The Newbrain Forum

portretro · 12-07-2022 00:28:45

{strange forum ... you get logged out if you take too long to finish your post and so your post is lost ... good thing I always copy/paste it before "submitting" just in case something like that happens}

Greetings from Australia.

I got my first NewBrain a month or two ago from the Netherlands.

On initial power up it was working - VFD was correct, BASIC was running.

Then the RIFA blew, then the machine stopped working. I believe they are unrelated.

I've fixed the PSU up (replaced all capacitors) and now can measure 13.5V, 7V and -5V unloaded.

I made some early progress on the machine itself ... the reset/startup capacitors had leaked ... so got it booting after replacing those.

The screen would then show random garbage. Since then I think I've started to chase my tail.

I noticed several other electrolytics had corrosion around them, so have replaced most of them: all three on memory board, all on the keyboard and most on the main board. I haven't yet replaced the ones near the cassette ports as they looked okay and I assumed they were related to tape input.

As the machine was working, then failed, I'm guessing it has to be due to the capacitor leakage causing some problem ... still.

I now observe the following:

[1] VFD displays random garbage and pressing keys can make more random garbage appear
[2] Screen remains black after replacing original ROMs with 3 x 27C64s with "version 2.0" - prior it would show B&W garbage
[3] Sometimes when powered on the Z80 address lines are stuck, sometimes I see activity ... so maybe the Z80 isn't coming out of reset properly? Either case, screen remains black.

Summary:

[1] Replaced most electrolytics due to leakage/corrosion (except 94/95/104)
[2] 74LS245 (421) was getting very warm (50C) so replaced with 74HCT245, now cool
[3] 74LS153 (403) was replaced as A12 on the Z80 seemed stuck - now working
[4] Replaced original 8/16KB ROMs with 3 x 27C64s with "version 2.0" from emulator - with the original ROMs I would see garbage on the screen, now I see just a blank screen. ROMs are AB (406), CD (407), EF (408)
[5] Replaced two resistors (28, 129) due to corrosion
[6] LM358P (401) was cleaned & re-seated in IC socket due to corrosion

Video of my initial troubleshooting here:
https://youtu.be/fnzsDMvMMXY

... and some more:
https://youtu.be/OIAWtzaw-M8

Photos of board here:
https://www.dropbox.com/sh/72t5bt0f9aid … 5BlAa?dl=0

Any ideas what I should try next?

Thanks

Brett.

Last edited by portretro (12-07-2022 01:25:13)

kluder · 12-07-2022 13:16:41

Hi

You write 13.5V, 7V and -5V on the PSU sad
This indicates that you have a bad transistor in the output on the minus side
The two plus voltages are OK but the minus voltage is wrong
There must be + 13.5V, + 7V and - 12V

Greetings
Klaus

use google to translate

portretro · 26-07-2022 12:17:02

Hey Klaus,

Thanks for replying.

I'm a bit confused now - I've been going off Thomas's troubleshooting where he says it should be +5V, -5V and +12V:

http://www.8bit-homecomputermuseum.at/r … brain.html

I re-checked and I'm definitely getting +/-5V and +12-14V on the board at the same positions Thomas indicates.

I've decided to try to make a 32KB SRAM replacement board, so I've removed the existing memory board.

Here's my latest update wink

https://youtu.be/e-BDFOF9eoQ

/Brett

portretro · 27-07-2022 08:48:30

Ahh, apologies Klaus, I see where I've caused the confusion.

The output from the PSU is -12V but it's converted to +12V in the power section of the NB.

I had "assumed" that power out from PSU was the same as power measured inside the NB (except, under load)!

Still some strange behaviour. I'm going to re-do the startup/reset capacitors as sometimes the machine will just not boot at all.

And when it does boot, I get garbage.

As I said, I've removed the RAM board ... thinking now of just trying to add the SRAM via the expansion port rather than trying to reproduce the CAS/RAS generation logic. The Spectrum design made it look pretty simple as this logic is contained within the ULA ...

Just trying to understand the EXRM0/1/2 and RAMENB signal purposes. I know the expansion unit gave extra 64KB (so 96KB in total) ... is this possible via the expansion port (3x32KB) ... ?

/Brett

kluder · 03-08-2022 08:37:12

Hi

First a small correction, I assume you mean converted to -5V on the mainboard. smile

I don't know if you have seen the diagram I posted in "Newbrain PCB layout schematics"
https://newbrainemu.eu/components/com_a … -Board.pdf

Some of the circuits that are included in monitor signals are 433, 436, 450, 453, 454 and 444, as well as that some signals come from the RAM board which are also used in connection with the monitor signal, it can also be RAM circuits that are to blame for the bad picture.

Even the Expansion box is equipped with 8 x TMS 4164 DRAM circuits (65536x1bit).
The machine should be able to handle up to 2Mb RAM, but there will still only be 32Kb for basic and 64Kb for CPM, the rest can be used for a RAM DISK.

I don't think you can get it to run without the RAM board in that there are some signals coming from the RAM board that must be used on the mainboard and the CPU starts by checking the RAM circuits that are on the RAM board after which it continues with the expansion box according to that software located in EPROM/ROM circuits 406, 407 and 408.

If you manage to make a RAM board equipped with SRAM, I will be very interested in that diagram, as I myself have made a new RAM board equipped with 256K x 4 CMOS DRAM

Greetings
Klaus

use google to translate

The Newbrain Forum

#1 12-07-2022 00:28:45

#2 12-07-2022 13:16:41

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#3 26-07-2022 12:17:02

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#4 27-07-2022 08:48:30

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#5 03-08-2022 08:37:12

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