So these 3 Superbrains followed me home...

[Witchy]

All I did was drive down to see forum member Zippysticks and they must've snuck into my car while we were chatting :D

This means that, for now, I'm home to a dismantled SB1 QD and a pair of complete SBII QDs which are well known to JonB and StephenM.



I've been after a Superbrain for 20 years so when the chance came up to get 3 of them I couldn't refuse, though not all of them are for me.

SBII with missing keys is the most complete and boots fine. Initially the 2nd SBII (on the left in the 2nd pic) screamed when the CRT board was connected so I assumed that the flyback had died even though the noise was definitely more of a scream than the whine of a failure. I borrowed the flyback from the SB1 (made sure the part numbers were the same) and also got a scream so the fault lay with the analogue board - indeed swapping that from SB1 got me a nice wobbly green display and successful boot.

One thing I had noticed with the original board was that not only did a component on it scream but it also took down the power rails so the machine wouldn't boot. Disconnect the CRT and feed it a floppy and it would boot and let me DIR. Well done to the PSU for not melting down while this was going on!

Got the noisy board on the bench earlier to test various components and thought I'd better check here to see if anyone else had worked on these boards before I start reinventing the wheel? Much searching didn't show anything up CRT wise apart from experimentations with a screen from an Amstrad GT65...

Cheers!

 

[acollins22]

Crikey that's a nice find.

I used those back in about '82-'84 and really liked them. Cracking machines.

Given their obvious fickle nature, do you think one might sneak into my car too?

(You can't blame a chap for trying )


Cheers,

Andy.

 

[Witchy]

Hehehe!

To answer my own question, not only has JonB already done the legwork on the analogue board but he's also discovered it's the same board as used in the Northstar Horizon so we have a full schematic and parts breakdown. Splendid.

When I first saw the internal shots of the CRT cage of the SB I remarked that it looked very DEC VT100 like, and sure enough the company that made the boards, Elston, also did the VT100 and Kaypro. Mind, if the flyback in SB2-2 HAD been dead having all this knowledge wouldn't really help because the last known company to make flybacks, HR Diemen, don't do a VT100 one either. I emailed the fine folk at donberg.ie with all the part numbers and they drew a blank too.

I've already found a dead IN7474A zener so now I need to find what failed to cause it to pack up. It would seem that a voltage regulator may be at fault, or possibly the HOT. Easy enough to remove and test.

 

[JonB]

Ah, so you found my threads (there are some on StarDot too) - especially the one where I track down the value of that pesky Zener... well done.

Should you find any machine broken beyond repair, I would like you to know that I am looking for a Superbrain case to house my fully functional SB II board. I'm also after a flyback transformer. I have a (broken) video board but no LOPT - for the SBII; my SB I is fine.

You might also be interested to know that I have uIDE working on both machines, booting and all CP/M drivers / updated ROM. More info here: http://www.cpcwiki.eu/index.php/UIDE_Universal_IDE_adapter_cards_for_Z-80_computers and there is a uRTC board for the SB I as well: http://www.cpcwiki.eu/index.php/URTC-8_Universal_RTC_for_Z80_computers.

There are demos on my YouToobZ channel. Terribly amateur and unscripted but you'll get the gist:

uIDE Superbrain demos: https://www.youtube.com/playlist?list=PL2L62b7tPEXvcFZYvyflF4LreJgGOxYUl

uRTC Superbrain demo: https://www.youtube.com/watch?v=POBRlgAFxmo


Shameless plugging, I know!

 

[alan8086]

@witchy - thanks for posting this, I too have a functional Superbrain, thanks in part to a lot of JonB's help. You have an audience of at least two now as anything Superbrain is interesting reading for me

 

[harry]

I too have a SB , ( on the list of things to do!) which gives the same screeching problem as witchy has described, And as I have had little time to investigate its cause, I am very interested to follow any progress or fault finding to cure this problem, please let us know your findings.

Harry

 

[JonB]

Well this is good stuff. I now know of (lemme see now...) at least six UK superbrain owners. We should form a club!

 

[geneb]

I may need to lean on you guys at some point: https://i.imgur.com/o4qgDE9.jpg :D

It got dropped VERY hard - tore out both front screws and the board attached to the left of the display is shifted down about 3/8". (you can see the bent standoffs)

g.

 

[JonB]

I think the screw hole can probably be repaired with car filler (I think it is called "bondo" in the US). The board to the left of the screen (I guess it is a hard drive interface) will need the mounting screws straightened and all connectors checked. I cannot see any other damage from the picture, but does it still boot, and does the HDD still work? If not, then you have problems.

 

[Witchy]

Evening all!

I've lost track of where I am with this, too many other things going on despite my kitchen being full of Superbrains. Some troubleshooting last weekend showed up a dead BU406 with corresponding dead IN4747A, replacements for which have both arrived. I did also try and buy a replacement 10uF 35V bipolar cap but like Jon I ended up with a couple of tiny ones. A search of CPC's website didn't reveal anything close to the original size still available either. I don't think there's anything wrong with the original but both my fluke meter and component tester rate it at around 60uF which seems mighty high, unless Nichicon had a lack of 10uF stock at the time and just relabelled anything with a higher rating. ESR is quite high but it's not open or shorted so it'll do for now.

No other dead components that I could see so once I've swapped the 2 failures I'll see if I still get a scream or a screen :D

I still need to resurrect my floppy writing PC too; I only have one boot disk that I think Jon created originally.

Cheers!

 

[JonB]

You should ask someone to send you one...

 

[alan8086]

I did also try and buy a replacement 10uF 35V bipolar cap but like Jon I ended up with a couple of tiny ones.

Cheers!

This is what I use as a replacement for the non polar electrolytic cap...



You'll have to click on the thumbnail - snip was too small.

I got 5 off eBay a year or so ago - my SB display has been fine and the component itself seems to have enough current throughput so that it doesn't overheat like those tiny modern electrolytics do.

Regarding your blown BU406 - one of my video boards had the same issue. The 15pF cap directly behind BU406's heatsink had gone short circuit - marked C438 in this board:



The one in the pic is a modern replacement - the original was a smaller green plastic axial tube thing, had hard black sludge leaking from one end. Once I replaced it and the BU406, I had a picture - which rapidly shrank in width as smoke poured from that aforementioned non polar electrolytic. Once that was replaced, I had a great SB display!

 

[Witchy]

This is what I use as a replacement for the non polar electrolytic cap...

Ah yes! I didn't think of those, I'll have a search through my spares bins to see if I've got anything suitable. Nice one!

 

[JonB]

I pulled one from another monitor. Not terribly good practice I know, but the screen is working now. You can also make one up by putting two normally polarised caps back to back (that is + to - / - to + if you get my drift) though I haven't tried it myself.

 

[Witchy]

Bah. Replaced the BU406, CR412 and put the ok-but-seemingly-far-too-much-overrated bipolar cap back in and I still get a scream. Fortunately the components I replaced are still OK so the fault obviously lies elsewhere...but where.

Wait, what's this? 22uF 50V Capacitor on a wonk in the HV circuit path. Let's just pull that out...



The Screaming Cap! The one next to it (470uF 25V) also tests low with high ESR so we'll swap that one too. Suddenly...



Picture is a bit wobbly on the horizontal as it is with its original analogue board - the one currently in is from the SB1.

So! That's 2 almost-fully working SB2s and a supposedly working SB1 but at least the SB1's CRT tray is fully functional now. Just need to sort out its lumpy capacitored PSU! Oh, and work out how to put it back together

 

[JonB]

Well done Witchy!

Now you just need a copy of the proper boot disk for this machine (which doesn't leave the screen in reverse video). I have it - so if you PM me your address you might get a nice pre-Xmas present. Oh, and I'll stick the uIDE SB II utility suite on there as well. You never know

Also have it as an image file if you can stick it on a disk.

 

[Witchy]

Funnily enough I'm just having a go at writing some disks as we speak, namely the SB II Boot one plus a couple from Don Maslin's archive. Not having much success but I suspect head incompatibility between the writing drive in my floppy imaging PC and the SB's. Since the drive from the SB1 is currently loose I'll try hooking that up to the PC and see where we go. I've actually got another drive out of a Superbrain that I can't remember the origin of (to me, obviously its ultimate origin was a Superbrain) so I'll see if I can find that too.

Settings I'm using: 40 track, single sided, double step off. Disk is unreadable in a real machine or in 22disk with SUP3 as the definition.

 

[JonB]

Funnily enough I'm just having a go at writing some disks as we speak, namely the SB II Boot one plus a couple from Don Maslin's archive. Not having much success but I suspect head incompatibility between the writing drive in my floppy imaging PC and the SB's. Since the drive from the SB1 is currently loose I'll try hooking that up to the PC and see where we go. I've actually got another drive out of a Superbrain that I can't remember the origin of (to me, obviously its ultimate origin was a Superbrain) so I'll see if I can find that too.

Settings I'm using: 40 track, single sided, double step off. Disk is unreadable in a real machine or in 22disk with SUP3 as the definition.

I have a GoTek on my SB II board as no working TM100 drive units.

AFAIK the only SBII boot disk in the Maslin archive boots to reverse screen. Zippysticks and I had to hack a working disk together, then when I reverse engineered the hidden BIOS I found where it sets the video chip up and fixed it properly. That’s why I’m offering to send you a disk.

I think you need double step on.

 

[harry]

Good work Witchy, glad you got your screen back, I must find the time to dig out the screecher, and checkout the caps , I originally thought the flyback was needing replacing, but your findings now give me new hope! thanks for sharing.


harry

 

[Witchy]

I've forgotten to keep this thread up to date, though I know nobody's hanging on tenterhooks to see how I got on

The SBII QD is a happy bunny aside from drive B, which is fine as the spare space can house a Gotek.

The SB1 came to me as a kit of parts so as well as fixing it I had to work out how it all went back together, particularly the honking great toroidal step-down transformer. It's PSU was dead so much troubleshooting and component testing eventually revealed the MJE13004 power transistor to be utterly without life, and one of the caps was out of tolerance so I swapped that too. Surprisingly all the board itself needed was a cleanup and a small repair to an LS123 that had a capacitor soldered across 2 of the pins. That is now booting from a gotek too. Pics tell a better story that words sometimes, so...

SB1 fixing, the garbled screen is because I'm booting from an SBII image, thanks JonB!


NORMAL.COM makes the screen look like this:


This is my SBII booting from the same images.


Now the fun part - where to put them on display on account of them being HUGE :mrgreen: