Non booting BBC/B Issue 7 - Continuous beep

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Sniffer
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Non booting BBC/B Issue 7 - Continuous beep

Post by Sniffer »

I've googled myself to death almost and read through tons of articles, including one here but still no joy. So if anybody can help ....

I have an issue 7 BBC/B that when powered on gives a continuous beep (or beeps). I say 'or beeps' because it sounds like I get the first beep and the second but all rolled into one so to speak. The tone(s) just continue on and on, nothing on screen, one led, no keyboard activity.

I'm using a known good keyboard, cable and power supply.

I got a good BBC/B board, also Issue 7, onto the bench, powered it up and all is good. I then removed all the socketed ICs from the faulty board (excepting the smaller disc based ones) and started one by one replacing the ones in the good board, each time powering on. In the end I have the good board with all the socketed ICs from the bad board and it works fine.

The ICs were IC51(OS), IC101(Basic), IC52(DFS), IC6, IC14, IC69, IC1, IC2, IC3, IC78, IC7, IC73.

I had already tried booting without vaious socketed ICs installed such as both Vias but no joy. I even removed the soldered SN74LS245N at IC at IC72.

Is this a ram issue? If so, how to I tell? Any ideas?

Cheers,
Mike.
Last edited by Sniffer on Mon Apr 08, 2019 1:46 pm, edited 1 time in total.
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Re: Non bootong BBC/B Issue 7 - Continuous beep

Post by DutchAcorn »

I don’t see you mention trying S25 south? See this (very recent) post: viewtopic.php?f=3&t=16852&view=unread#p232947
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Re: Non bootong BBC/B Issue 7 - Continuous beep

Post by Kazzie »

Given that you've swapped out all the socketed chips, testing the RAM is a sensible next step.

As a Model B, your machine has two banks of RAM (/CAS0 and /CAS1), each providing 16k of RAM. The Model A used the same board, and could be configured to use only one bank (/CAS1).

The 6502 CPU has few on-board registers, but it is designed to use the lowest area of the system's RAM as "zero-page" registers. If you have a fault in the RAM used for the zero page, this can cause the CPU to read incorrect values, execute incorrect values, and hang/crash. The OS normally mutes the sound chip very early in the boot process; the fact that your first beep is continuous indicates that the OS doesn't get that far.

Assuming that there's a fault with one of the eight chips that make up the /CAS0 bank (affecting the zero page), you can try disabling them, and using /CAS1 only, by configuring your machine as a Model A. This can be done by moving the jumper on switch S25, which is near the RGB connector on the motherboard. For a Model B it's in the North position (toward the back of the case), so move it to the South position and see if your machine boots.

If you get a successful boot (with 16k of RAM), then we can move on to working which of the eight chips in /CAS0 is faulty, and needs replacing.

See also the post that DutchAcorn linked to as I was typing. ;)
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Re: Non bootong BBC/B Issue 7 - Continuous beep

Post by Sniffer »

I don’t see you mention trying S25 south? See this (very recent) post: viewtopic.php?f=3&t=16852&view=unread#p232947
Sorry, yes, I tried that, no change. I didn't try with S25 off completely though. Thanks for the link, I'll check it out.
Last edited by Sniffer on Thu Apr 04, 2019 11:03 am, edited 1 time in total.
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Re: Non bootong BBC/B Issue 7 - Continuous beep

Post by Sniffer »

Well, I can't explain this but, I put all the ICs back (actually the ICs from the good board because I left this faulty boards original ICs in the good board, as it was working) and tried booting with S25 completely off but again no change. So, I tried with S25 south, which I had already tried, and hey presto, a 16K beeb. Unbelievable, I was up until 2.30am working on this :)

So, I guess now then that at least one of the chips in /CAS0 is faulty?

Editted just to add that the continuos beep still remains. Is that to be expected?
Last edited by Sniffer on Thu Apr 04, 2019 11:28 am, edited 1 time in total.
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Re: Non bootong BBC/B Issue 7 - Continuous beep

Post by DutchAcorn »

No, it’s not. Does the keyboard work? Did you (correctly) replace the system VIA (6522, IC3)?
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Re: Non bootong BBC/B Issue 7 - Continuous beep

Post by Kazzie »

The sound chip (IC18) is controlled by the System VIA (IC3), so if the VIA's not there, the OS can't tell the sound chip to shut up (as it normally does at boot).
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Re: Non bootong BBC/B Issue 7 - Continuous beep

Post by Sniffer »

No, it’s not. Does the keyboard work? Did you (correctly) replace the system VIA (6522, IC3)?
Yep, both VIAs in situ. Keyboard Caps Lock & Shift Lock leds lit, keyboard does NOT respond until break is pressed. At this point I can use the keyboard but apart from the default MODE, no others seem to be available (Bad MODE) except 5.
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Re: Non bootong BBC/B Issue 7 - Continuous beep

Post by Sniffer »

The sound chip (IC18) is controlled by the System VIA (IC3), so if the VIA's not there, the OS can't tell the sound chip to shut up (as it normally does at boot).
The implication being that the System Via is missing or faulty? It's not though so maybe a board/track problem. I'll take a closer look.
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Re: Non bootong BBC/B Issue 7 - Continuous beep

Post by Kazzie »

Sniffer wrote:
Thu Apr 04, 2019 12:53 pm
The sound chip (IC18) is controlled by the System VIA (IC3), so if the VIA's not there, the OS can't tell the sound chip to shut up (as it normally does at boot).
The implication being that the System Via is missing or faulty? It's not though so maybe a board/track problem. I'll take a closer look.
Some of the I/O lines used by the System VIA to communicate with the keyboard and sound chip are latched by IC32 (74ls259), which is in turn clocked by one half of IC31 (74ls74), so they're also worth checking.
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Re: Non bootong BBC/B Issue 7 - Continuous beep

Post by Sniffer »

By the way, thank you both for your inputs thus far, it's very miuch appreciated.
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Re: Non bootong BBC/B Issue 7 - Continuous beep

Post by Sniffer »

Some of the I/O lines used by the System VIA to communicate with the keyboard and sound chip are latched by IC32 (74ls259), which is in turn clocked by one half of IC31 (74ls74), so they're also worth checking.
Ok, thanks. Before I go through the pain of desoldering and replacing those I think I might have some bad caps. I'll change them out and come back.
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Re: Non bootong BBC/B Issue 7 - Continuous beep

Post by DutchAcorn »

Sniffer wrote:
Thu Apr 04, 2019 12:51 pm
No, it’s not. Does the keyboard work? Did you (correctly) replace the system VIA (6522, IC3)?
Yep, both VIAs in situ. Keyboard Caps Lock & Shift Lock leds lit, keyboard does NOT respond until break is pressed. At this point I can use the keyboard but apart from the default MODE, no others seem to be available (Bad MODE) except 5.
The bad mode is because you effectively have a 16K BBC, reducing the number of screen modes available.
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Re: Non bootong BBC/B Issue 7 - Continuous beep

Post by DutchAcorn »

Does the beeeeb stop after you press break?
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Re: Non bootong BBC/B Issue 7 - Continuous beep

Post by Sniffer »

Does the beeeeb stop after you press break?
Only if I remove the speaker lead :D
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Re: Non bootong BBC/B Issue 7 - Continuous beep

Post by Sniffer »

Does the beeeeb stop after you press break?
On switch on the monotonous tone continues. On pressing 'BREAK' I get a second tone which fades away quickly and is replaced by an undulating whistle, the first tone keeps going. At this point, when pressing keys, its like playing some kind of wierd electronic instrument.
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Re: Non bootong BBC/B Issue 7 - Continuous beep

Post by Kazzie »

Sniffer wrote:
Thu Apr 04, 2019 12:51 pm
No, it’s not. Does the keyboard work? Did you (correctly) replace the system VIA (6522, IC3)?
Yep, both VIAs in situ. Keyboard Caps Lock & Shift Lock leds lit, keyboard does NOT respond until break is pressed. At this point I can use the keyboard but apart from the default MODE, no others seem to be available (Bad MODE) except 5.
I missed this post earlier.

The fact that you get both the Caps and Shift LEDs lit means that the VIA in IC3 and the IC32 latch are working (to some extent).

Since switching S25 to get your machine booting, have you tried switching the two VIAs (IC3 and IC69) around?
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Re: Non bootong BBC/B Issue 7 - Continuous beep

Post by Kazzie »

Sniffer wrote:
Thu Apr 04, 2019 4:44 pm
Does the beeeeb stop after you press break?
On switch on the monotonous tone continues. On pressing 'BREAK' I get a second tone which fades away quickly and is replaced by an undulating whistle, the first tone keeps going. At this point, when pressing keys, its like playing some kind of wierd electronic instrument.
That's interesting.

The keyboard and the sound chip are both connected to IC3's PORT A (PA0-PA7), but only one should be enabled at any time, using control signals from PORT B (latched via IC32).

The fact that pressing buttons on the keyboard makes noises come out of the speaker suggests that the sound chip's 'input enable' pin isn't being deactivated properly (either because IC32 fails to do so, or it isn't told to do so by the IC3 VIA).

If swapping the VIAs around doesn't improve things, then suspicion will fall back on IC32 outputting dodgy values, or possibly not being clocked correctly by IC31.
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Re: Non bootong BBC/B Issue 7 - Continuous beep

Post by Sniffer »

Since switching S25 to get your machine booting, have you tried switching the two VIAs (IC3 and IC69) around?
No, I haqdn't. I was pretty confident in both as they came from the good board. However, I thought I'd do it anyway and sadly, using the chip puller has pulled the pastic housing of IC3 off the board. :shock:

All the pins are intact on the board itself. I'll attempt to pop it back but I'm not confident of a result.
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Re: Non bootong BBC/B Issue 7 - Continuous beep

Post by DutchAcorn »

Sniffer wrote:
Thu Apr 04, 2019 7:06 pm
No, I haqdn't. I was pretty confident in both as they came from the good board. However, I thought I'd do it anyway and sadly, using the chip puller has pulled the pastic housing of IC3 off the board. :shock:

All the pins are intact on the board itself. I'll attempt to pop it back but I'm not confident of a result.
That’s too bad. :(

It’s generally better to use a flat head screwdriver and use a pencil to support the screwdriver so you can use it as a lever. Then gently wedge one end up, then the other end. This way you have full control on the force you’re applying.
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Re: Non bootong BBC/B Issue 7 - Continuous beep

Post by marcusjambler »

pulled the pastic housing of IC3 off
Yep... That can happen

It may be time to replace that socket rather than put it back because it will probaly keep doing it

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Re: Non bootong BBC/B Issue 7 - Continuous beep

Post by Sniffer »

That’s too bad.
Anyway, these things are sent to try us. Had a good look under where the plastic housing was, it seems somebody in the past has had a bit of a dig with a screwdriver (which is what I often use) or some such implement as there is some gouging and the track from pin 1, IC25 is damaged. I cant be sure it's cut but it could be.

I'm useless at reading electronic schematics so maybe somebody can help me out? From the scematic of the board, pin 1 of IC25 links to '2MHZE'. Can I use that link to check if the track is cut? If so how?
It may be time to replace that socket rather than put it back because it will probaly keep doing it
You're probably right but initially, for the purposes of this excersie, I'm gonna superglue the housing to some of the pins :) Making sure of course that it doesn't get stuck to the board.
Last edited by Sniffer on Thu Apr 04, 2019 8:13 pm, edited 4 times in total.
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Re: Non bootong BBC/B Issue 7 - Continuous beep

Post by Kazzie »

Sniffer wrote:
Thu Apr 04, 2019 8:11 pm
Had a good look under where the plastic housing was, it seems somebody in the past has had a bit of a dig with a screwdriver (which is what I often use) or some such implement as there is some gouging and the track from pin 1, IC25 is damaged. I cant be sure it's cut but it could be.

I'm useless at reading electronic schematics so maybe somebody can help me out? From the scematic of the board, pin 1 of IC25 links to '2MHZE'. Can I use that link to check if the track is cut? If so how?
IC25 (at least the half that uses pin 1) is used to enable the output of the ROM chips, including the OS ROM. The fact that you can get the machine to boot as far as accepting keyboard input seggests that this track isn't broken.

To check it and make sure, you could use a multimeter to do a continuity check between IC25 pin 1 and IC37 pin 10 (the generator of the 2MHZE signal) or any other pin that's labelled as being linked directly to 2MHZE, such as IC7 pin 10, IC59 pin 24, etc.
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Re: Non bootong BBC/B Issue 7 - Continuous beep

Post by Sniffer »

To check it and make sure, you could use a multimeter to do a continuity check between IC25 pin 1 and IC37 pin 10 (the generator of the 2MHZE signal) or any other pin that's labelled as being linked directly to 2MHZE, such as IC7 pin 10, IC59 pin 24, etc.
Great, thanks. Track not broken. So I'll set about repairing IC3's housing.
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Re: Non bootong BBC/B Issue 7 - Continuous beep

Post by Sniffer »

Fixed the housing and I've now tried 3 or 4 Vias in IC3, all with the same result. So I'll focus on IC31 & IC32.
Last edited by Sniffer on Thu Apr 04, 2019 9:15 pm, edited 1 time in total.
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Re: Non bootong BBC/B Issue 7 - Continuous beep

Post by Sniffer »

Posting from my phone as no internet due to Zen maint.

Thought I'd give the 74ls04 inverter a go whilst im waiting for some bits.

I get a sort of booted 32k beeb, but still the continuous beep but also a messed up screen. Attaching a foto.
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Re: Non bootong BBC/B Issue 7 - Continuous beep

Post by 1024MAK »

That suggests a problem with either the RAM providing the screen memory or the data lines (or IC15, a 74LS273) between the screen memory and the teletext chip. Also possible is a fault with IC14, a 74LS245 transceiver. Also possible is an address line problem with the RAM or the feeds to it.

Can you change to MODE 6?

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Re: Non bootong BBC/B Issue 7 - Continuous beep

Post by DutchAcorn »

I’d say this makes IC62 the prime suspect.

Bit 1 seems to be pulled high, making an A show as a C and space as a “. As this is CAS0 (using the inverter), bit 1 is stored in IC62.
Last edited by DutchAcorn on Fri Apr 05, 2019 7:40 am, edited 1 time in total.
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Re: Non bootong BBC/B Issue 7 - Continuous beep

Post by Sniffer »

That suggests a problem with either the RAM providing the screen memory or the data lines (or IC15, a 74LS273) between the screen memory and the teletext chip. Also possible is a fault with IC14, a 74LS245 transceiver. Also possible is an address line problem with the RAM or the feeds to it.

Can you change to MODE 6?
Already swapped IC14 and completely removed IC72 (as this was suggested as a possible cause. I'll as IC15 to the list of replaceees! It was late last night when I tested this and didn't try any mode changes. Will try shortly.
I’d say this makes IC62 the prime suspect.
I assumed CAS0 was faulty when I got the 16k boot with S25 south so I'll look to run the mem test as soon as I can. Ihave some HM48168P-7 chips which seem pin compatible with the installed ram. Would they be ok as replacements?

I still have the continuous tone and musical keyboard to resolve of course.
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Re: Non bootong BBC/B Issue 7 - Continuous beep

Post by Sniffer »

With the inverter fitted the symptoms are similar to those with S25 south.

Cannot type until break is pressed . Then th 32k message disappears and typing is possible, albeit with musical tones.
The beeeeeeeep continues.
Typing in the startup MODE (cant remember what mode that is) produces random characters.
Mode changing is possible. Despite the randome chars, the MODE 6 command is accepted.
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