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BBC Model B No2. Will it live again!

Posted: 18 Apr 2021 18:47
by rubber_jonnie
So whilst I have focused a lot on the ST series over the years, some of you may have noticed that my retro net is cast quite wide with quite a varied selection of machines, but with a definite Atari focus, because they were my childhood and teenage years machines, along with the Sinclair machines that I'd had before.

At the time, the BBC was too expensive for my parents to buy, hence the much cheaper Sinclairs over the years. I know they'd have bought me one if they could, they are great parents, but I expect this was a scenario repeated across the UK.

The other machines that I remember fondly, are those that I used in the computer classes at school, namely an Apple II and BBC Model Bs.

I've yet to get an Apple II, mostly because prices are silly now, and I'd rather try and find machines that better fit my budget. So I decided to head down the route of the BBC Model B, and bought one earlier in the year for about £90 including delivery.

This No1 machine unfortunately ended up staying in it's box because of the house move, and once we got settled in in our temporary accommodation, I just felt I needed to get my hands on another, and for £55 got a really grubby No2 machine to help keep my retro juices flowing. I suppose I could have got another ST, but I have rather a lot of them and fancied a different challenge. Plus I brought my MSTE with me which helps in that direction.

So this is how BBC Model B looked on unboxing:

Dirty_Case.jpg

With the keyboard particularly grubby:

Dirty_Keyboard.jpg

At this point it felt rather like a barn find car!

I popped off the keycaps to find a host of tiny dust bunnies, which were banished with a stiff brush:

KBD_Unclean.jpg

And the PCB wiped down with a floor wipe, which I find very good for getting rid of surface dirt.

Further examination of the KBD found a number of bodges and some missing LEDs for caps lock etc:

KBD_Bodges_1.jpg
KBD_Bodges_2.jpg

And after pulling the mobo out so I could wash the top/bottom cases, there was even evidence of a previous magic smoke incident, though the mobo seemed undamaged:

Magic_Smoke.jpg

So I've spent a couple of hours scrubbing this afternoon and the case and keys are looking a lot better:

Clean_Case.jpg

As you can see, colour wise it isn't great, but at least now it is clean and I don't heave when I touch it!! I have the opportunity to leave it outside in the sun at the moment, so I may opt to try leaving it out to see if the colour improves.

Clean_Keys.jpg

And I'm sure once I reassemble the keys, the KBD will have a much better look and feel.

Given there is evidence of some rework on the KBD and mobo, I'm going to have to get the IPA out and clean off what looks like 30 year old crusted on flux, so fingers crossed it'll all be good.

The KBD bodge wires are going to have to be replaced at some point, but I will leave stuff like that until I'm in my new workshop, unless of course my hand is forced because it doesn't work at all.

Next on the agenda is the PSU, and I'm going to do a basic recap, which is the 2 RIFAs (Otherwise known as magic smoke generators) and the switching PSU starter cap. This is what seems to be recommended as a starting point by the 'experts' though I feel I'm likely to do a full recap once I'm in my new workshop.

Re: BBC Model B No2. Will it live again!

Posted: 21 Apr 2021 23:33
by rubber_jonnie
In an effort to not hijack @paulj's blog post, I've jumped over here and added what @stephen_usher said:

"Oh, that's not good. That means that the sound chip isn't working at all as the initial beep is caused by it powering up in a random mode and unconfigured by the system. It could be the op-amp circuit, check the -5V. I had one of the smaller caps go in the PSU and it sent the -5V into a bad state. It could also be the ULA near the cassette port, which interfaces with the op-amp chip. Try removing it."

The -5v seems to be OK and actually at a steady -5v, however the +5v seems low, under 4.8v under load and not much higher even off load. I don't know how well the Beeb deals with voltages that low but I've certainly seen instabilities in contemporary machines with voltages at that level.

I can certainly try removing the ULA, but really wanted to see what the sound was doing as I don't currently have a suitable TV lead. I need to get the schematics, and I have the service manual, but the other thing it really needs is a good clean up because there is a lot of oxidisation on all the chips and sockets. There are a few other things that don't truly look good, like a black and red lead appearing from the mainboard near the modulator. Makes me wonder if there was some sort of mod there which is now gone.

Re: BBC Model B No2. Will it live again!

Posted: 22 Apr 2021 06:59
by cmorley
rubber_jonnie wrote: 21 Apr 2021 23:33 +5v seems low, under 4.8v under load and not much higher even off load.
That's standard. It is possible (fairly easy in fact) to adjust the PSU output by changing the feedback reisitors - one is usually cropped out from factory - but it is hardly worth it.

Re: BBC Model B No2. Will it live again!

Posted: 22 Apr 2021 08:27
by rubber_jonnie
cmorley wrote: 22 Apr 2021 06:59
rubber_jonnie wrote: 21 Apr 2021 23:33 +5v seems low, under 4.8v under load and not much higher even off load.
That's standard. It is possible (fairly easy in fact) to adjust the PSU output by changing the feedback reisitors - one is usually cropped out from factory - but it is hardly worth it.
Thanks, then I have to start looking at other things.

Re: BBC Model B No2. Will it live again!

Posted: 22 Apr 2021 08:48
by cmorley
If you have no initial beep then check that the volume hasn't been turned right down. Contrary to numerous youtubers' assertions that the beebs are too noisy and don't have a volume control, they all do it is VR1. You might be missing clocks though... check on the schematic & test for your 16MHz clock and all the sub clocks generated by the video ULA.

Re: BBC Model B No2. Will it live again!

Posted: 22 Apr 2021 09:37
by rubber_jonnie
cmorley wrote: 22 Apr 2021 08:48 If you have no initial beep then check that the volume hasn't been turned right down. Contrary to numerous youtubers' assertions that the beebs are too noisy and don't have a volume control, they all do it is VR1. You might be missing clocks though... check on the schematic & test for your 16MHz clock and all the sub clocks generated by the video ULA.
Thanks, sounds like a great place to start. This one may end up taking a back seat if it's in need of my oscilloscope, as thats in storage until I can get in the new house.

I do see a pot marked VR2, but not VR1. Issue 4 board BTW

Re: BBC Model B No2. Will it live again!

Posted: 22 Apr 2021 10:49
by rubber_jonnie
Well, I have the schematic now and the service manual, so I just need to keep plugging away at it.

Re: BBC Model B No2. Will it live again!

Posted: 22 Apr 2021 12:38
by cmorley
rubber_jonnie wrote: 22 Apr 2021 09:37 I do see a pot marked VR2, but not VR1. Issue 4 board BTW
39790620603_c2c93ac1e5_b.jpg

Re: BBC Model B No2. Will it live again!

Posted: 22 Apr 2021 12:55
by rubber_jonnie
cmorley wrote: 22 Apr 2021 12:38
rubber_jonnie wrote: 22 Apr 2021 09:37 I do see a pot marked VR2, but not VR1. Issue 4 board BTW
39790620603_c2c93ac1e5_b.jpg
So, the plot thickens, I just had a look there, and it looks like the pot has been removed, and in it's place there are 3 wires, cut off almost at board level!!

I am wondering if it had any audio out mods, but I expect with nothing connected to any of the 3 pins, there won't be any audio out. I presume if I were to jump the centre pin to either of the wiper pins, I'd get no volume or full volume respectively?

Re: BBC Model B No2. Will it live again!

Posted: 22 Apr 2021 13:04
by cmorley
Sounds like a remote mounted volume control mod. Some people mount a pot at the back of the machine. If the pot is missing then you will get no audio... if CAPS LOCK doesn't work then it is probably "long beep"ing but you just can't hear it (catch all term for not passing the POST).