[Solved!] Bbc master configuration

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pau1ie
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[Solved!] Bbc master configuration

Post by pau1ie » Sat Nov 24, 2018 3:18 pm

I replaced the power supply in my master with the meanwell following the waitingforfriday instructions, and it worked fine when I tested it. However, having put it back where it lives and connected it up, now it always boots to terminal. Even if i do *co. Lang 12 then *status shows lang = 15, and ctrl-break boots back to terminal.

What should i check? Where is the config stored physically? I checked the battery and that seemed ok, but i wouldn't expect those symptoms to be caused by a low battery.
Last edited by pau1ie on Sun Nov 25, 2018 12:41 pm, edited 1 time in total.

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AndyF
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Re: Bbc master configuration

Post by AndyF » Sat Nov 24, 2018 4:21 pm

I think (and I'm not 100%) even if the battery was awol or dead unless you power cycle the machine itself, (that being turn it off with the mains switch) then any config you have set *should* stay set until said power loss. A hard break should not cause it to jumble itself up.

I wonder if its struggling to set it due to random corruption. To quote from the Service Manual about a reset of this:
Turn the power off and back on again whilst holding down the R key.
Then press CTRL F BREAK. All previous configuration commands will be reset
to zero.
I think it means "Hold CTRL+F and press BREAK" :lol:

If its no better then I'd suspect the chip itself or its supply perhaps. IC11 from a cursory glance at the service manual, just says "IC6818 RTC CMOS IC 11"
Last edited by AndyF on Sat Nov 24, 2018 4:22 pm, edited 1 time in total.
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KenLowe
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Re: Bbc master configuration

Post by KenLowe » Sat Nov 24, 2018 5:04 pm

A similar issue was discussed in this thread:

viewtopic.php?f=3&t=15745

Not sure if it was ever fully resolved, but there are several pointers to the possible cause.

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pau1ie
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Re: Bbc master configuration

Post by pau1ie » Sat Nov 24, 2018 11:13 pm

Thanks both, especially for the similar thread.

I notice Jungsi helpfully posted this extract from the service manual:
7.1.9 All the keys are working,but CTRL BREAK behaves as BREAK.

(CTRL BREAK should invoke the configuration settings.)

1 Check that all power-down components around the CMOS clock/RAM chip IC11 are present and have the correct value.

On some early computers,C11 needs to be 100uF instead of 10uF,D8 should be changed to IN4001,and Q3 should have a 1k8 resistor between its base and emitter.

...

7.1.10 The computer powers up in terminal mode,and *configure will not change it,

1 Check 1 and 2 in the section above.

2 Check all connections to the clock/RAM chip IC11 for open circuits or short circuits.
I am not sure what is meant by "Power-down components", but anyway:
R17 5M6 - 5.9M
R24: 100K - Measures at 97.8K
R29: 10K - Measures at 9.8K
R154: 1K8 Measures at 1.53K
D8 is a diode.
C7 - Not short circuit.
C11 - 100 uF


IC11 Pins
1 - Not connected
2 - Connected to R17 and X1
3 - Connected to C7 and R24
4 - SA0 - Connected to IC12 10, IC16 34
5-11 I will leave for now
12 doesn't seem to be mentioned, but connects to ground.
13 - Connected to R29 and Q3
14 - Connected to IC8 pin 17
15 - Leave for now
16 - Not mentioned
17 - Leave for now
18 - Connected to pin 22 and IC6 pin 34
19 - Connected to LK4
20 - +5VB
21 - TP1
22 - Connected to ping 18 above
23 - Not connected
24 - Vcc - PL8

So all the easy stuff around looks sane.

Now when I test I get a different result whenever I CTRL-BREAK. Different filesystem, pretty much always not a language. Sometimes I get "Bad sum" Key repeat is generally too fast to type but occasionally not. When not, the display freezes - I guess it is sending the output to the printer and is waiting for a response?

I am going to work on the assumption that IC11 has turned into a random number generator somehow. Before dinner it seemed to always return high for everything, e.g. language 15 like in the thread referenced above.

I expect I will have to buy a new 146818 chip. I see a few on Ebay. I suppose my concern is that with the stories of chips being rebadged, how would I know if it didn't work whether it was the chip I had replaced or another problem on the motherboard.

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pau1ie
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Re: Bbc master configuration

Post by pau1ie » Sat Nov 24, 2018 11:15 pm

Another thing at the back of my mind is that often the date would have junk in it when I checked, and occasionally I would get symptoms like this. so maybe this has been failing for some time?

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1024MAK
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Re: Bbc master configuration

Post by 1024MAK » Sun Nov 25, 2018 12:03 am

On IC11 (146818) test the following with a multimeter on voltage. Test with the mains power switched off. Then again with the mains turned on.

Pin 24 (+) to pin 12 (-)
Pin 20 (+) to pin 12 (-)

Do you have a logic probe or oscilloscope?

Mark

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pau1ie
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Re: Bbc master configuration

Post by pau1ie » Sun Nov 25, 2018 12:41 pm

Power off (Battery connected
Pin 24 (+) to pin 12 (-) - 3.14V
Pin 20 (+) to pin 12 (-) - 0v

Power on (Battery connected
Pin 24 (+) to pin 12 (-) - 4.00V
Pin 20 (+) to pin 12 (-) - 4.45v
1024MAK wrote:
Sun Nov 25, 2018 12:03 am
Do you have a logic probe or oscilloscope?
Not really. I have a DSO138 but the switches are dodgy (I have to keep my finger on it to get a signal) - I am looking for replacements. I am happy to buy something that isn't too expensive if you can suggest something. I'm rather keen to get this working again as it is my only BBC computer!

Thinking about it a little though, pin 24 at 4v, and pin 20 at 4.45v seems low. I am pretty sure I measured the power supply at 5v, but in any case it is too low now. Altered it to 5v again, and all is well! I typed in all the configure commands in the welcome guide and it is much happier!

Thanks all, and especially 1024MAK for the steer in the right direction!

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