Category Archives: Retro Computers

BBC B Computer – Power Supply

As the BBC B is faulty and needs to be repaired, I’ve decided to start repairing the power supply. From reading some forums and the BBC B service manual it is very common for the input capacitors and some of the secondary smoothing capacitors to fail.

The power supply and capacitors here are 35 years old. Old electrolytic capacitors do fail over longer period of time.

The 2 X-Class capacitors indicated above are the input stage capacitors that tend to fail over time. Although these haven’t failed yet I am going to replace them.

These are the 2 capacitors that I have removed from the power supply. As you can see they are cracked.

…It has been a couple of weeks since this post was last updated and the replacement parts have arrived. I have decided to replace all the electrolytic capacitors on the power supply.

Here above we see all the parts that were ordered from Radionics. When you look at the YouTube video, you will hear that these new parts are smaller than their original parts.

Above we can see the parts that I am replacing in the power supply. All of these will be heading for the bin.

Before fitting the power supply back into the BBC Micro, I test the power supply. As this power supply is switch mode power supply, it requires a minimum load for the power supply to operate. In the above image you can see I am getting a supply voltage of 5.335v where it should be closer to 5v.

When the load is connected, we can see that the output voltage is 5.061v, which is much closer to the desired output voltage.

I check both the 12V and -5V outputs on the power supply and record the values of 12.07V and -4.673V for both of these outputs.

Once the testing is completed, I install the power supply back into the BBC Micro, and power it on. The dual beep at startup surprises me and indicates that all appears to be good.

I connect the BBC Micro to my desktop VGA monitor via a Composite Video to VGA adaptor and to my surprise I see the BBC Micro splash screen.

After typing *HELP and pressing ENTER, the BBC B Micro returns a list of the installed ROMS. Here I can see that DFS (Disk Filing System), Teletext Telesoft firmware and VIEW (Word processor) ROM’s are fitted.

Now that the BBC Micro is up and running, it is time to run a little BASIC program to show that it all works.

And yes, it appears that I have a working BBC B Micro.


BBC B Computer – First Inspection

I am excited to share that I just received a BBC B home computer, a machine that has been on my wish list for over 25 years! This one came with a floppy drive and teletext controller. I also got about 30ish 5.25″ floppy disks.

I remember first using a BBC B computer in a friend’s house back in early 1984 and yes, he did have Elite (which was loaded from a cassette tape). It was the first time I had seen a 3D wireframe game on a computer. I remember thinking how wonderful it was and wondered what the future of computer games would be like, once technology got better.

While I was in third year in secondary school, I remember when the dean of my year asked what computers the school should get for the new computer room. At that point the school only had an Apple II computer. Some students were suggesting Commodore 64’s, while a few of us suggested the Beeb. I was very happy when a few months later we got told that we were getting Beeb’s.

I remember counting the days to the delivery date and feeling very sad (Sheldon sad) when I found out that we would not have the computers until after the Christmas holidays.

When we eventually got access to the computer lab, we found out that we had a network of 16 BBC B’s for workstations, a BBC Master with double drives for the server and another BBC B with an Epson Dot Matrix printer.

Many an evening was spent after school in the computer lab, writing programs in BASIC and learning my craft. At home I was using my Dad’s Epson HX-20 portable computer and friend’s computers. Although I was repairing faulty computers for friends and family, I didn’t have enough cash to buy my own.

So for a couple of years I wrote games for the Beeb, which my classmates would play when we had computer class. I always told myself that someday I would get a BBC B. When I left school in ’87 my dad got an IBM XT Clone for work. I started my collection of computers, which never included a BBC B until now.

So, here it is, an old yellowed BBC B computer. It has that old computer smell, so I hope that all is good inside, which may not be the case considering the age of this machine, so it looks like I’ll have to RetroBright the case, top and bottom.

I can’t wait any longer. I have the screwdriver out and it is time to open the computer to have a sneak peak inside.

At first glance it all looks good inside, although pretty dusty and in need of an Econet interface upgrade. Next step is to power up the computer to see if it works. This does not give the hoped for result – I get a constant tone from the computer speaker – the most common cause of this type of problem is faulty RAM.

This is not good, so before I do any more damage, I power off the computer and order a service manual for the Beeb which was a reproduction of the original manual. Further inspection of the computer shows up a damaged tube IDC connector.

I think I have a spare 40pin IDC connecter in stock, so this is on the list of things to fix for this computer.

The date stamp on the power supply puts a manufactured date of week 22 of 1982. This makes this computer 35 years old. I do check the voltages on the power supply and verify that I measure  -5V and +5V, so it all looks good here, but I have no guarantee that it is a clean supply.

We can see from the main board that this is a revision 3 PCB.

After taking the board out and looking at the underside, I don’t see any bodge wires or hacks.

This is a good sign as it shows that no-one has tried to make any upgrades to this PCB. I hope that this means that the problems that exist are only minor issues.


Epson PX-8 Battery Rebuild

After checking and rebuilding the battery for the Epson HX-20 (see the link at the bottom of this post), I then decided to check the battery packs in the two Epson PX-8 computers that I have.

These were both in poor condition and needed to be replaced. This got me thinking of all the old retro computers out there that are slowly getting destroyed by leaky batteries. So if you have one of these computers buried in a wardrobe, garage or attic; please take it out and check its battery, as if you leave it too late, the battery acid just may corrode the main PCB to a state that makes it beyond repair.

Neither end of these batteries are in useable condition. I’m not going to open them, I am just going to bin them responsibly (local shops take in old batteries) and build new battery packs. If you don’t want to build them, you can purchase from several online suppliers.

After about an hour of repeating the same process that was used when making the new battery pack for the HX-20, I now have a new battery pack for the PX-8. Only one more battery pack to build, but I need to get a couple more of the 2/3 C Cell rechargeable batteries, to complete the last pack.

At least the connector was not as damaged as the connector on  the HX-20 battery pack. So now I have a good battery pack that can be used when I start the repair process on the PX-8 computer.

HX-20 Battery Pack Rebuild 

HX-20 Battery Refit

After searching through my part bins, I was able to find a contact that I fitted into the connector for the battery pack, to replace the contact that was corroded. I fitted some spacers between each of the cell, then wrapped the battery pack in some tape to seal it.

Once the pack was assembled, it was fitted back into the HX-20. While reassembling the computer, I spotted a dry joint on the DC jack. So once again I disassembled the computer to remove the main PCB.

Once the PCB was out, I removed the DC jack, cleaned the contact for the DC jack, cleaned the pads on the PCB, reassembled the computer and tested it. The computer beeped when it was powered on, but I got no text on the LCD. So scratching my head, I disassembled the computer and checked the connectors. They all looked good. Then I remembered that the computer may need to be reset, as the battery had been out of the laptop for a few days.

So while the LCD was not showing anything when powered on, I remembered that you need to press “CTRL & @” to initialise the computer. After entering in a dummy date time of “01010101101” and pressing “Enter”, the computer reset and the LCD became active.

Yeah, the battery has been replaced and the computer is working again. 👍

Replacement Battery Pack for Epson HX-20

After disassembling the HX-20 the other day, and discovering that the Ni-Cad battery pack is leaking after 35 years,  I purchased some 2/3 C size cell rechargeable batteries. I purchased a total of 10 of these cells, as I plan to make a new battery pack for the HX-20 and one for a Epson PX-8’s that I also have. I suspect that if the batteries in the HX-20 are in this state then the ones in the PX-8 are not far behind.

Blog Post showing the teardown of HX-20.

Here is a link to these batteries on Amazon for those of you who want to replace their batteries in the computer.

After verifying that they were the correct size, I started to break apart the old battery pack to see how far gone  the batteries were. If you’re doing this at home, lay out some newspaper and wear gloves, as battery acid is nasty stuff.

Once I removed the yellow plastic shrink wrap from the battery pack, it was very clear that the protective foam on the negative side of the battery pack terminal was saturated (battery acid) and the terminal was corroded.

When the protective foam was released the lead to the negative side of the battery pack just fell off. It obviously hadn’t had a solid connection to the negative battery terminal for a long while.

Just to see how far the corrosion had gone, I cut into the wires going to the connector for the main PCB. I was unable to find any good clean tinned or copper wire – it was tarnished all the way to the connector. This means that I need make up a new connection going to the main PCB (I may just replace the headder  to the main PCB with PCB molex headder and replace the connection on the battery pack).

Curiosity got the better of me and I removed the last of the red shrink wrap on the old battery pack, just to see how bad the batteries were. The walls of two of the batteries were paper thin and were corroded in places completely. In fact I found two holes in the battery shells which are visible above. Can you spot them?  These batteries had seen better days and I was very happy that there were getting replaced.

It was very easy to wire up the new batteries. I just had to tack solder the positive lead of one cell to the negative lead of the other cell in series. Next step is to organise some spacers for in-between the cell, so the casings don’t short together, then to wrap the cells and install back in the computer.

– [Batt1] +  – [Batt 2] + – [Batt 3] + – [Batt4] +

We just have 4 x 1.2V Cells in series giving 4.8V. Once I get the new connectors for the battery pack and the main PCB. I’ll re-fiit the battery pack back in the HX-20, charge it up  and start testing.