugged, dependable, terrific BASIC interpreter, in-line assembler. The list goes on and on.
This machine came to me from my friend Tris. He is another retro-computer collector but unlike most of us, knows when enough is enough and decided the his BBC Master would be enough and gave me this non-functioning unit.
Under the lid.
The machine has an Issue 4 mother board. I think this is a bit unusual. From memory I think Issue 3 and Issue 7 were the most numerous. Another thing it has is a lid lined with aluminium foil. Curious. I wonder if it gave out too much interference for the TV or somesuch.
When power is applied, I here one beep. There should be two. With a monitor connected there is a nice raster on the screen and a flashing cursor. Nothing more.
A look inside shows that the keyboard connector is very tired, with a number of connections broken. I tried to reconnect the broken ones. There was a brief bit of activity as the words “BBC Micro…” started appearing on the screen. _VERY_ Slowly. I tried a reset and things got worse. The beep didn’t end.
Now I’ve seen that the machine is close to working and so my strategy is this.
Replace the keyboard cable – I have one on order.
Replace the capacitors in the power supply – It won’t hurt.