I got a BBC B a couple of months ago and it's had a lot of problems. The current situation is that if I use an under powered Apple AC adaptor (5.2V at 2.4A) it kind of works, though sometimes takes several attempts and will probably lock up at some point and then may take 10-20 mins to get working again.
If I use a Pi4 5.1V 3A supply, I just get a constant beep, though I have had it work for a few minutes occasionally.
If I measure the voltage in the circuit I'm getting 4.89V at the VCC points when it's not working with the Pi PSU, though I only get 4.64v with the Apple one.
I've ordered a cheap oscilloscope (FNIRSI DSO153 for £20), so I'll be able to do more tests once that arrives, but I've tested voltages on 11 of the main chips (excluding IC69) and they looked okay to me (VCC good, and various voltages on the other pins).
Is this a known problem, or has anyone got any ideas?
BBC B mostly works with worse power, but not with correct one.
BBC B mostly works with worse power, but not with correct one.
"The eighties aren't over. I'll say when the eighties are over. The eighties are never over! You don't just turn off the eighties!" - Jack Howitzer, GTA V
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maniacminer
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Re: BBC B mostly works with worse power, but not with correct one.
That doesn't sound like a PSU problem but a marginal voltage threshold in the logic - multiplexers, flip-flops and buffers are particularly vulnerable to this effect. There are *lots* of reasons for this behaviour, the usual place to start is the buffer between the CPU and RAM IC14/74LS245 as it is known to go bad, you might be lucky and have that in a socket. There's a long list of fault finding in a sticky thread at the top of this forum that should help you.
Big Model B Econet,Master 512,Electron,A3000,A540,Atom,Unilab 3-Chip Plus,6502,Z80,65C816,80186,32016,Matchbox,ARM7TDMI,Master 10/100,PiCoPro,Teletext,Music500,PiSCSI,Challenger3,Gotek,VideoNuLA,GoSDC,GoMMC,Integra-B,RGB2HDMIv4,Epson LQ-850 (for the win!)
Re: BBC B mostly works with worse power, but not with correct one.
Thanks.
It is socketed
Is there an easy way to test this with a multimeter (or oscilloscope), or should I just order one from eBay and hope that's the problem? With it being socketed it's probably the easiest thing to do.
Does it need to be the exact same one - Texas Instruments SN74LS245N, or are there better replacements these days, or compatibles, as I'm seeing several similar names (like 245PC instead of 245N, etc)?
Is there a good place to get things like this from apart from eBay? The postage from "proper" sellers is ranging from £12 to £66 for a single chip!
It is socketed
Is there an easy way to test this with a multimeter (or oscilloscope), or should I just order one from eBay and hope that's the problem? With it being socketed it's probably the easiest thing to do.
Does it need to be the exact same one - Texas Instruments SN74LS245N, or are there better replacements these days, or compatibles, as I'm seeing several similar names (like 245PC instead of 245N, etc)?
Is there a good place to get things like this from apart from eBay? The postage from "proper" sellers is ranging from £12 to £66 for a single chip!
"The eighties aren't over. I'll say when the eighties are over. The eighties are never over! You don't just turn off the eighties!" - Jack Howitzer, GTA V
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maniacminer
- Posts: 1330
- Joined: Thu Sep 21, 2017 2:59 am
- Location: Cambridge / Singapore
- Contact:
Re: BBC B mostly works with worse power, but not with correct one.
Component suppliers such as Mouser, Farnell, Digikey and RS sell 74LS logic, you don't have to use Texas Instruments, the N of the part just means plastic DIL package. Come to think about it, there is another 74LS245 driving the 1MHz bus, if that's in a socket you could try that one and see if anything changes.
It's best to look the circuit over with an oscilloscope, a multimeter won't help much. Don't try to use other families of logic for IC14 as it's part of an assumption made by the designer of the circuitry to have the timing response of a 74LS245 in that spot.
I've tried with various levels of success and failure 74F, 74S, 74HCT, 74ACT and 74FCT with faster logic there's a possibility of glitches being taken seriously by the system and lock ups, corruption and burn outs. I've had a lot of laughs building Beebs from scratch using different combinations of logic and seeing how squirrelly things get
It's best to look the circuit over with an oscilloscope, a multimeter won't help much. Don't try to use other families of logic for IC14 as it's part of an assumption made by the designer of the circuitry to have the timing response of a 74LS245 in that spot.
I've tried with various levels of success and failure 74F, 74S, 74HCT, 74ACT and 74FCT with faster logic there's a possibility of glitches being taken seriously by the system and lock ups, corruption and burn outs. I've had a lot of laughs building Beebs from scratch using different combinations of logic and seeing how squirrelly things get
Big Model B Econet,Master 512,Electron,A3000,A540,Atom,Unilab 3-Chip Plus,6502,Z80,65C816,80186,32016,Matchbox,ARM7TDMI,Master 10/100,PiCoPro,Teletext,Music500,PiSCSI,Challenger3,Gotek,VideoNuLA,GoSDC,GoMMC,Integra-B,RGB2HDMIv4,Epson LQ-850 (for the win!)
Re: BBC B mostly works with worse power, but not with correct one.
Thanks for the replies. I think I have some success, though I've though this in the past and it didn't last.
Yesterday I was reading through a lot of the fault finding thread, doing more checks with a multi-meter (just to check that pins were actually doing anything, and resets were being acknowledged), then I decided to swap IC3 and 69 (the 6522s). I'd done this in the past, but I've cleaned a lot of other chips, switches, etc since then. I cleaned them and swapped them over, and I got it to start. I switched off and it started a second time. This morning I just put it on and it started! It's looking so far like it was a bad contact, or maybe one of the 6522s is faulty but isn't affecting anything I've noticed so far.
I'm still on my lower powered supply. If it's still okay later I'll swap to my proper 3A supply and see what happens.
I just need to get the colour composite working now. My extremely dodgy attempt (while I'm waiting for the pin headers to arrive) using a paperclip isn't working as it's still black and white. C59 is populated, so I guess my contact isn't good enough. I was hoping to record through comp for my Youtube videos. Especially for Uncle Claude, if I can loop it.
[edit] The colour has just started flashing on and off, so I just need a proper header and jumper on it.
Yesterday I was reading through a lot of the fault finding thread, doing more checks with a multi-meter (just to check that pins were actually doing anything, and resets were being acknowledged), then I decided to swap IC3 and 69 (the 6522s). I'd done this in the past, but I've cleaned a lot of other chips, switches, etc since then. I cleaned them and swapped them over, and I got it to start. I switched off and it started a second time. This morning I just put it on and it started! It's looking so far like it was a bad contact, or maybe one of the 6522s is faulty but isn't affecting anything I've noticed so far.
I'm still on my lower powered supply. If it's still okay later I'll swap to my proper 3A supply and see what happens.
I just need to get the colour composite working now. My extremely dodgy attempt (while I'm waiting for the pin headers to arrive) using a paperclip isn't working as it's still black and white. C59 is populated, so I guess my contact isn't good enough. I was hoping to record through comp for my Youtube videos. Especially for Uncle Claude, if I can loop it.
[edit] The colour has just started flashing on and off, so I just need a proper header and jumper on it.
"The eighties aren't over. I'll say when the eighties are over. The eighties are never over! You don't just turn off the eighties!" - Jack Howitzer, GTA V