Jump to content
Due to a large amount of spamers, accounts will now have to be approved by the Admins so please be patient. ×
IGNORED

BBC Micro Model B repair


Recommended Posts

You might have noticed that I'm getting a few of my vintage computers restored lately. I'm trying to get through some of the backlog while putting a bit of cash away to finish the Thomas pin.

I have to thank @darkjedi for this one, a totally dead BBC Micro Model B. He'd replaced the rifa caps in the power supply but it still wasn't working correctly. The 5v line would start at about 2v and slowly creep up to 4v.

I had a look and found burn marks under one of the electrolytic caps. The cap had been replaced but I suspected that the electrolytics were still an issue as there was a mix of different brands of various quality. cc42e4efdef7a5be6cd3fab448bd9c8e.jpg

I had checked the other components in the vicinity of the burn mark and they all appeared to be ok. I ordered a fresh set of new (Nippon Chemicon) caps and waited, and waited. When they finally arrived and were fitted I had a healthy power supply putting out all the correct voltages.

I'd already replaced the missing 6522 via chip so was hopeful the machine would now boot. Not bloody likely. No video signal and a continuous tone from the speaker. Bugger.I found a YouTube video about a black screen repair for a BBC Micro Model B which suggested that the ram could be faulty.

The model B is a 32k machine but you can select just the first 16k by changing a jumper. I did this and it still had the fault. I checked the lower ram with my logic probe and one of the data out pins was stuck high so I socketed and replaced that chip.

With the machine still in the 16k mode I powered it on and got this error code.

2b3d5629e315273ce7892b41f63c195f.jpg

After doing a bit more Googling I learnt that the S21 jumper is sometimes removed to add an expansion rom board and if the board is removed and the jumper not replaced this error will occur. Yes the Rom board had been removed and so had the jumper which was now replaced and the machine worked fine in 16k mode.

7317576f6fe3be5faeea134307feaf88.jpg

The next thing to try was the full 32k so I set it back to 32k mode and you guessed it, it wouldn't work. There is an inverter board and bit of code that'll tell you which chip is faulty but as I couldn't wait for the info to come from the UK so I removed all the second bank ram, set the machine back into 16k mode and tried each chip in the newly added socket and yep one was a dud. Eight new sockets and one replacement ram chip later and the machine is fully operational in the 32k mode albeit with a bit of a hiccup with the composite cables.[emoji6]a62c8b115b6ccf2bf54e447734c4d3c4.jpg

Now perfect after replacing the internal wiring to the socket and using a new video cable.[emoji16]1e8313d136959eb54cd7641bc2a10b8c.jpg

Cheers Trev.

 

Sent from my SM-G930F using Tapatalk

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...