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Making some progress with the 16K Microbee - Printable Version

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RE: Making some progress with the 16K Microbee - CheshireNoir - 21-11-2021

So further work has been carried out. I got the 'scope out and probed backwards. I didn't seem to be getting any signal at the connector but did see some at the other end of R20. I replaced R20, and then the connector (It was more than a bit loose and more than a bit oxidised, despite deoxit) but neither change made a difference.
Now I have a theory here, but I wanted to check it past the more experienced people here.
This is what I'm getting at the point where TR2, R18 and R20 meet (looking at the "microbee motherboard issue 2" schematic).
[attachment=300]
"black" background.
[attachment=301]
A line of text pixels

These look, to me, to be a signal, but they seem WAY too quiet. 250mV before they go through R20. Nothing out the other side,
My theory is that TR2 is not amplifying enough. Should I replace TR2? Is something else in that mess of analog possibly pulling things down too far? Should I look at swapping R17 to a different value, as it's marked as being the video level adjustment?

I don't want to continue replacing scattergun style if I can avoid it.

I've reflowed absolutely everything in that area to no avail. I've probed around with a non conductive probe in case something is "loose". No changes.
I've checked to make sure nothing seems to be shorting out between R20 and the connector. (If it is, it's nigh invisible)

It feels like I must be really close!

Chesh

EDIT: I just jumpered around R20 and got a signal. This seems to confirm the signal level is too low.


RE: Making some progress with the 16K Microbee - Ernest - 22-11-2021

Sounds like an open circuit collector to TR2 or no 5V to the transistor.

Ernest


RE: Making some progress with the 16K Microbee - CheshireNoir - 22-11-2021

Thanks Ernest,
I'll have a look at (and around) TR2 tonight.

Chesh


RE: Making some progress with the 16K Microbee - CheshireNoir - 23-11-2021

TR2 is getting 5v on the Collector fine. I swapped it, figuring it was the most likely culprit, but no! It's still not working. R18 looked out of spec (800ohm rather than 330ohm) but swapping that didn't help either. I think I'll need to look at C20 next.

Chesh


RE: Making some progress with the 16K Microbee - someone - 24-11-2021

(20-11-2021, 02:49 PM)CheshireNoir Wrote: ...and the Retro gods taketh away. Stuck it back in the case and I'm back to no video. WTF?

If it doesn't cold boot beep either, its likely to be poor coreboard to baseboard connection or misalignment.


RE: Making some progress with the 16K Microbee - CheshireNoir - 24-11-2021

It's still cold boot beeping. I also get picture if I jumper past R20.
Tonight I'll be doing the video mods from the Microbee Hardware Notebook. (Page 63, B3)
Looks like mine has never had the video circuit upgraded.

Chesh


RE: Making some progress with the 16K Microbee - CheshireNoir - 24-11-2021

The video fix, well, fixed it.
I have a picture. Consistently.
Pity the only letter of the word "load" that works is "o" and even that is intermittent :-D
I'm thinking long term I'll get a keyboard replacement kit, apply that to the PC85B and use the keys to replace all the bad switches on this one.

Thanks everyone for the advice, hints and encouragement.

Chesh


RE: Making some progress with the 16K Microbee - ChickenMan - 24-11-2021

Glad to hear all is finally working well, good job Smile Another Microbee survives Smile


RE: Making some progress with the 16K Microbee - CheshireNoir - 25-11-2021

I've reached out, asking for when the keyboard kits will be back in stock :-)

I also write it all up: https://ilike8bits.com/2021/11/25/three-bee-three-bee/

Chesh


RE: Making some progress with the 16K Microbee - CheshireNoir - 26-11-2021

Final update: I desoldered all the dead keys, disassembled them and resoldered and it completely works! I loaded up a game from "tape" and that works as well. All in all a successful refurb.
It doesn't have a top case and the "ctrl" key is missing, but otherwise it works.

BWAHAHAH!

erm.

Chesh