B+

discuss both original and modern hardware for the bbc micro/electron
philb
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B+

Post by philb »

Just trying to resurrect a B+ (or at least I think it's a B+) that I've had sitting in a corner for quite some time.

I've replaced the primary side capacitors in the power supply and it seems to power up fine. A few questions...

The startup banner says "BBC Computer 32k" which sounds rather more like a B than a B+. I'm not sure if this means that what I have is actually a B variant that happens to look more like a B+ board, or whether it's a B+ but with the wrong OS ROM fitted. Or maybe I'm just remembering wrong what it's supposed to look like.

There's quite a lot of digital noise from the loudspeaker. Is that likely to mean that I should have replaced the secondary side caps in the PSU as well? The oscilloscope in the office is away being repaired so I can't easily check for ripple at the moment but I could just change the parts out speculatively and see if it helps.

What version of (A)DFS is appropriate for a B+? I'm also planning to fit the Econet interface assuming I can find the components. Does it use the same NFS as a B?

Thanks
Phil
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danielj
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Re: B+

Post by danielj »

B+ should say "Acorn OS 64k" - is it possible you have a US/German B?

d.
philb
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Re: B+

Post by philb »

Yeah, that's what I was wondering. It looks pretty much like I remembered a B+ being; it's got the ROM sockets at the back left and the footprint for the extra RAM header in the bottom right, so I'd jumped to the conclusion that's what it was. But I don't think I've ever seen a US or German model B to compare with. I'll take a photo later.
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Re: B+

Post by danielj »

philb
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Re: B+

Post by philb »

Mm, fairly sure it's not that. My best guess at the moment is that it really is a B+ but it's had the wrong OS ROM fitted at some point for whatever reason. Might have to exhume my EPROM programmer... !
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Re: B+

Post by danielj »

philb
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Re: B+

Post by philb »

Yup, that's the puppy.
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DutchAcorn
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Re: B+

Post by DutchAcorn »

A B+ normally has a combined OS and BASIC ROM. Can you share a photo of the board?
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1024MAK
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Re: B+

Post by 1024MAK »

Noise from the speaker is common on Model B and Master 128 machines. I can’t say I have noticed one way or the other if my B+ is better or not than my other machines.

The noise comes primarily from the digital switching signals of the DRAM, ROMs, CPU, 6845 CRTC etc...

It is however worthwhile checking the -5V supply voltage (purple wire) and the electrolytic capacitors (on the main board) for the filtered +5V and -5V supplies to the audio chips.

If you have a good quality digital multimeter that can ignore a DC voltage when switched to an AC voltage range, has a 200mV AC range, and has at least an audio frequency bandwidth on the AC 200mV range, then this can be used to get an idea of how much ripple and noise is on a DC power rail. But note that normally only expensive multimeters meet this criteria. A cheap and cheerful multimeter is no good.

As to the ROM chips, you need the correct 32k byte ROM (or EPROM) with both the OS and BASIC. For the disk system, read this page. I think any of the Econet filing system ROMs should work as long as the ROM/EPROM chip is compatible with the socket you are trying to use it in.

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Re: B+

Post by BeebMaster »

The standard NFS (3.34, 3.60 etc) or DNFS can be used in the BBC B+, the DFS portion of DNFS will be disabled if it doesn't detect the 8271 controller. Also ANFS 4.18 can be used.

There's provision on the B+ motherboard for either an 8271 or 1770 based disc interface and the service manual describes how to fit either (but not both, sadly, as the components overlap). It's a common misconception that B+es came with 1770 DFS as standard. At least one machine escaped from the factory with no disc interface at all:

Image
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Re: B+

Post by jgharston »

What does *FX 0 tell you?

Code: Select all

$ bbcbasic
PDP11 BBC BASIC IV Version 0.32
(C) Copyright J.G.Harston 1989,2005-2020
>_
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Re: B+

Post by philb »

IMG_20201010_154704.jpg
Here it is.
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hoglet
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Re: B+

Post by hoglet »

It looks like it has OS 1.20 installed (the ROM says PB04 which is OS 1.20)

Possibly that was done to improve compatibility with games.

It also appears to be lacking a Basic II ROM, though in a B+ that would normally be included with the OS in a 32K 23256 Masked ROM.

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Re: B+

Post by BeebMaster »

Interesting, it has no UHF modulator.

What disc interface does it have?
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DutchAcorn
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Re: B+

Post by DutchAcorn »

You should be getting a banner saying

BBC Computer 32K
Language?

Attached is a zipped image for the 32K combined OS and BASIC ROM that you can put in a 32K EPROM and replace the currently installed OS with.
Attachments
BBC+OSBASIC.zip
(25.19 KiB) Downloaded 12 times
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philb
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Re: B+

Post by philb »

Ah excellent, thanks. I'll give that a try as soon as I can locate a blank EPROM and my programmer.
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Re: B+

Post by philb »

And it has a 1770 disk interface fitted by the looks of things. I couldn't immediately spot a footprint for an 8271 on the board but I didn't look very closely. I know it's supposed to have both as options.

Edit, I just noticed the dual footprint in the photo above, doh. :-)
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Re: B+

Post by philb »

1024MAK, thanks for the audio noise notes. I think this machine is slightly worse than I remember others being, but I don't have any other working BBCs on hand right now to compare it with. (I do, however, have a large pile of machines to try...)

I will have a rummage around and see if I can find a capable enough DMM to do a noise measurement, but I suspect it will be easiest to wait until the oscilloscope comes back from Agilent. Next time I have the board out of the case anyway I might change the 5V filter caps just in case.
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rmbrowngr
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Re: B+

Post by rmbrowngr »

If you’re interested retroclinic has a switchable multi os available for the b+. I bought one for my b+.

And my speaker is noisy as well. I’d like to get rid of buzzing noise.
Richard B
Acorn Electrons issue 4 and 6, MRB, Plus 1, AP6, AP5, Pegasus 400, BeebSCSI, Gotek, Raspberry Pi, GoSDC MBE.
BBC B+ 64K (128K upgraded) with Duel OS, Raspberry Pi and Gotek.
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Re: B+

Post by Coeus »

rmbrowngr wrote:
Sat Oct 10, 2020 9:22 pm
And my speaker is noisy as well. I’d like to get rid of buzzing noise.
Do we know the mechanism by which the noise is picked up. I think someone said it is due to digital signals being picked up by the audio amplifier but is it being inducted into the amplifier's input? Or does the amplifier have poor power supply noise rejection? Could it be improved with some local decoupling?
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Re: B+

Post by Bobbi »

If it is the same as Model B, it is noise pickup from the audio in line which is part of the user port. The usual fix is to add a tie-down resistor.
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Re: B+

Post by robheaton »

Bobbi wrote:
Sat Oct 10, 2020 10:24 pm
If it is the same as Model B, it is noise pickup from the audio in line which is part of the user port. The usual fix is to add a tie-down resistor.
What value of resistor should be used and where should it be added?
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Bobbi
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Re: B+

Post by Bobbi »

There are definitely threads on it here. Let me see if I can find ...

Mentioned in passing here: viewtopic.php?f=3&t=20263&p=284195&hili ... se#p284195
philb
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Re: B+

Post by philb »

Just had a rummage around at the office and in a slightly unexpected stroke of good luck I found one of these:
IMG_20201011_134155.jpg
So now I just need to figure out how to get my laptop to talk to my EPROM programmer. Hopefully that will be a mere formality.
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Re: B+

Post by philb »

Okay, great, success. Getting the EPROM programmer working under DOSBox was fairly easy; I programmed the OS+BASIC image, put it in the B+, and I now have "Acorn OS 64K" and a BASIC prompt.

So, excellent :-) Now I just need to figure out some storage.
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Re: B+

Post by BeebMaster »

Get one of these, you'll have years of fun just trying to format it....
DoubleWinchester8.jpg
It can be yours for just the 1984 Acorn R.R.P....adjusted for inflation of course!
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1024MAK
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Re: B+

Post by 1024MAK »

Bobbi wrote:
Sat Oct 10, 2020 10:24 pm
If it is the same as Model B, it is noise pickup from the audio in line which is part of the user port. The usual fix is to add a tie-down resistor.
Actually the 1MHz bus port... And R44 (a pull down resistor) should already be fitted.

BBC B+ reducing noise from speaker

Remove link S17 if fitted or cut PCB track (on the underside I think) (service manual says: “S17 PCB track, made North: North configures 1MHz bus audio for input, South for output”.

Renew C51, C52 (both 10uF 16V).

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Bobbi
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Re: B+

Post by Bobbi »

Sorry 1MHz bus. My bad.

I didn't know that audio line could be configured to be an input or an output - very handy!
philb
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Re: B+

Post by philb »

BeebMaster wrote:
Sun Oct 11, 2020 4:01 pm
It can be yours for just the 1984 Acorn R.R.P....adjusted for inflation of course!
Haha. Couldn't afford it BITD, can't afford it now. Though I do have a couple of FileStore E20s that I guess are much the same...

For immediate B+ purposes I've bought myself a Gotek. I think I might look at a hard disk type solution for the Master though.

Also ordered some new parts to replace the filter caps on the motherboard, we'll see if that helps with my noisy speaker.
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Re: B+

Post by philb »

Replaced C51 and C52 (also did X1 while I was about it) but it doesn't seem to have helped much.

I was originally planning to renew all the digital decoupling caps but I seem to have somehow ordered the wrong parts (radial not axial) and I'm not sufficiently convinced it will help anything to want to take the machine to bits again at the moment. So I think I'm going to assume that the current level of digital noise is "normal".
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