BBC Model B No2. Will it live again!

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rubber_jonnie
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BBC Model B No2. Will it live again!

Post by rubber_jonnie » Sun Apr 18, 2021 6:47 pm

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
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With the keyboard particularly grubby:

Dirty_Keyboard.jpg
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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
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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
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KBD_Bodges_2.jpg
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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
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So I've spent a couple of hours scrubbing this afternoon and the case and keys are looking a lot better:

Clean_Case.jpg
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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
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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.
Collector of old Atari things:
800XL + Ultimate 1MB, 2x 1010 cassette, 1050 + Happy mod, 65XE (128k +APE Warp) & XC12, SIO2SD, 2600jr, 7800 and Lynx II
Atari 520ST (1Meg), 1040STF long button floppy, 2x 1040 STFMs + Gotek, 2x 4160STE with 32Mhz booster, ROM switcher, Mega ST1, Mega ST4, not to mention various bare ST boards for testing including a PAK 68/2 :)
Plus the rest..
Amiga stuff, Mac stuff, Sinclair stuff etc...
www.electronicnothingness.co.uk

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

Post by rubber_jonnie » Wed Apr 21, 2021 11:33 pm

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.
Collector of old Atari things:
800XL + Ultimate 1MB, 2x 1010 cassette, 1050 + Happy mod, 65XE (128k +APE Warp) & XC12, SIO2SD, 2600jr, 7800 and Lynx II
Atari 520ST (1Meg), 1040STF long button floppy, 2x 1040 STFMs + Gotek, 2x 4160STE with 32Mhz booster, ROM switcher, Mega ST1, Mega ST4, not to mention various bare ST boards for testing including a PAK 68/2 :)
Plus the rest..
Amiga stuff, Mac stuff, Sinclair stuff etc...
www.electronicnothingness.co.uk

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

Post by cmorley » Thu Apr 22, 2021 6:59 am

rubber_jonnie wrote:
Wed Apr 21, 2021 11:33 pm
+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.

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

Post by rubber_jonnie » Thu Apr 22, 2021 8:27 am

cmorley wrote:
Thu Apr 22, 2021 6:59 am
rubber_jonnie wrote:
Wed Apr 21, 2021 11:33 pm
+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.
Collector of old Atari things:
800XL + Ultimate 1MB, 2x 1010 cassette, 1050 + Happy mod, 65XE (128k +APE Warp) & XC12, SIO2SD, 2600jr, 7800 and Lynx II
Atari 520ST (1Meg), 1040STF long button floppy, 2x 1040 STFMs + Gotek, 2x 4160STE with 32Mhz booster, ROM switcher, Mega ST1, Mega ST4, not to mention various bare ST boards for testing including a PAK 68/2 :)
Plus the rest..
Amiga stuff, Mac stuff, Sinclair stuff etc...
www.electronicnothingness.co.uk

cmorley
Posts: 223
Joined: Tue May 28, 2019 5:46 pm

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

Post by cmorley » Thu Apr 22, 2021 8:48 am

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.

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

Post by rubber_jonnie » Thu Apr 22, 2021 9:37 am

cmorley wrote:
Thu Apr 22, 2021 8:48 am
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
Collector of old Atari things:
800XL + Ultimate 1MB, 2x 1010 cassette, 1050 + Happy mod, 65XE (128k +APE Warp) & XC12, SIO2SD, 2600jr, 7800 and Lynx II
Atari 520ST (1Meg), 1040STF long button floppy, 2x 1040 STFMs + Gotek, 2x 4160STE with 32Mhz booster, ROM switcher, Mega ST1, Mega ST4, not to mention various bare ST boards for testing including a PAK 68/2 :)
Plus the rest..
Amiga stuff, Mac stuff, Sinclair stuff etc...
www.electronicnothingness.co.uk

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

Post by rubber_jonnie » Thu Apr 22, 2021 10:49 am

Well, I have the schematic now and the service manual, so I just need to keep plugging away at it.
Collector of old Atari things:
800XL + Ultimate 1MB, 2x 1010 cassette, 1050 + Happy mod, 65XE (128k +APE Warp) & XC12, SIO2SD, 2600jr, 7800 and Lynx II
Atari 520ST (1Meg), 1040STF long button floppy, 2x 1040 STFMs + Gotek, 2x 4160STE with 32Mhz booster, ROM switcher, Mega ST1, Mega ST4, not to mention various bare ST boards for testing including a PAK 68/2 :)
Plus the rest..
Amiga stuff, Mac stuff, Sinclair stuff etc...
www.electronicnothingness.co.uk

cmorley
Posts: 223
Joined: Tue May 28, 2019 5:46 pm

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

Post by cmorley » Thu Apr 22, 2021 12:38 pm

rubber_jonnie wrote:
Thu Apr 22, 2021 9:37 am
I do see a pot marked VR2, but not VR1. Issue 4 board BTW
39790620603_c2c93ac1e5_b.jpg
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Re: BBC Model B No2. Will it live again!

Post by rubber_jonnie » Thu Apr 22, 2021 12:55 pm

cmorley wrote:
Thu Apr 22, 2021 12:38 pm
rubber_jonnie wrote:
Thu Apr 22, 2021 9:37 am
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?
Collector of old Atari things:
800XL + Ultimate 1MB, 2x 1010 cassette, 1050 + Happy mod, 65XE (128k +APE Warp) & XC12, SIO2SD, 2600jr, 7800 and Lynx II
Atari 520ST (1Meg), 1040STF long button floppy, 2x 1040 STFMs + Gotek, 2x 4160STE with 32Mhz booster, ROM switcher, Mega ST1, Mega ST4, not to mention various bare ST boards for testing including a PAK 68/2 :)
Plus the rest..
Amiga stuff, Mac stuff, Sinclair stuff etc...
www.electronicnothingness.co.uk

cmorley
Posts: 223
Joined: Tue May 28, 2019 5:46 pm

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

Post by cmorley » Thu Apr 22, 2021 1:04 pm

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).

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