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Superbrain QD repair

A

alan8086

Experienced Member
Joined
Oct 1, 2016
Messages
331
Location
Burnley, UK
Hi - I've been attempting repairs again on my Superbrain QD for the nth time in 11 years. I thought I'd plot the course on here, post some vids/pictures etc.

JonB very kindly tried various repairs on one of my SB boards but had to send it back in the end. I have managed to repair one of my PSU boards to the extent that it supplies the correct voltages to the board now. I'm trying again with the original logic board that JonB hasn't seen - the one that came with the SB. At the mo, the disk drives spin and my Z80 ICE reports a short on address bus 3 - which to me sounds like a good place to start with a logic probe, current tracer etc, tracing which components may affect the path of address bus 3 from the schematics. I also note a 700ohm reading between ground and +12V - maybe the component that shorted the adress bus? maybe - if I'm very lucky!

My knowledge of digital logic fault analysis is weak - following JonB's posts about his SB repairs, I quickly get lost. However, I have the tools and hopefully this experience should teach me what I need to know.

First vid is just me waffling on for 12 mins showing spare parts and what came with the SB, software disks etc. I promise to speak up next time and speak quicker! I sound asleep!

This is the first and longest of 4 vids I did today.


Please ignore the Computer Repair branding on my Google account logo - the company no longer exists!
 
Great to see another Superbrain user in the UK !
I have three of them two Superbrain II QD and a Superbrain with hard disk fitted - currently only one working.

Be interested to get some images of the floppies uploaded when you get round to them so see what useful and interesting software you have.

Keep at the repairs !

Jon is very helpful and knowledgeable on these & conveniently close to where I live too and he helped get one of my Superbrains sorted with a boot disk that I know he has now improved upon.
 
Are yours just three out of a big collection of other machines or do you have a particular interest in the Superbrain? I'll have to put my Dave Dunfield rig back together over the weekend - I'll try and see what, if anything is on those disks. Not sure if a Kryoflux device might restore them if the data has faded? i'll have to read up on them. What software do you have?


zippysticks said:
Great to see another Superbrain user in the UK !
I have three of them two Superbrain II QD and a Superbrain with hard disk fitted - currently only one working.

Be interested to get some images of the floppies uploaded when you get round to them so see what useful and interesting software you have.

Keep at the repairs !

Jon is very helpful and knowledgeable on these & conveniently close to where I live too and he helped get one of my Superbrains sorted with a boot disk that I know he has now improved upon.
Click to expand...
 
alan8086 said:
Are yours just three out of a big collection of other machines or do you have a particular interest in the Superbrain? I'll have to put my Dave Dunfield rig back together over the weekend - I'll try and see what, if anything is on those disks. Not sure if a Kryoflux device might restore them if the data has faded? i'll have to read up on them. What software do you have?
Click to expand...

Mine are three out of a big collection of other machines (mostly Commodore).
Software wise, I don't have anything other than a Superbrain II boot disk that Jonb helped me create. He has recently managed to improve on this build and get useful utilities including Kermit installed.
One of my Superbrains has a hard drive installed but I have not yet got around to finding out if that works or has any useful software installed on it - too many other half baked projects on the go.

I have a Kryoflux if you get around to wanting to try it on any of the disks.
 
You have Kryoflux?! If I cant make images of the disks, I may as well pass them on to you if you are willing to have a go?
 
In the mean time....

My latest purchase!!

https://www.ebay.co.uk/itm/123602450740

Its an ABI Electronics ICT - 24 Diagnostic chip tester - rebranded as an RS Components unit. I asked the seller what comes up on the screen when powered on. 'CHIPS' was the correct answer!

http://www.jammarcade.net/abi-ict-24-digital-ic-tester-tech-info/

This guy in the above site has the manual and ROM dumps available for download. ABI's first chip tester! Not far behind what my DDS 40 XP can do, maybe it will be of use to help fix it? As well as the SB and IMS 8000.
 
One more vid - forgot to mention the spare chips I have. Just a short one so not too painfull!!

I'll try and do some proper fault diagnosis on Sunday, pics and conclusions etc.
 
Hi Alan

I'm looking for a spare video board for the SB, preferebly with flyback transformer and defelector coils. I recently acquired a Superbrian II motherboard that works (!!). I have it running with an ASTEC PSU (actually the one from my SB I that used to drive the now zapped HDD) and HxC floppy emulator.

I documented all the diagnostics here: https://stardot.org.uk/forums/viewtopic.php?f=45&t=16316

..and I found that the video output is very non standard, so I am struggling to get a CRT monitor to give a good picture. So far I tried a GT65 (Amstrad) and a cute little 9" monitor but neither of them have enough adjustment to compensate for the SB II's overwide video output. I cannot see the last 6 columns of the display.

Can you help?

Also, I have a PC that can read / write Superbrain disks... I could try to read your disks. Nashua disks were known to be of very low quality by the way.

@zippysticks, I've got some disk images from CCH but they look to be SB I disks. Have a few interesting bits on them: JBIOS, JBIOS 2 MicorMode SuperBIOS 1.7 and a replacement 2.2 BDOS. Mostly with source...
 
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It's OK Alan. I'm up to my neck in retro computers and my SB I and II both work, so I have no real cause for envy.

You seen to be gathering tools. That IC tester looks a bit tasty... what's the plan? Will you use it to check all the logic chips on the board? If it's the one you sent me, I have a few ideas where you can start.

That midi device looks like it connects to the SB serial port. I bet it is pretty simple inside. We could reverse engineer it!

Regarding the Z80 ICE, I have already produced an 8080 ICE copy using a different board layout and a 3.3v CPLD. I'm planning to try to do the same with the Z80 ICE so you can possibly get your hands on one at some point.
 
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Hi Jon, I've just read through your stardot posts. Trouble is, if I gave you a video board - your various post might get a lot less interesting and convoluted! I never imagined such a lash up of old and new tech to be possible!
 
....oh, my plan with the ABI chip tester, Id like to do some traditional fault diagnosis - my polar instruments device plus logic probes and schematics should help there. Once I get desperate enough and if I get no where doing it the proper way, will I then start desoldering chips one at a time to test them!
 
Last non - testing vid and last test kit vid. I'm only posting all this in case anyone has any guidance regarding my use of what I have in getting to a working Superbrain.

 
No need to give me one, I’ll pay you, or you might consider loaning me one.

It’s either that or try to find a CRT monitor with enough deflection control to accommodate the SB’s horizontal overscan (which is excessive). It’s tricky enough finding s monochrome monitor..
 
alan8086 said:
Hi Jon, I've just read through your stardot posts. Trouble is, if I gave you a video board - your various post might get a lot less interesting and convoluted! I never imagined such a lash up of old and new tech to be possible!
Click to expand...

In fact, it is all old: the monitor guts came from an Amstrad GT65 (old green screen mono monitor supplied with the CPC464), the PSU is contemporary with my SB I (was used to provide 12v to the hard drive) and the sync mixer is made of LS7400 series ICs. The only thing that isn't old is the FDD emulator, and these devices are well known (jolly useful, too).

I'm currently trying to guess how best to endow the GT65 with horizontal width adjustment. Errrmmmm.... not my area of expertise at all, but as long as I observe a few basic safety precautions I should be OK.
 
Well, I managed to hack the GT65 to show a narrower screen after all, with help from the guys at CPCWiki; so it looks like I'm OK monitor-wise for now. Offer still stands, though.

But back to you, Alan: I'm waiting for the next update!
 
Not much of an update - I've got my Suoerbrain spread out over that box I sent the board to you in:

WP_20190129_14_32_58_Pro_LI.jpg

I've burned one of the ROM dumps from one of your VCFED posts - a basic QD BIOS. Burned it on to a 2816 and the adapter board you supplied.

I can count through the ROM part of the address map ok with my polar instruments device.
It reports errors at every RAM address if you do the same on the RAM test - suggesting an issue with access to the RAM rather than every chip being faulty. Or maybe my device doesn't like Superbrains! The Learn test seems to create the address map ok - where ROM ends and RAM begins etc.

The Shorts test says there is a short on address line 3 - I traced the route from adr3 pin on the Z80 from the schematics. I've logic probed/pulsed a nearby inverter and one other chip but found no issue with them.

More probing to do and I'm wondering if it's worth de soldering all the RAM on both boards and testing it when my ABI tester arrives. I would have a pile of proven RAM that could be socketed back into the board. I would then 'only' have glue logic to worry about as I have good Z80s, working ROM and many of the socketed VLSI chips to substitute.
 
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Oh, I also lost the 15v going to the VDU board - now found it again. The 15v regulator leads got chopped where I managed to pinch them between the board and the case. I have good voltages going to the vdu driver board and a glowing crt heater. No HT but I suppose without any video signal I get no Horizontal or Vertical drive signal or anything to drive the flyback transformer?
 
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