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#1 06-07-2021 21:32:03

nama
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Help needed - Newbrain AD partial boot

I’m hoping someone can help me. I’m presently repairing a Newbrain AD and I need some assistance.

I have replaced all the caps and did troubleshooting on the power circuit as I was not getting 12V.

I now have all the voltages and the machine partially boots.
On power-up the built in display makes it some distance through the startup sequenc, but then hangs.
i.e. sometimes it may show 'READY' prompt, sometimes just 'READ', sometimes 'NEWBRA', sometimes the flashing '_' cursor…etc.
The machine then seems to hang. Pressing the keyboard does not display anything.

After it hangs on a word (or partial word) from the splash screen (e.g. NEWBRAIN) and I press keys the 'NEWBRAIN' message doesn’t disappear but often some random characters appear as if over the top of the word(s), and the intensity of the characters gets brighter. Releasing the keys and the 'NEWBRAIN' message remains. A little hard to explain.

The composite monitor seems to have sync issues as far as I can tell. Not sure whats going on here. lots of movement. No characters visible.

Hopefully someone here may be able to give me some advice on how to proceed with testing, or has an idea as to what could be happening.

Much thanks for your time.

Last edited by nama (07-07-2021 00:35:04)

 

#2 07-07-2021 01:13:06

nama
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Re: Help needed - Newbrain AD partial boot

I will also add that after getting the voltages fixed I let the machine run for a few minutes to asses whether there are some obvious hot ICs. Everything seemed very cool, with the exception of the 74ls245 buffer between RAMboard and motherboard which got slightly warm. RAM stayed cool.

I have also tried the technique where you can short pin 1 and 6 on the rear connector to initiate a boot from the second bank or RAM. No real change in the startup could be seen.

I posted some videos here:
https://www.youtube.com/watch?v=OpPDns-IT8I

and here:
https://www.youtube.com/watch?v=1bftgg1AIRI

Last edited by nama (07-07-2021 05:17:55)

 

#3 10-07-2021 23:41:07

ellvis
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Re: Help needed - Newbrain AD partial boot

Hi nama,

not much of ideas myself, but try to look here, it may help you: http://www.8bit-homecomputermuseum.at/r … epair.html

Good luck!

ellvis

 

#4 11-07-2021 06:07:02

nama
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Re: Help needed - Newbrain AD partial boot

Thanks Ellvis,
I think I've scoured the internet for every scrap of information about the Newbrain I can find. I have come across that page many many times.

Thanks though.

 

#5 11-08-2021 06:34:01

nama
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Re: Help needed - Newbrain AD partial boot

Two steps forward, one step back...an update:

Been working very hard in this:
SO of you recall I was getting the machine to almost boot, but it would then always crash/hang. There was also no sync from the video output, so I decided to trace the video signal back from the video out jack and finally I found an 74LS06 @ 435 had dead outputs. replaced this the screen then had sync, but still no image/text. I was however very surprised to see the machine (via the VFD display) was working. keyboard was working too.

Probing around the video generation circuitry I found a 74LS157 @ 444 with dead outputs. Again I replaced and now the video works and I get the splash screen!!!!

Doing a PRINT FREE only shows 13k of the 32k is being addresses, so definitely some RAM issues to look into.

Now this is where the problem start again. I had the machine turned on and was looking around the RAM then I noticed the VFD screen wasn't showing anything, and the keyboard was dead (Composite out still works and shows the machine is actually still running). I've checked the output of the DC/DC board and it looks ok. So what chip deals with both the keyboard and the VFD ???? Maybe the COP420 ???.

• UPDATE - I checked AC and DC voltages on the DC/DC board and they look ok.

Now I'm a bit stuck. so as always any advice would be greatly appreciated.

Last edited by nama (11-08-2021 23:36:46)

 

#6 14-08-2021 02:37:57

nama
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Re: Help needed - Newbrain AD partial boot

Another update:
The reason the keyboard and the VFD display wasn't working was because the MC14584 Schmitt Trigger inverter was dead at 417...DOn't have an exact replacement so for now I've socketed and dropped in a 74HC14.
Now back to finding that RAM issue.


PRINT FREE   =  13991

POKE 2000,0
PRINT PEEK (20000)  =  1 (Not 0 as expected)

I think this likely means the RAM at 416 is bad? That might make sense as its the chip closest to the leaked capacitor. (FYI - All caps have been replaced and area cleaned with IPA).

 

#7 14-08-2021 22:32:12

nama
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Re: Help needed - Newbrain AD partial boot

Finally the last thing was to replace 4116 @ 416. Now I have a fully functioning Newbrain AD (although I have not checked the cassette ports yet)
I have some MC14584 on order to replace the temporary install of the 74HC14, then I can stick it all back in the case... if I can remember how to.

Philip.

 

#8 16-08-2021 09:37:55

nama
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Re: Help needed - Newbrain AD partial boot

 
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