BBD B+ Issues..please help

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nama
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BBD B+ Issues..please help

Post by nama » Fri Sep 28, 2018 8:06 am

Hi Guys,
Seems like every time I turn on my BBC B+ recently it has a new problem that need fixing.

Today I went to play a few games and have the following issues:

- Keyboard not working. It will sometime work very briefly after a restart but only for about a second. Then either everything goes a little crazy, and the keyboard stops, or the keyboard just stops. I have attached the image showing what happens sometime when I press a key after restart (in this case the 'J' Key). Starts ok, then suddenly seems to throw in a bunch or random 'returns', and after a second the keyboard is un-responsive.
IMG_7240.JPG
- Occasionally I get a continuously Beep on restart. A 'Break' stops the beep
- I've changed out the 6522, but no change to the issue.
- Issues seem to get worse if the machine is left on for a minute or so.

Was hoping someone may be able to point me in the right direction for a fix.

Thanks in advance

Philip
Last edited by nama on Fri Sep 28, 2018 8:14 am, edited 1 time in total.

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Lardo Boffin
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Re: BBD B+ Issues..please help

Post by Lardo Boffin » Fri Sep 28, 2018 8:53 am

Have you ever replaced the keyboard connector cable? Mark at Retroclinic can supply new ones.
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nama
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Re: BBD B+ Issues..please help

Post by nama » Fri Sep 28, 2018 8:56 am

No I haven't...Although I might. But I'm not sure it's a connection issue.

Cheers

Philip

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Re: BBD B+ Issues..please help

Post by Kazzie » Fri Sep 28, 2018 9:02 am

On the basis of what you've described, I'd strongly suspect the 6522 System VIA chip (IC20 on a B+). This chip is responsible for interfacing the computer to the keyboard. A faulty or missing System VIA can cause a continuous beep and no prompt on screen (but with the usual OS and memory message displayed).

There are two VIAs in a B or B+: one for the system's own use, and another for the user (user port, printer, custom timers, etc). In a regular B, it's easy to swap the two over because they're sat in sockets; a faulty user VIA will affect your ability to use the user and printer ports, but the system will boot and the keyboard will operate fine. It seems from this image that they're also socketed in a B+, so try swapping IC10 and IC20 over, and see if it boots trouble-free for you.
Last edited by Kazzie on Fri Sep 28, 2018 9:02 am, edited 1 time in total.
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Re: BBD B+ Issues..please help

Post by flaxcottage » Fri Sep 28, 2018 10:39 am

Both VIAs in my B+ were soldered to the motherboard. One broke and was a pig to get out but its replacement is now socketed.
- John
Image

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Lardo Boffin
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Re: BBD B+ Issues..please help

Post by Lardo Boffin » Fri Sep 28, 2018 12:24 pm

nama wrote:
Fri Sep 28, 2018 8:56 am
No I haven't...Although I might. But I'm not sure it's a connection issue.

Cheers

Philip
Assuming the cable has no polarity you could take it out and out it back in the other way. If it makes no difference it’s probably not the cable.
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DutchAcorn
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Re: BBD B+ Issues..please help

Post by DutchAcorn » Fri Sep 28, 2018 3:48 pm

If the symptoms get worse when the bbc has warmed up you may be able to find a failing ic using coolant spray. I’d start with logic around the 6522 (IC20).

You could also check C3.
Paul

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Re: BBD B+ Issues..please help

Post by nama » Sat Sep 29, 2018 4:16 am

Thanks for all the replies and thoughts.
I think I'll do what Lardo recommends and eliminate the keyboard cable as the culprit first and if that doesn't fix things, I'll look into the 6522 and surrounding circuit.

Will contact Retroclinic, and get back asap.

Philip
Last edited by nama on Sat Sep 29, 2018 4:16 am, edited 1 time in total.

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Re: BBD B+ Issues..please help

Post by nama » Sat Oct 13, 2018 8:41 am

Damn!!!

My new keyboard connector arrived but that failed to fix my issue.
Unfortunately my second 6522 is not socketed. Nearly everything is un-socketed in the B+.
Tomorrow I'll replace the 103 capacitor @ C3

Cheers

Philip

nama
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Re: BBD B+ Issues..please help

Post by nama » Sat Oct 13, 2018 11:15 pm

I changed the C3 capacitor, but that didn't help. I didn't think it would, but worth a try.

Just a couple more observations in the hopes that someone may be able to help me narrow things down:

- The system seems to work for longer from a cold start, but not much longer.
- The system will lock up and most pulsing will cease. I can tell when it locks as I hear the buzz from the speaker, and when that stops I know the system has locked up.
- Sometimes it will just lock up, but other times it seems to throw in a bunch of rapid fire returns before locking up. I've noticed when these rapid fire returns happen there is a burst of activity on pin 14 of IC30 (74259) just before the system locks up.
Screen Shot 2018-10-14 at 12.07.03 PM.jpg
Quite frustrating obviously, and having everything not socketed makes it more difficult to fix. Don't really want to go shotgun replacing components.

Hurumph!

Phil

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Re: BBD B+ Issues..please help

Post by paulv » Sat Oct 13, 2018 11:44 pm

If it's a flaky chip with potential heat issues, why not try some freezer spray on each chip in turn in the keyboard control circuit to see if cooling the chip prolongs the period of functional use? That seems easier than replacing them all one by one....

Paul

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1024MAK
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Re: BBD B+ Issues..please help

Post by 1024MAK » Sun Oct 14, 2018 1:58 am

Yes, that is the next step. Freezer spray (or an air duster spay can uses upside down)...

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nama
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Re: BBD B+ Issues..please help

Post by nama » Mon Oct 15, 2018 1:13 am

Hi guys, Thanks for the replies.
I got some freeze spay over the weekend, but once I started to get into tracking down the fix, I realised that maybe it isn't as heat related as I first thought....in other words, the spray didn't help be track down the fault. The fault just seems random.
I'm fairly sure it's something funky around the 6522 circuit. I just replaced the 6522 a few months back so that is socketed now. I've have a couple spare 6522's so the first thing was to try changing that out, but no luck. I've ordered a 74LS259 for IC30 (IC32 on a regular B).
Thought I'd start there.

I'll take a video later this evening and post it on Youtube.

Phil
Last edited by nama on Mon Oct 15, 2018 1:14 am, edited 1 time in total.


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Re: BBD B+ Issues..please help

Post by nama » Mon Oct 15, 2018 8:50 pm

Quick Question about the 74LS259.

I've been probing the output pins Q0 - Q7 on IC30 74LS259 at power-up. I see briefly see high low pulses on all outputs except Q2 and Q6 which always seem to be stuck low.
Does anyone know if I should expect something to happen on Q2 and Q6 on power-up.
Screen Shot 2018-10-16 at 9.34.37 AM.png

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Re: BBD B+ Issues..please help

Post by Coeus » Mon Oct 15, 2018 9:06 pm

nama wrote:
Mon Oct 15, 2018 8:50 pm
Does anyone know if I should expect something to happen on Q2 and Q6 on power-up.
These are the values programmed into this IC on power-up, obtained by having B-Em print them out. These are the values that should appear on the outputs, not on the inputs:

Code: Select all

80	1010000
C0	1011010
E0	1011010
F0	1011010
F8	1100010
FC	1100011
FE	1100011
FF	1100011
F7	1100001
B7	1100001
37	0100101
3F	0100111
3E	0100111
3F	0100111
3E	0100111
3F	0100111
3E	0100111
3F	0100111
3E	0100111
3F	0100111
3E	0100111
3F	0100111
3E	0100111
3F	0100111
3E	0100111
3F	0100111
3E	0100111
3F	0100111
3E	0100111
3F	0100111
3E	0100111
3F	0100111
3E	0100111
3F	0100111
2F	0011101
27	0011011
A7	1100001
AF	1100011
A7	1100001
AF	1100011
A7	1100001
AF	1100011
2F	0011101
27	0011011
A7	1100001
AF	1100011
A7	1100001
AF	1100011
AE	1100011
AF	1100011
A7	1100001
AF	1100011
A7	1100001
AF	1100011
A7	1100001
AF	1100011
A7	1100001
AF	1100011
A7	1100001
AF	1100011
A7	1100001
AF	1100011
A7	1100001
AF	1100011
A7	1100001
AF	1100011
A7	1100001
AF	1100011
A7	1100001
AF	1100011
A7	1100001
AF	1100011
AE	1100011
AF	1100011
AE	1100011
AF	1100011
A7	1100001
AF	1100011
A7	1100001
AF	1100011
A7	1100001
AF	1100011
A7	1100001
AF	1100011
A7	1100001
AF	1100011
A7	1100001
AF	1100011
A7	1100001
AF	1100011
A7	1100001
AF	1100011
A7	1100001
AF	1100011
A7	1100001
AF	1100011
A7	1100001
AF	1100011
A7	1100001
AF	1100011
A7	1100001
AF	1100011
AE	1100011
AF	1100011
AE	1100011
AF	1100011
AE	1100011
AF	1100011

nama
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Re: BBD B+ Issues..please help

Post by nama » Mon Oct 15, 2018 11:41 pm

Hi Coeus,
Thanks for the reply.
So hooking up a logic analyser to Q0 - Q7, and looking at the output hex on powerup?
I can do that.

Phil

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Re: BBD B+ Issues..please help

Post by nama » Tue Oct 16, 2018 8:49 am

OK... I think I found something.

I hooked up my logic analyser to IC30 and I can see the following data on the outputs:

Code: Select all

84
C4
E4
F4
FC
FE
FF
F7
B7
37
3F
3E
3F
3E...
That seems to indicate that the 3rd bit is stuck high. The 3rd bit seems to correspond to Q2. Looking at the output of Q2 it starts low, and at power-up it jumps high and just stays high with no pulsing.

What do you guys think?
What should I check next?

Phil
Last edited by nama on Tue Oct 16, 2018 8:51 am, edited 2 times in total.

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Re: BBD B+ Issues..please help

Post by johnkenyon » Tue Oct 16, 2018 9:37 am

nama wrote:
Tue Oct 16, 2018 8:49 am
OK... I think I found something.

I hooked up my logic analyser to IC30 and I can see the following data on the outputs:

That seems to indicate that the 3rd bit is stuck high. The 3rd bit seems to correspond to Q2. Looking at the output of Q2 it starts low, and at power-up it jumps high and just stays high with no pulsing.

What do you guys think?
What should I check next?

Phil
Q1 and Q2 feed the speech processor - not that unusual for it to remain inactive (=high) once the MOS has set everything up.

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hoglet
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Re: BBD B+ Issues..please help

Post by hoglet » Tue Oct 16, 2018 12:26 pm

nama wrote:
Tue Oct 16, 2018 8:49 am
I hooked up my logic analyser to IC30 and I can see the following data on the outputs:
Out of interest, what logic analyser do you have?


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