Problems with newly acquired BBC Micro

[BertZonn]

Hi, I have some questions about the BBC Acorn B I bought yesterday. First of all, I will introduce myself; I'm Bert and I live in Belgium. I love vintage computers and that is the reason I bought this BBC Acorn. But the are some problems.

When I switch on the computer I have a blanc screen en there is a constant tone/sound coming from the loudspeaker. After I press the 'BREAK' key the BBC boots again and I get the normal opening screen. The sound from the speaker stops. What could this be?

The second problem is with the floppydrive. When I put a disk in and I type the command *. the drive starts spinning but the read/write head don't move. What could be the problem?

And the last question is about the ROMS. There are 4 ROMS placed in the sockets but how do I know which ROM does what? The ROMS have numbers on them but when I enter these numbers in Google I come up empty.

What I understand is that there is 1 ROM for basic and there is 1 ROM for the system.
How can I identify which ROM is doing what.

I have ordered new capacitors for the PSU because I know the Rifas will blow up for sure sometime with a lot of smoke. So I will not switch the BBC on untill the capacitors are replaced (The same in Revox A77 taperecorders, there are 4 Rifas in this recorder and they will go bang!)

I'll try to upload a picture for the ROMS:


If anyone can tell me how to identify the ROMS I a bit closer to my goal.

[Kazzie]

Hi, I have some questions about the BBC Acorn B I bought yesterday. First of all, I will introduce myself; I'm Bert and I live in Belgium. I love vintage computers and that is the reason I bought this BBC Acorn. But the are some problems.

Hi, welcome, and congratulations on your new acquisition!

With the ROMs, the left-most (lowest in your picture) is the OS (or MOS). The other four sockets are all interchangable, so you have to use the markings.

One is labelled "view" , this is a word processor on ROM. For the others, you'll need to look up the codes on a list such as this one: http://mdfs.net/Docs/Comp/BBC/Resources/MaskROM . The top one, labelled PB05 is Basic II, the last one may be a DFS ROM, for operating a Floppy Drive Controller chip, but I haven't checked.

The long beep you get is what the sound chip does by default until the OS tells it to shut up. The fact that the break key gets things started implies that the CPU doesn't get a good, clean reset signal when you turn the power on, but a second reset signal from the break key is good enough. I'd suspect either a slightly low voltage from the power supply when it's turned on, or a fault around the 555 timer used to generate the reset signal. There is one capacitor in the power supply (C9) that I understand can cause problems when powering up, I'm not sure if it's a likely cause for this exact scenario.

Our fault-finding thread is a good place to look for people who've had similar problems, and the solution they found.

Do you have any equipment like a voltmeter or soldering iron, and have you any experience in using circuit diagrams? Either way, we can help you work out what needs fixing.

Edit: It'd be a good idea to start a new thread for sorting out your reset problems.

[BertZonn]

Hi Kazzie, thanks a lot for this information and the links!! This will help me to further investigate MY BBC Computer. I've seen a lot of Youtube items about this computer and I think I will like it a lot. Time for me to explore the BBC Acorn.

[BertZonn]

So, if I'm correct, from top to bottom I have the following ROMS:

1 HN613128PB05 ACORN COMPUTERS LTD - BASIC II
2 Empty
3 CN61014N = Disk Filing System (Acorn DFS)
4 ROM HN613128PB04 ACORN COMPUTERS LTD - OS 1.20

And the marking PB04 on the one on the bottom refers to ROM HN613128PB04

Hope my capacitors will arrive soon.
Maybe in time I will replace ALL capacitors. In the PSU and on the mainboard.

[Kazzie]

So, if I'm correct, from top to bottom I have the following ROMS:

1 HN613128PB05 ACORN COMPUTERS LTD - BASIC II
2 Empty
3 CN61014N = Disk Filing System (Acorn DFS)
4 ROM HN613128PB04 ACORN COMPUTERS LTD - OS 1.20

And the marking PB04 on the one on the bottom refers to ROM HN613128PB04

Hope my capacitors will arrive soon.
Maybe in time I will replace ALL capacitors. In the PSU and on the mainboard.

You missed out View in socket 4, but that list looks correct.

Our experience is that the capacitors on the mainboard don't need routine replacement, but there are three in the PSU (including C9) that do. A few people sell ready-made capacitor replacement kits for the PSUs, on places like eBay.

[BertZonn]

You're right, I missed one.

[1024MAK]

If you type

Code: Select all

*HELP

that will show some details of some of the ROM chips.

Mark

[BertZonn]

Ok, I will try that. I have nu clue (yet) about all the commands that are possible on the BBC Acorn. Haven't read a book yet. First the computer must work then I will study (and ask here )

[DutchAcorn]

Actually the very first thing to do is to replace the capacitors in the PSU. They will fail producing a foul smell and muck in the PSU. It can be seconds, minutes or perhaps even a few hours before they fail, but they will fail (unless you have a made in Hong Kong model).

[BertZonn]

Well, today the capacitors arrived with the post. This evening I've replaced the old capacitors so I dared to switch on my BBC. Same startup sound. The speaker kept on sounding the first tone and only after pressing the break key, the BBC would start up. Still wonder how to fix that problem.
Maybe the 555 is bad?

The floppydrive did not respond and gave error 14. I removed the ribboncable and plugged it in again and then I got some response. The read/write head moved and I was able to read the disk but it was still giving errors. I swapped the 2 7438 chips and then the BBC would not start up so I think that at least 1 of those 7438's is faulty. Just ordered 2 new ones on Ebay to be on the safe side.

Don't know if 1 of the VIA's could be bad. Can I use the 6522 from a commodore computer or diskdrive to check or are those chips not compatible?
Those are also called VIA's and have the same number (6522).

It seems the BBC has DFS 1.20 and also OS 1.2

Thanks for you answer(s) !!

[Kazzie]

For any suspicions on the 6522, you can swap over the two that are already in the Beeb. Only the System VIA (IC3) is needed for the OS to boot happily; the User VIA is only used for expansion ports and some software. If the symptoms change or go away, then you know to order a replacement.

For the reset circuit, there may be a fault with the 555 (IC16), the inverter (IC33), or any of the passive components surrounding it. (It could be as simple as a poor solder joint.)

What sort of test equipment / experience do you have?

[retroclinic]

....Only the System VIA (IC3) is needed for the OS to boot happily....

Just to point out that the OS will boot without either VIA being present. Of course the keyboard and sound won't work, but removing both (along with the ADC, Serial ULA, 6850 if it's socketd, and all ROMs apart from the OS) is a good method to use for a machine that shows no signs of booting at all.

[BertZonn]

For any suspicions on the 6522, you can swap over the two that are already in the Beeb. Only the System VIA (IC3) is needed for the OS to boot happily; the User VIA is only used for expansion ports and some software. If the symptoms change or go away, then you know to order a replacement.

For the reset circuit, there may be a fault with the 555 (IC16), the inverter (IC33), or any of the passive components surrounding it. (It could be as simple as a poor solder joint.)
bbcResetCircuit.png
What sort of test equipment / experience do you have?

I am quite experienced in repairing PCB's. I've done some Commodores (PET/2001, C=64 etc)
I'm also in repairing vintage audio equipment.
I have some 6522 in stock but they come from Commodore floppydrives and I was wondering if these VIA's are Interchangeable.
I've read that the 'Beep' is quite picky about some CPU's and I don't know if the Beep is picky about the VIA's. There is one way to find out and that is to try it out. I will do so this evening.

The tools I have are: Fluke 75 DMM, small Chinese DIY DSO138 scope, a Chinese DIY component tester and a Chinese DIY frequency counter.

[1024MAK]

The W65C22S VIA is definitely not compatible, but other 6522 or 65C22 chips may work okay. Only way to find out is to try...

The reason that the Beeb may not like some 6502 microprocessors (mainly CMOS versions) is due to two things, the data and address busses are heavily loaded and with a different chip with different output characteristics or slightly different signal timing, this may cause problems. The second is that the NMOS 6502 has undocumented instructions and the various CMOS types have different operations for these instruction codes. Although this makes no difference to the OS and BASIC, it may affect some games.

The most common problem with the lack of a power on reset (which is the problem you have) is with the passive components around IC16 or contamination of the PCB in this area.

First, check the board visually in the area of IC16 (NE555) and R13 (1MΩ), R21 (1MΩ), C8 (100nF) or C24 (100nF). Check especially for contamination and dirt. Then measure the values of the resistors.
Also check R20 (1MΩ) and C10 (10nF) (reset for IC3, the system 6522 VIA).

Mark

[retroclinic]

The W65C22S VIA is definitely not compatible, but other 6522 or 65C22 chips may work okay

I've been playing with the new ones. The W65C22N works, but seems to give spurious IRQs from the keyboard when keys are pressed or the options switches are set. I need to play more and find out if this can be cured, as there is a fix for some keyboards that do this by putting a 100pf cap between pins 7 and 8 of the LS30 at the edge of the KB, I've not tried that yet on the new VIA.

The new VIA (N version, I've not tried the S) however does work flawlessly in both positions on a Master.

Thanks, Mark.

[1024MAK]

This page details the differences with the W65C22S VIA. With some modifications it may be possible to get it to work.

I’m not surprised that the W65C22N works in a Master, as some Masters were originally manufactured with a CMOS version.

Mark

[BertZonn]

The W65C22S VIA is definitely not compatible, but other 6522 or 65C22 chips may work okay. Only way to find out is to try...

The reason that the Beeb may not like some 6502 microprocessors (mainly CMOS versions) is due to two things, the data and address busses are heavily loaded and with a different chip with different output characteristics or slightly different signal timing, this may cause problems. The second is that the NMOS 6502 has undocumented instructions and the various CMOS types have different operations for these instruction codes. Although this makes no difference to the OS and BASIC, it may affect some games.

The most common problem with the lack of a power on reset (which is the problem you have) is with the passive components around IC16 or contamination of the PCB in this area.

First, check the board visually in the area of IC16 (NE555) and R13 (1MΩ), R21 (1MΩ), C8 (100nF) or C24 (100nF). Check especially for contamination and dirt. Then measure the values of the resistors.
Also check R20 (1MΩ) and C10 (10nF) (reset for IC3, the system 6522 VIA).

Mark

About the CPU; I've switched the original Rockwell CPU with the CPU from a VIC-1541 floppydrive (MOS) and both devices seem to work. It does NOT cure the problem with the boot-beeb but it causes no additional issuis. I have ordered a brand new CPU in China that is supposed to be made by Rockwell. I wonder if it is really Rockwell or rebranded.

There was some dirt (white stuff/fungus?) near the D-SUB connector but the PCB looks to be in good shape. No suspicious marks on the PCB. Maybe I wil clean the PCB with IPA to be sure. I will check the resistors and capacitors near the NE555 and also R20 and C10. I will also swap the 2 Via's just to see if it improves/changes the booting behaviour of the 'Beeb'.

I'll keep you informed. I think a lot of the problems were caused by bad connections of the IC's and the connector to the floppydrive.
I wiggled most of the chips in their sockets and the problems slowly disappeared but not for 100%

[1024MAK]

Rockwell as a semiconductor manufacturer has not existed for about twenty years, so it is extremely unlikely that you will get a ‘brand new’ 6502. What you are likely to get will be a remarked (relabelled) chip. These are chips that have been pulled out of old equipment and cleaned up. It may be a NMOS version or a CMOS version. The people who do this don’t appear to know or care about the differences. Same with the speed rating, the marking may not be the same as the original speed rating of the chip, it may be a faster part, or more likely a slower part.

A member here, Dave H. can sell you a spare NMOS 6502 or a NMOS 6522 chip. He tests each chip to make sure they work.

Mark

[BertZonn]

Today I received the CPU. The markings 'show' it is a Rockwell and it looked like brand new so I gave it a try. I removed the installed CPU that came out of the Commodore 1541 floppy drive and I pushed the 'new' Rockwell 6502 CPU into the socket. The BEEB booted after pressing the BREAK key so the CPU was accepted with no problems. So if you are looking for a cheap 6502 CPU; I bought this one on Ebay for $2,48 and an additional shipping of $2,00. Good buy I guess.

Here is the link for the CPU: https://www.benl.ebay.be/itm/1PCS-R6502 ... 2749.l2648

After that I was still wondering why the BEEB would not go to the next step after switching it on. The first beep kept on going and only after pressing the BREAK key the BEEB would start up. I took the motherboard out and I resoldered most of the connections near the NE555. Apparently there must have been a bad soldering joint because after resoldering the NE555 the BEEB worked like new; A short low pitched tone and then a higher pitched tone.

The floppydrive also works fine now. I replaced the 2 SN7438 chips to be on the save side. The problems with the floppy drive could also be caused by a bad connection in the ribbon cable.

Because of the age of the disks I'm not able to read all of them. I sometimes I got error codes 18 and 14 but I blame this on the age of the disks. Also 'BAD PROGRAM' errors occured.

I want to thank all of you for helping me with this one. I've never owned a BEEB so I had to start from scratch. But I survived and so does the BEEB.
Thanks again!

[1024MAK]

I’m glad you got it going

Have fun

Mark

[eelco108]

Good to hear you got your Beeb to boot!

The Rockwell 6502A is probably fake though - the date code 0448 (week 48 of the year 2004) is very unlikely as Rockwell ceased their electronics part business (and much else) in 2001 ...

It could still be an older MOS 6502, or a 65C02. They might think more money can be made from a 'vintage' 6502A than from a 65C02.

You could try running a game like Zalaga that uses 'illegal' opcodes and see whether it runs

[eelco108]

This one is funny: date code seems OK, but ....

https://www.ebay.co.uk/itm/2PCS-Rockwel ... 2975923768

[eelco108]

Myself, I was stupid enough to get these:
https://www.ebay.co.uk/itm/5pcs-R6502AP ... 1355648682

These did not work, one of them was not even a CPU (you can try and let them "run free" - just NOPs hardwired on the databus - to test this using a simple circuit and an oscillosope).
I then learned about the date code etc. (which is the year 2011 here!) - a bit too late.

I did get my money back though - they hate people complaining and giving bad feedback, so a refund is quick and easy.